What is Long-term memory?

There are three main types of memories: sensory memory, short-term memory, and long-term memory. Sensory memory in short is the ability to hold sensory information for a short period of time, for example looking at an object and being able to remember what it looked like moments after. Short-term memory is memory that allows a person to recall a short period of time; this can be a few seconds to a minute. Short-term memory allows people to remember what happened during that short time span without actually practicing the memory. Long-term memory has a much larger capacity than the prior two and actually stores information from both these types of memories to create a long lasting and large memory. Long-term memory is the largest target for research involving selective memory erasure.

Memory erasure

Memory erasure is the selective artificial removal of memories or associations from the mind.

There are many reasons that research is being done on the selective removal of memories. Potential patients for this research include patients suffering from psychiatric disorders such as post traumatic stress disorder, or substance use disorder, among others.

Memory erasure has been shown to be possible in some experimental conditions; some of the techniques currently being investigated are: drug-induced amnesia, selective memory suppression, destruction of neurons, interruption of memory, reconsolidation, and the disruption of specific molecular mechanisms.

Research focused on gaining a better understanding of what memories are has been going on for many years, in this way so has research in memory erasure. The basis for the recent history for memory erasure has been focused on determining how the brain actively keeps memories stored and retrieves them. There have been several instances where researchers found drugs that when applied to certain areas of the brain, usually the amygdala, have relative success in being able to erase some memories. As early as 2009 researchers were able to trace and destroy neurons involved in supporting the specific type of memory that they were trying to erase. This caused the erasure of the target memory.

Aside from the biotechnology approach to studying memory, research in psychiatry on how memories work has also been going on for several years. There has been some studies that show that some behavioral therapy can erase bad memories. Although there has been some evidence that psychodynamic therapy and other energy techniques can help with forgetting memories among other psychiatric issues there is no proven therapeutic approach for trying to erase bad memories.

There are several different types of possible patients that have the potential to draw great benefit from the selective memory erasure; these include people suffering from drug addiction, or posttraumatic stress disorder (PTSD). PTSD patients may include war veterans, people who witnessed horrific events, victims of violent crimes and many other possibly traumatic events. These potential patients have unwanted memories that can be absolutely devastating to their daily lives and cause them to not be able to function properly.

Along with patients suffering from these severe circumstances the idea of selective memory erasure is a very attractive idea for many. Thus making the practical use of this technology something that could be used by many people.

Within long-term memory there are also different types of long-term memories. Implicit memory is one type of long-term memory that is generally described as the ability to remember how to use objects or specific movements of the body (e.g. using a hammer). Another type of long-term memory, explicit memory, refers to memories that can be consciously drawn upon by a person to remember. Explicit memory can be split into further subcategories, one being episodic memory, which is the memory of specific events and the information surrounding it, as well as semantic memory, which is the ability to remember factual information (e.g. what numbers mean).

A type of memory of main concern for memory erasure are emotional memories. These memories often involve several different aspects of information in them that can come from a variety of the different categories of memories mentioned above. These emotional memories are very powerful memories and can have very strong physiological effects on a person. An example of an emotional memory can be found in patients suffering from PTSD, for these patients a traumatic event has left a lasting emotional memory that can have powerful effects on a person even without them consciously retrieving the memory.

Drug-induced amnesia is the idea of selectively losing or inhibiting the creation of memories using drugs. Amnesia can be used as a treatment for patients who have experienced psychological trauma or for medical procedures where full anesthesia is not an option. Drug-induced amnesia is also a side-effect of other drugs like alcohol and rohypnol.

There are other drugs that also can cause their users to be put in an amnesic state, where they experience some type of amnesia because of their use. Examples of these drugs include Triazolam, Midazolam and Diazepam.

There is a growing amount of information that has shown that memory depends largely on the brain's synaptic plasticity, with a large part of this being dependent on its ability to maintain long-term potentiation (LTP). Studies on LTP have also started to indicate that there are several molecular mechanisms that may be at the basis of memory storage.
Thus a more recent approach to erasing memories and the associations the brain makes with objects is disrupting specific molecular mechanisms in the brain that are actively keeping memories active.

Recovering methamphetamine (METH) addicts have reported that the sight of certain objects such as a lighter, gum or drug paraphernalia can cause massive cravings that can sometimes lead to a break in their mental strength and cause them to relapse. This indicates that long-term memories can be called upon by various different associations that were made with the memory without the conscious effort of the person. With an increasing belief that memories are largely supported by functional and structural plasticity deriving from F-actin polymerization in postsynaptic dendritic spines at excitatory synapses. Recent research has been done to target this F-actin polymerization by using direct actin depolymerization or a myosin II inhibitor to disrupt the polymerized F-actin associated with METH memory associations. The study indicated types of associations can be disrupted days to weeks after consolidation. Although the depolymerization techniques had no effect on food reward based associations or shock based associations the results demonstrate the idea that meth associated memories' actin cytoskeleton is constantly changing making it uniquely sensitive to depolymerization during the maintenance phase. This is some of the first evidence showing that memories made with different associations are actively maintained using different molecular substrates. These results also show that the actin cytoskeleton may be a promising target for selective disruption of unwanted long-term memories.

Selective memory suppression is the idea that someone can consciously block an unwanted memory. There are many different therapeutic techniques or training that has been done to test this idea with some success. Many of these techniques focus on blocking the retrieval of a memory using different suppression techniques to slowly teach the brain to suppress the memory. Although some of these techniques have been useful for some people it has not been shown to be a clear cut solution to forgetting memories. Because these memories are not truly erased but merely suppressed the question of how permanent the solution is and what actually happens to the memories can be troubling for some.

Selective memory suppression is also something that can occur without a person being consciously aware of suppressing the creation and retrieval of unwanted memories. When this occurs without the person knowing it is usually referred to as memory inhibition; the memory itself is called a repressed memory.

One of the ways scientists have attempted to erase these memories through suppression is by interrupting the reconsolidation of a memory. Memory consolidation of a memory is when a person recalls a memory, usually a fearful one, it becomes susceptible to alteration, and then gets stored again. This has led many researchers to believe that this time period is the best time for memories to be altered or erased. Studies have shown that through behavioral training results showed that they were able to erase memories by tampering with memories during the reconsolidation phase.

With evidence showing that different memories excite different neurons or system of neurons in the brain the technique of destroying select neurons in the brain to erase specific memories is also being researched. Studies have started to investigate the possibility of using distinct toxins along with biotechnology that allows the researchers to see which areas of the brain are being used during the reward learning process of making a memory to destroy target neurons. In a paper published in 2009, authors showed that neurons in the lateral amygdala that had a higher level of cyclic adenosine monophosphate response element-binding protein (CREB) were activated primarily over other neurons by fear memory expression. This indicated to them that these neurons were directly involved in the making of the memory trace for that fear memory. They then proceeded to train mice using auditory fear training to produce a fear memory. They proceeded to check which of the neurons were overexpressing CREB and then, using a inducible diphtheria-toxin strategy, they destroyed those neurons, resulting in persistent and strong memory erasure of the fear memory.

Due to the lack of understanding of the brain this technique of destroying neurons may have a much larger effect on the patient than just the removal of the intended memories. Due to this complex nature of the brain treatment that would stun the neurons instead of destroying them could be another approach that could be taken.

As with most new technologies the idea of being able to erase memories comes with many ethical questions. One ethical question that arises is the idea that although there are some extremely painful memories that some people (for example PTSD patients) would like to be rid of, not all unpleasant memories are bad. The ability to soften or erase memories could have drastic effects on how society functions. The ability to remember unpleasant effects from one's past has a huge impact on the future actions they may take. Remembering and learning from past mistakes is crucial in the emotional development of a person and helps to ensure they do not repeat previous errors. The ability to erase memory could also have a massive impact on the law. When it comes to determining the outcome of a trial, the ability to modify memory could have a massive impact on the judicial system. Another ethical question that arises is to how the government will use this technology and what restrictions would need to be put in place. Some worry that if soldiers can go into battle knowing that the memories created during that time period can simply be erased they may not uphold military morale and standards.

Memory erasure has also been a common topic of interest in science fiction and other fiction. Several notable comics, TV shows and movies feature mindwipes, including "Telefon", "Total Recall", "Men in Black", "Eternal Sunshine of the Spotless Mind", "The Bourne Identity", and "Dollhouse". Novels that feature memory erasure include "The Invincible" by Stanisław Lem and some of the "Harry Potter" novels (including "Harry Potter and the Chamber of Secrets") by J. K. Rowling, also "The Giver" by Lois Lowry deals with this subject. Several works by Philip K. Dick are about mindwipes, including "Paycheck" and "We Can Remember It for You Wholesale" (inspiration for "Total Recall").

While short-term memory encodes information acoustically, long-term memory encodes it semantically: Baddeley (1966) discovered that, after 20 minutes, test subjects had the most difficulty recalling a collection of words that had similar meanings (e.g. big, large, great, huge) long-term. Another part of long-term memory is episodic memory, "which attempts to capture information such as 'what', 'when' and 'where. With episodic memory, individuals are able to recall specific events such as birthday parties and weddings.

Memory

Memory is the faculty of the brain by which information is encoded, stored, and retrieved when needed.

Memory is vital to experiences, it is the retention of information over time for the purpose of influencing future action. If we could not remember past events, we could not learn or develop language, relationships, or personal identity (Eysenck, 2012).

Often memory is understood as an informational processing system with explicit and implicit functioning that is made up of a sensory processor, short-term (or working) memory, and long-term memory (Baddely, 2007). This can be related to the neuron.
The sensory processor allows information from the outside world to be sensed in the form of chemical and physical stimuli and attended to with various levels of focus and intent. Working memory serves as an encoding and retrieval processor. Information in the form of stimuli is encoded in accordance with explicit or implicit functions by the working memory processor. The working memory also retrieves information from previously stored material. Finally, the function of long-term memory is to store data through various categorical models or systems (Baddely, 2007).

Explicit and implicit functions of memory are also known as declarative and non-declarative systems (Squire, 2009). These systems involve the purposeful intention of memory retrieval and storage, or lack thereof.
Declarative, or explicit, memory is the conscious storage and recollection of data (Graf & Schacter, 1985). Under declarative memory resides semantic and episodic memory. Semantic memory refers to memory that is encoded with specific meaning (Eysenck, 2012), while episodic memory refers to information that is encoded along a spatial and temporal plane (Schacter & Addis, 2007; Szpunar, 2010). Declarative memory is usually the primary process thought of when referencing memory (Eysenck, 2012).

Non-declarative, or implicit, memory is the unconscious storage and recollection of information (Foerde & Poldrack, 2009). An example of a non-declarative process would be the unconscious learning or retrieval of information by way of procedural memory, or a priming phenomenon (Eysenck, 2012; Foerde & Poldrack, 2009; Tulving & Schacter, 1990). Priming is the process of subliminally arousing specific responses from memory and shows that not all memory is consciously activated (Tulving & Schacter, 1990), whereas procedural memory is the slow and gradual learning of skills that often occurs without conscious attention to learning (Eysenck, 2012; Foerde & Poldrack, 2009).

Memory is not a perfect processor, and is affected by many factors. The ways by which information is encoded, stored, and retrieved can all be corrupted. The amount of attention given new stimuli can diminish the amount of information that becomes encoded for storage (Eysenck, 2012). Also, the storage process can become corrupted by physical damage to areas of the brain that are associated with memory storage, such as the hippocampus (Squire, 2009). Finally, the retrieval of information from long-term memory can be disrupted because of decay within long-term memory (Eysenck, 2012). Normal functioning, decay over time, and brain damage all affect the accuracy and capacity of memory.

Memory loss is usually described as forgetfulness or amnesia.

Sensory memory holds sensory information less than one second after an item is perceived. The ability to look at an item and remember what it looked like with just a split second of observation, or memorization, is the example of sensory memory. It is out of cognitive control and is an automatic response. With very short presentations, participants often report that they seem to "see" more than they can actually report. The first experiments exploring this form of sensory memory were precisely conducted by George Sperling (1963) using the "partial report paradigm". Subjects were presented with a grid of 12 letters, arranged into three rows of four. After a brief presentation, subjects were then played either a high, medium or low tone, cuing them which of the rows to report. Based on these partial report experiments, Sperling was able to show that the capacity of sensory memory was approximately 12 items, but that it degraded very quickly (within a few hundred milliseconds). Because this form of memory degrades so quickly, participants would see the display but be unable to report all of the items (12 in the "whole report" procedure) before they decayed. This type of memory cannot be prolonged via rehearsal.

Three types of sensory memories exist. Iconic memory is a fast decaying store of visual information; a type of sensory memory that briefly stores an image which has been perceived for a small duration. Echoic memory is a fast decaying store of auditory information, another type of sensory memory that briefly stores sounds that have been perceived for short durations. Haptic memory is a type of sensory memory that represents a database for touch stimuli.

Short-term memory is also known as working memory. Short-term memory allows recall for a period of several seconds to a minute without rehearsal. Its capacity is also very limited: George A. Miller (1956), when working at Bell Laboratories, conducted experiments showing that the store of short-term memory was 7±2 items (the title of his famous paper, "The magical number 7±2"). Modern estimates of the capacity of short-term memory are lower, typically of the order of 4–5 items; however, memory capacity can be increased through a process called chunking. For example, in recalling a ten-digit telephone number, a person could chunk the digits into three groups: first, the area code (such as 123), then a three-digit chunk (456) and lastly a four-digit chunk (7890). This method of remembering telephone numbers is far more effective than attempting to remember a string of 10 digits; this is because we are able to chunk the information into meaningful groups of numbers. This may be reflected in some countries in the tendency to display telephone numbers as several chunks of two to four numbers.

Short-term memory is believed to rely mostly on an acoustic code for storing information, and to a lesser extent a visual code. Conrad (1964) found that test subjects had more difficulty recalling collections of letters that were acoustically similar (e.g. E, P, D). Confusion with recalling acoustically similar letters rather than visually similar letters implies that the letters were encoded acoustically. Conrad's (1964) study, however, deals with the encoding of written text; thus, while memory of written language may rely on acoustic components, generalisations to all forms of memory cannot be made.

The storage in sensory memory and short-term memory generally has a strictly limited capacity and duration, which means that information is not retained indefinitely. By contrast, long-term memory can store much larger quantities of information for potentially unlimited duration (sometimes a whole life span). Its capacity is immeasurable. For example, given a random seven-digit number we may remember it for only a few seconds before forgetting, suggesting it was stored in our short-term memory. On the other hand, we can remember telephone numbers for many years through repetition; this information is said to be stored in long-term memory.

Short-term memory is supported by transient patterns of neuronal communication, dependent on regions of the frontal lobe (especially dorsolateral prefrontal cortex) and the parietal lobe. Long-term memory, on the other hand, is maintained by more stable and permanent changes in neural connections widely spread throughout the brain. The hippocampus is essential (for learning new information) to the consolidation of information from short-term to long-term memory, although it does not seem to store information itself. It was thought that without the hippocampus new memories were unable to be stored into long-term memory and that there would be a very short attention span, as first gleaned from patient Henry Molaison after what was thought to be the full removal of both his hippocampi. More recent examination of his brain, post-mortem, shows that the hippocampus was more intact than first thought, throwing theories drawn from the initial data into question. The hippocampus may be involved in changing neural connections for a period of three months or more after the initial learning.

Research has suggested that long-term memory storage in humans may be maintained by DNA methylation, and the 'prion' gene.

The multi-store model (also known as Atkinson–Shiffrin memory model) was first described in 1968 by Atkinson and Shiffrin.

The multi-store model has been criticised for being too simplistic. For instance, long-term memory is believed to be actually made up of multiple subcomponents, such as episodic and procedural memory. It also proposes that rehearsal is the only mechanism by which information eventually reaches long-term storage, but evidence shows us capable of remembering things without rehearsal.

The model also shows all the memory stores as being a single unit whereas research into this shows differently. For example, short-term memory can be broken up into different units such as visual information and acoustic information. In a study by Zlonoga and Gerber (1986), patient 'KF' demonstrated certain deviations from the Atkinson–Shiffrin model. Patient KF was brain damaged, displaying difficulties regarding short-term memory. Recognition of sounds such as spoken numbers, letters, words and easily identifiable noises (such as doorbells and cats meowing) were all impacted. Visual short-term memory was unaffected, suggesting a dichotomy between visual and audial memory.

In 1974 Baddeley and Hitch proposed a "working memory model" that replaced the general concept of short-term memory with an active maintenance of information in the short-term storage. In this model, working memory consists of three basic stores: the central executive, the phonological loop and the visuo-spatial sketchpad. In 2000 this model was expanded with the multimodal episodic buffer (Baddeley's model of working memory).

The central executive essentially acts as an attention sensory store. It channels information to the three component processes: the phonological loop, the visuo-spatial sketchpad, and the episodic buffer.

The phonological loop stores auditory information by silently rehearsing sounds or words in a continuous loop: the articulatory process (for example the repetition of a telephone number over and over again). A short list of data is easier to remember.

The visuospatial sketchpad stores visual and spatial information. It is engaged when performing spatial tasks (such as judging distances) or visual ones (such as counting the windows on a house or imagining images).

The episodic buffer is dedicated to linking information across domains to form integrated units of visual, spatial, and verbal information and chronological ordering (e.g., the memory of a story or a movie scene). The episodic buffer is also assumed to have links to long-term memory and semantical meaning.

The working memory model explains many practical observations, such as why it is easier to do two different tasks (one verbal and one visual) than two similar tasks (e.g., two visual), and the aforementioned word-length effect. Working memory is also the premise for what allows us to do everyday activities involving thought. It is the section of memory where we carry out thought processes and use them to learn and reason about topics.

Researchers distinguish between recognition and recall memory. Recognition memory tasks require individuals to indicate whether they have encountered a stimulus (such as a picture or a word) before. Recall memory tasks require participants to retrieve previously learned information. For example, individuals might be asked to produce a series of actions they have seen before or to say a list of words they have heard before.

Topographic memory involves the ability to orient oneself in space, to recognize and follow an itinerary, or to recognize familiar places. Getting lost when traveling alone is an example of the failure of topographic memory.

Flashbulb memories are clear episodic memories of unique and highly emotional events. People remembering where they were or what they were doing when they first heard the news of President Kennedy's assassination, the Sydney Siege or of 9/11 are examples of flashbulb memories.

Anderson (1976) divides long-term memory into "declarative (explicit)" and "procedural (implicit)" memories.

Declarative memory requires conscious recall, in that some conscious process must call back the information. It is sometimes called "explicit memory", since it consists of information that is explicitly stored and retrieved.

Declarative memory can be further sub-divided into semantic memory, concerning principles and facts taken independent of context; and episodic memory, concerning information specific to a particular context, such as a time and place. Semantic memory allows the encoding of abstract knowledge about the world, such as "Paris is the capital of France". Episodic memory, on the other hand, is used for more personal memories, such as the sensations, emotions, and personal associations of a particular place or time. Episodic memories often reflect the "firsts" in life such as a first kiss, first day of school or first time winning a championship. These are key events in one's life that can be remembered clearly. Autobiographical memory – memory for particular events within one's own life – is generally viewed as either equivalent to, or a subset of, episodic memory. Visual memory is part of memory preserving some characteristics of our senses pertaining to visual experience. One is able to place in memory information that resembles objects, places, animals or people in sort of a mental image. Visual memory can result in priming and it is assumed some kind of perceptual representational system underlies this phenomenon.

In contrast, procedural memory (or "implicit memory") is not based on the conscious recall of information, but on implicit learning. It can best be summarized as remembering how to do something. Procedural memory is primarily employed in learning motor skills and should be considered a subset of implicit memory. It is revealed when one does better in a given task due only to repetition – no new explicit memories have been formed, but one is unconsciously accessing aspects of those previous experiences. Procedural memory involved in motor learning depends on the cerebellum and basal ganglia.

A characteristic of procedural memory is that the things remembered are automatically translated into actions, and thus sometimes difficult to describe. Some examples of procedural memory include the ability to ride a bike or tie shoelaces.

Another major way to distinguish different memory functions is whether the content to be remembered is in the past, retrospective memory, or in the future, prospective memory. Thus, retrospective memory as a category includes semantic, episodic and autobiographical memory. In contrast, prospective memory is memory for future intentions, or "remembering to remember" (Winograd, 1988). Prospective memory can be further broken down into event- and time-based prospective remembering. Time-based prospective memories are triggered by a time-cue, such as going to the doctor (action) at 4pm (cue). Event-based prospective memories are intentions triggered by cues, such as remembering to post a letter (action) after seeing a mailbox (cue). Cues do not need to be related to the action (as the mailbox/letter example), and lists, sticky-notes, knotted handkerchiefs, or string around the finger all exemplify cues that people use as strategies to enhance prospective memory.

Infants do not have the language ability to report on their memories and so verbal reports cannot be used to assess very young children's memory. Throughout the years, however, researchers have adapted and developed a number of measures for assessing both infants' recognition memory and their recall memory. Habituation and operant conditioning techniques have been used to assess infants' recognition memory and the deferred and elicited imitation techniques have been used to assess infants' recall memory.

Techniques used to assess infants' recognition memory include the following:

Techniques used to assess infants' recall memory include the following:

Researchers use a variety of tasks to assess older children and adults' memory. Some examples are:Brain areas involved in the neuroanatomy of memory such as the hippocampus, the amygdala, the striatum, or the mammillary bodies are thought to be involved in specific types of memory. For example, the hippocampus is believed to be involved in spatial learning and declarative learning, while the amygdala is thought to be involved in emotional memory.

Damage to certain areas in patients and animal models and subsequent memory deficits is a primary source of information. However, rather than implicating a specific area, it could be that damage to adjacent areas, or to a pathway "traveling" through the area is actually responsible for the observed deficit. Further, it is not sufficient to describe memory, and its counterpart, learning, as solely dependent on specific brain regions. Learning and memory are usually attributed to changes in neuronal synapses, thought to be mediated by long-term potentiation and long-term depression. However, this has been questioned on computational as well as neurophysiological grounds by the cognitive scientist Charles R. Gallistel and others.

In general, the more emotionally charged an event or experience is, the better it is remembered; this phenomenon is known as the memory enhancement effect. Patients with amygdala damage, however, do not show a memory enhancement effect.

Hebb distinguished between short-term and long-term memory. He postulated that any memory that stayed in short-term storage for a long enough time would be consolidated into a long-term memory. Later research showed this to be false. Research has shown that direct injections of cortisol or epinephrine help the storage of recent experiences. This is also true for stimulation of the amygdala. This proves that excitement enhances memory by the stimulation of hormones that affect the amygdala. Excessive or prolonged stress (with prolonged cortisol) may hurt memory storage. Patients with amygdalar damage are no more likely to remember emotionally charged words than nonemotionally charged ones. The hippocampus is important for explicit memory. The hippocampus is also important for memory consolidation. The hippocampus receives input from different parts of the cortex and sends its output out to different parts of the brain also. The input comes from secondary and tertiary sensory areas that have processed the information a lot already. Hippocampal damage may also cause memory loss and problems with memory storage. This memory loss includes retrograde amnesia which is the loss of memory for events that occurred shortly before the time of brain damage.

Cognitive neuroscientists consider memory as the retention, reactivation, and reconstruction of the experience-independent internal representation. The term of internal representation implies that such definition of memory contains two components: the expression of memory at the behavioral or conscious level, and the underpinning physical neural changes (Dudai 2007). The latter component is also called engram or memory traces (Semon 1904). Some neuroscientists and psychologists mistakenly equate the concept of engram and memory, broadly conceiving all persisting after-effects of experiences as memory; others argue against this notion that memory does not exist until it is revealed in behavior or thought (Moscovitch 2007).

One question that is crucial in cognitive neuroscience is how information and mental experiences are coded and represented in the brain. Scientists have gained much knowledge about the neuronal codes from the studies of plasticity, but most of such research has been focused on simple learning in simple neuronal circuits; it is considerably less clear about the neuronal changes involved in more complex examples of memory, particularly declarative memory that requires the storage of facts and events (Byrne 2007). Convergence-divergence zones might be the neural networks where memories are stored and retrieved. Considering that there are several kinds of memory, depending on types of represented knowledge, underlying mechanisms, processes functions and modes of acquisition, it is likely that different brain areas support different memory systems and that they are in mutual relationships in neuronal networks: "components of memory representation are distributed widely across different parts of the brain as mediated by multiple neocortical circuits".

Study of the genetics of human memory is in its infancy. A notable initial success was the association of APOE with memory dysfunction in Alzheimer's Disease. The search for genes associated with normally varying memory continues. One of the first candidates for normal variation in memory is the protein "KIBRA," which appears to be associated with the rate at which material is forgotten over a delay period. There has been some evidence that memories are stored in the nucleus of neurons.

Up until the mid-1980s it was assumed that infants could not encode, retain, and retrieve information. A growing body of research now indicates that infants as young as 6-months can recall information after a 24-hour delay. Furthermore, research has revealed that as infants grow older they can store information for longer periods of time; 6-month-olds can recall information after a 24-hour period, 9-month-olds after up to five weeks, and 20-month-olds after as long as twelve months. In addition, studies have shown that with age, infants can store information faster. Whereas 14-month-olds can recall a three-step sequence after being exposed to it once, 6-month-olds need approximately six exposures in order to be able to remember it.

Although 6-month-olds can recall information over the short-term, they have difficulty recalling the temporal order of information. It is only by 9 months of age that infants can recall the actions of a two-step sequence in the correct temporal order – that is, recalling step 1 and then step 2. In other words, when asked to imitate a two-step action sequence (such as putting a toy car in the base and pushing in the plunger to make the toy roll to the other end), 9-month-olds tend to imitate the actions of the sequence in the correct order (step 1 and then step 2). Younger infants (6-month-olds) can only recall one step of a two-step sequence. Researchers have suggested that these age differences are probably due to the fact that the dentate gyrus of the hippocampus and the frontal components of the neural network are not fully developed at the age of 6-months.

In fact, the term 'infantile amnesia' refers to the phenomenon of accelerated forgetting during infancy. Importantly, infantile amnesia is not unique to humans, and preclinical research (using rodent models) provides insight into the precise neurobiology of this phenomenon. A review of the literature from behavioral neuroscientist Dr Jee Hyun Kim suggests that accelerated forgetting during early life is at least partly due to rapid growth of the brain during this period.

One of the key concerns of older adults is the experience of memory loss, especially as it is one of the hallmark symptoms of Alzheimer's disease. However, memory loss is qualitatively different in normal aging from the kind of memory loss associated with a diagnosis of Alzheimer's (Budson & Price, 2005). Research has revealed that individuals' performance on memory tasks that rely on frontal regions declines with age. Older adults tend to exhibit deficits on tasks that involve knowing the temporal order in which they learned information; source memory tasks that require them to remember the specific circumstances or context in which they learned information; and prospective memory tasks that involve remembering to perform an act at a future time. Older adults can manage their problems with prospective memory by using appointment books, for example.

Gene transcription profiles were determined for the human frontal cortex of individuals from age 26 to 106 years. Numerous genes were identified with reduced expression after age 40, and especially after age 70. Genes that play central roles in memory and learning were among those showing the most significant reduction with age. There was also a marked increase in DNA damage, likely oxidative damage, in the promoters of those genes with reduced expression. It was suggested that DNA damage may reduce the expression of selectively vulnerable genes involved in memory and learning.

Much of the current knowledge of memory has come from studying "memory disorders", particularly amnesia. Loss of memory is known as amnesia. Amnesia can result from extensive damage to: (a) the regions of the medial temporal lobe, such as the hippocampus, dentate gyrus, subiculum, amygdala, the parahippocampal, entorhinal, and perirhinal cortices or the (b) midline diencephalic region, specifically the dorsomedial nucleus of the thalamus and the mammillary bodies of the hypothalamus. There are many sorts of amnesia, and by studying their different forms, it has become possible to observe apparent defects in individual sub-systems of the brain's memory systems, and thus hypothesize their function in the normally working brain. Other neurological disorders such as Alzheimer's disease and Parkinson's disease can also affect memory and cognition.
Hyperthymesia, or hyperthymesic syndrome, is a disorder that affects an individual's autobiographical memory, essentially meaning that they cannot forget small details that otherwise would not be stored. Korsakoff's syndrome, also known as Korsakoff's psychosis, amnesic-confabulatory syndrome, is an organic brain disease that adversely affects memory by widespread loss or shrinkage of neurons within the prefrontal cortex.

While not a disorder, a common "temporary" failure of word retrieval from memory is the tip-of-the-tongue phenomenon. Sufferers of Anomic aphasia (also called Nominal aphasia or Anomia), however, do experience the tip-of-the-tongue phenomenon on an ongoing basis due to damage to the frontal and parietal lobes of the brain.

Interference can hamper memorization and retrieval. There is retroactive interference, when learning new information makes it harder to recall old information and proactive interference, where prior learning disrupts recall of new information. Although interference can lead to forgetting, it is important to keep in mind that there are situations when old information can facilitate learning of new information. Knowing Latin, for instance, can help an individual learn a related language such as French – this phenomenon is known as positive transfer.

Stress has a significant effect on memory formation and learning. In response to stressful situations, the brain releases hormones and neurotransmitters (ex. glucocorticoids and catecholamines) which affect memory encoding processes in the hippocampus. Behavioural research on animals shows that chronic stress produces adrenal hormones which impact the hippocampal structure in the brains of rats. An experimental study by German cognitive psychologists L. Schwabe and O. Wolf demonstrates how learning under stress also decreases memory recall in humans. In this study, 48 healthy female and male university students participated in either a stress test or a control group. Those randomly assigned to the stress test group had a hand immersed in ice cold water (the reputable SECPT or 'Socially Evaluated Cold Pressor Test') for up to three minutes, while being monitored and videotaped. Both the stress and control groups were then presented with 32 words to memorize. Twenty-four hours later, both groups were tested to see how many words they could remember (free recall) as well as how many they could recognize from a larger list of words (recognition performance). The results showed a clear impairment of memory performance in the stress test group, who recalled 30% fewer words than the control group. The researchers suggest that stress experienced during learning distracts people by diverting their attention during the memory encoding process.

However, memory performance can be enhanced when material is linked to the learning context, even when learning occurs under stress. A separate study by cognitive psychologists Schwabe and Wolf shows that when retention testing is done in a context similar to or congruent with the original learning task (i.e., in the same room), memory impairment and the detrimental effects of stress on learning can be attenuated. Seventy-two healthy female and male university students, randomly assigned to the SECPT stress test or to a control group, were asked to remember the locations of 15 pairs of picture cards – a computerized version of the card game "Concentration" or "Memory". The room in which the experiment took place was infused with the scent of vanilla, as odour is a strong cue for memory. Retention testing took place the following day, either in the same room with the vanilla scent again present, or in a different room without the fragrance. The memory performance of subjects who experienced stress during the object-location task decreased significantly when they were tested in an unfamiliar room without the vanilla scent (an incongruent context); however, the memory performance of stressed subjects showed no impairment when they were tested in the original room with the vanilla scent (a congruent context). All participants in the experiment, both stressed and unstressed, performed faster when the learning and retrieval contexts were similar.

This research on the effects of stress on memory may have practical implications for education, for eyewitness testimony and for psychotherapy: students may perform better when tested in their regular classroom rather than an exam room, eyewitnesses may recall details better at the scene of an event than in a courtroom, and persons suffering from post-traumatic stress may improve when helped to situate their memories of a traumatic event in an appropriate context.

Stressful life experiences may be a cause of memory loss as a person ages. Glucocorticoids that are released during stress, damage neurons that are located in the hippocampal region of the brain. Therefore, the more stressful situations that someone encounters, the more susceptible they are to memory loss later on. The CA1 neurons found in the hippocampus are destroyed due to glucocorticoids decreasing the release of glucose and the reuptake of glutamate. This high level of extracellular glutamate allows calcium to enter NMDA receptors which in return kills neurons. Stressful life experiences can also cause repression of memories where a person moves an unbearable memory to the unconscious mind. This directly relates to traumatic events in one's past such as kidnappings, being prisoners of war or sexual abuse as a child.

The more long term the exposure to stress is, the more impact it may have. However, short term exposure to stress also causes impairment in memory by interfering with the function of the hippocampus. Research shows that subjects placed in a stressful situation for a short amount of time still have blood glucocorticoid levels that have increased drastically when measured after the exposure is completed. When subjects are asked to complete a learning task after short term exposure they often have difficulties. Prenatal stress also hinders the ability to learn and memorize by disrupting the development of the hippocampus and can lead to unestablished long term potentiation in the offspring of severely stressed parents. Although the stress is applied prenatally, the offspring show increased levels of glucocorticoids when they are subjected to stress later on in life.

Making memories occurs through a three-step process, which can be enhanced by sleep. The three steps are as follows:
Sleep affects memory consolidation. During sleep, the neural connections in the brain are strengthened. This enhances the brain's abilities to stabilize and retain memories. There have been several studies which show that sleep improves the retention of memory, as memories are enhanced through active consolidation. System consolidation takes place during slow-wave sleep (SWS). This process implicates that memories are reactivated during sleep, but that the process doesn't enhance every memory. It also implicates that qualitative changes are made to the memories when they are transferred to long-term store during sleep. During sleep, the hippocampus replays the events of the day for the neocortex. The neocortex then reviews and processes memories, which moves them into long-term memory. When one does not get enough sleep it makes it more difficult to learn as these neural connections are not as strong, resulting in a lower retention rate of memories. Sleep deprivation makes it harder to focus, resulting in inefficient learning. Furthermore, some studies have shown that sleep deprivation can lead to false memories as the memories are not properly transferred to long-term memory.
One of the primary functions of sleep is thought to be the improvement of the consolidation of information, as several studies have demonstrated that memory depends on getting sufficient sleep between training and test. Additionally, data obtained from neuroimaging studies have shown activation patterns in the sleeping brain that mirror those recorded during the learning of tasks from the previous day, suggesting that new memories may be solidified through such rehearsal.

Although people often think that memory operates like recording equipment, it is not the case. The molecular mechanisms underlying the induction and maintenance of memory are very dynamic and comprise distinct phases covering a time window from seconds to even a lifetime. In fact, research has revealed that our memories are constructed: "current hypotheses suggest that constructive processes allow individuals to simulate and imagine future episodes, happenings, and scenarios. Since the future is not an exact repetition of the past, simulation of future episodes requires a complex system that can draw on the past in a manner that flexibly extracts and recombines elements of previous experiences – a constructive rather than a reproductive system." People can construct their memories when they encode them and/or when they recall them. To illustrate, consider a classic study conducted by Elizabeth Loftus and John Palmer (1974) in which people were instructed to watch a film of a traffic accident and then asked about what they saw. The researchers found that the people who were asked, "How fast were the cars going when they "smashed" into each other?" gave higher estimates than those who were asked, "How fast were the cars going when they "hit" each other?" Furthermore, when asked a week later whether they had seen broken glass in the film, those who had been asked the question with "smashed" were twice more likely to report that they had seen broken glass than those who had been asked the question with "hit". There was no broken glass depicted in the film. Thus, the wording of the questions distorted viewers' memories of the event. Importantly, the wording of the question led people to construct different memories of the event – those who were asked the question with "smashed" recalled a more serious car accident than they had actually seen. The findings of this experiment were replicated around the world, and researchers consistently demonstrated that when people were provided with misleading information they tended to misremember, a phenomenon known as the misinformation effect.

Research has revealed that asking individuals to repeatedly imagine actions that they have never performed or events that they have never experienced could result in false memories. For instance, Goff and Roediger (1998) asked participants to imagine that they performed an act (e.g., break a toothpick) and then later asked them whether they had done such a thing. Findings revealed that those participants who repeatedly imagined performing such an act were more likely to think that they had actually performed that act during the first session of the experiment. Similarly, Garry and her colleagues (1996) asked college students to report how certain they were that they experienced a number of events as children (e.g., broke a window with their hand) and then two weeks later asked them to imagine four of those events. The researchers found that one-fourth of the students asked to imagine the four events reported that they had actually experienced such events as children. That is, when asked to imagine the events they were more confident that they experienced the events.

Research reported in 2013 revealed that it is possible to artificially stimulate prior memories and artificially implant false memories in mice. Using optogenetics, a team of RIKEN-MIT scientists caused the mice to incorrectly associate a benign environment with a prior unpleasant experience from different surroundings. Some scientists believe that the study may have implications in studying false memory formation in humans, and in treating PTSD and schizophrenia.

A UCLA research study published in the June 2008 issue of the "American Journal of Geriatric Psychiatry" found that people can improve cognitive function and brain efficiency through simple lifestyle changes such as incorporating memory exercises, healthy eating, physical fitness and stress reduction into their daily lives. This study examined 17 subjects, (average age 53) with normal memory performance.
Eight subjects were asked to follow a "brain healthy" diet, relaxation, physical, and mental exercise (brain teasers and verbal memory training techniques). After 14 days, they showed greater word fluency (not memory) compared to their baseline performance. No long-term follow-up was conducted; it is therefore unclear if this intervention has lasting effects on memory.

There are a loosely associated group of mnemonic principles and techniques that can be used to vastly improve memory known as the art of memory.

The International Longevity Center released in 2001 a report which includes in pages 14–16 recommendations for keeping the mind in good functionality until advanced age. Some of the recommendations are to stay intellectually active through learning, training or reading, to keep physically active so to promote blood circulation to the brain, to socialize, to reduce stress, to keep sleep time regular, to avoid depression or emotional instability and to observe good nutrition.

Memorization is a method of learning that allows an individual to recall information verbatim. Rote learning is the method most often used. Methods of memorizing things have been the subject of much discussion over the years with some writers, such as Cosmos Rossellius using visual alphabets. The spacing effect shows that an individual is more likely to remember a list of items when rehearsal is spaced over an extended period of time. In contrast to this is cramming: an intensive memorization in a short period of time. Also relevant is the Zeigarnik effect which states that people remember uncompleted or interrupted tasks better than completed ones. The so-called Method of loci uses spatial memory to memorize non-spatial information.

Plants lack a specialized organ devoted to memory retention, and so plant memory has been a controversial topic in recent years. New advances in the field have identified the presence of neurotransmitters in plants, adding to the hypothesis that plants are capable of remembering. Action potentials, a physiological response characteristic of neurons, have been shown to have an influence on plants as well, including in wound responses and photosynthesis. In addition to these homologous features of memory systems in both plants and animals, plants have also been observed to encode, store and retrieve basic short-term memories.

One of the most well-studied plants to show rudimentary memory is the Venus flytrap. Native to the subtropical wetlands of the eastern United States, Venus Fly Traps have evolved the ability to obtain meat for sustenance, likely due to the lack of nitrogen in the soil. This is done by two trap-forming leaf tips that snap shut once triggered by a potential prey. On each lobe, three triggers hairs await stimulation. In order to maximize the benefit to cost ratio, the plant enables a rudimentary form of memory in which two trigger hairs must be stimulated within 30 seconds in order to result in trap closure. This system ensures that the trap only closes when potential prey is within grasp.

The time lapse between trigger hair stimulations suggests that the plant can remember an initial stimulus long enough for a second stimulus to initiate trap closure. This memory isn't encoded in a brain, as plants lack this specialized organ. Rather, information is stored in the form of cytoplasmic calcium levels. The first trigger causes a subthreshold cytoplasmic calcium influx. This initial trigger isn't enough to activate trap closure, and so a subsequent stimulus allows for a secondary influx of calcium. The latter calcium rise superimposes on the initial one, creating an action potential that passes threshold, resulting in trap closure. Researchers, to prove that an electrical threshold must be met to stimulate trap closure, excited a single trigger hair with a constant mechanical stimulus using Ag/AgCl electrodes. The trap closed after only a few seconds. This experiment gave evidence to demonstrate that the electrical threshold, not necessarily the number of trigger hair stimulations, was the contributing factor in Venus Fly Trap memory. It has been shown that trap closure can be blocked using uncouplers and inhibitors of voltage-gated channels. After trap closure, these electrical signals stimulate glandular production of jasmonic acid and hydrolases, allowing for digestion of the prey.

The field of plant neurobiology has gained a large amount of interest over the past decade, leading to an influx of research regarding plant memory. Although the Venus flytrap is one of the more highly studied, many other plants exhibit the capacity to remember, including the "Mimosa pudica" through an experiment conducted by Monica Gagliano and colleagues in 2013. As the field expands, it is likely that we will learn more about the capacity of a plant to remember.

Memory

Memory is the faculty of the brain by which information is encoded, stored, and retrieved when needed.

Memory is vital to experiences, it is the retention of information over time for the purpose of influencing future action. If we could not remember past events, we could not learn or develop language, relationships, or personal identity (Eysenck, 2012).

Often memory is understood as an informational processing system with explicit and implicit functioning that is made up of a sensory processor, short-term (or working) memory, and long-term memory (Baddely, 2007). This can be related to the neuron.
The sensory processor allows information from the outside world to be sensed in the form of chemical and physical stimuli and attended to with various levels of focus and intent. Working memory serves as an encoding and retrieval processor. Information in the form of stimuli is encoded in accordance with explicit or implicit functions by the working memory processor. The working memory also retrieves information from previously stored material. Finally, the function of long-term memory is to store data through various categorical models or systems (Baddely, 2007).

Explicit and implicit functions of memory are also known as declarative and non-declarative systems (Squire, 2009). These systems involve the purposeful intention of memory retrieval and storage, or lack thereof.
Declarative, or explicit, memory is the conscious storage and recollection of data (Graf & Schacter, 1985). Under declarative memory resides semantic and episodic memory. Semantic memory refers to memory that is encoded with specific meaning (Eysenck, 2012), while episodic memory refers to information that is encoded along a spatial and temporal plane (Schacter & Addis, 2007; Szpunar, 2010). Declarative memory is usually the primary process thought of when referencing memory (Eysenck, 2012).

Non-declarative, or implicit, memory is the unconscious storage and recollection of information (Foerde & Poldrack, 2009). An example of a non-declarative process would be the unconscious learning or retrieval of information by way of procedural memory, or a priming phenomenon (Eysenck, 2012; Foerde & Poldrack, 2009; Tulving & Schacter, 1990). Priming is the process of subliminally arousing specific responses from memory and shows that not all memory is consciously activated (Tulving & Schacter, 1990), whereas procedural memory is the slow and gradual learning of skills that often occurs without conscious attention to learning (Eysenck, 2012; Foerde & Poldrack, 2009).

Memory is not a perfect processor, and is affected by many factors. The ways by which information is encoded, stored, and retrieved can all be corrupted. The amount of attention given new stimuli can diminish the amount of information that becomes encoded for storage (Eysenck, 2012). Also, the storage process can become corrupted by physical damage to areas of the brain that are associated with memory storage, such as the hippocampus (Squire, 2009). Finally, the retrieval of information from long-term memory can be disrupted because of decay within long-term memory (Eysenck, 2012). Normal functioning, decay over time, and brain damage all affect the accuracy and capacity of memory.

Memory loss is usually described as forgetfulness or amnesia.

Three types of sensory memories exist. Iconic memory is a fast decaying store of visual information; a type of sensory memory that briefly stores an image which has been perceived for a small duration. Echoic memory is a fast decaying store of auditory information, another type of sensory memory that briefly stores sounds that have been perceived for short durations. Haptic memory is a type of sensory memory that represents a database for touch stimuli.

Short-term memory is believed to rely mostly on an acoustic code for storing information, and to a lesser extent a visual code. Conrad (1964) found that test subjects had more difficulty recalling collections of letters that were acoustically similar (e.g. E, P, D). Confusion with recalling acoustically similar letters rather than visually similar letters implies that the letters were encoded acoustically. Conrad's (1964) study, however, deals with the encoding of written text; thus, while memory of written language may rely on acoustic components, generalisations to all forms of memory cannot be made.

The storage in sensory memory and short-term memory generally has a strictly limited capacity and duration, which means that information is not retained indefinitely. By contrast, long-term memory can store much larger quantities of information for potentially unlimited duration (sometimes a whole life span). Its capacity is immeasurable. For example, given a random seven-digit number we may remember it for only a few seconds before forgetting, suggesting it was stored in our short-term memory. On the other hand, we can remember telephone numbers for many years through repetition; this information is said to be stored in long-term memory.

While short-term memory encodes information acoustically, long-term memory encodes it semantically: Baddeley (1966) discovered that, after 20 minutes, test subjects had the most difficulty recalling a collection of words that had similar meanings (e.g. big, large, great, huge) long-term. Another part of long-term memory is episodic memory, "which attempts to capture information such as 'what', 'when' and 'where. With episodic memory, individuals are able to recall specific events such as birthday parties and weddings.

Research has suggested that long-term memory storage in humans may be maintained by DNA methylation, and the 'prion' gene.

The multi-store model (also known as Atkinson–Shiffrin memory model) was first described in 1968 by Atkinson and Shiffrin.

The multi-store model has been criticised for being too simplistic. For instance, long-term memory is believed to be actually made up of multiple subcomponents, such as episodic and procedural memory. It also proposes that rehearsal is the only mechanism by which information eventually reaches long-term storage, but evidence shows us capable of remembering things without rehearsal.

The model also shows all the memory stores as being a single unit whereas research into this shows differently. For example, short-term memory can be broken up into different units such as visual information and acoustic information. In a study by Zlonoga and Gerber (1986), patient 'KF' demonstrated certain deviations from the Atkinson–Shiffrin model. Patient KF was brain damaged, displaying difficulties regarding short-term memory. Recognition of sounds such as spoken numbers, letters, words and easily identifiable noises (such as doorbells and cats meowing) were all impacted. Visual short-term memory was unaffected, suggesting a dichotomy between visual and audial memory.

In 1974 Baddeley and Hitch proposed a "working memory model" that replaced the general concept of short-term memory with an active maintenance of information in the short-term storage. In this model, working memory consists of three basic stores: the central executive, the phonological loop and the visuo-spatial sketchpad. In 2000 this model was expanded with the multimodal episodic buffer (Baddeley's model of working memory).

The central executive essentially acts as an attention sensory store. It channels information to the three component processes: the phonological loop, the visuo-spatial sketchpad, and the episodic buffer.

The phonological loop stores auditory information by silently rehearsing sounds or words in a continuous loop: the articulatory process (for example the repetition of a telephone number over and over again). A short list of data is easier to remember.

The visuospatial sketchpad stores visual and spatial information. It is engaged when performing spatial tasks (such as judging distances) or visual ones (such as counting the windows on a house or imagining images).

The episodic buffer is dedicated to linking information across domains to form integrated units of visual, spatial, and verbal information and chronological ordering (e.g., the memory of a story or a movie scene). The episodic buffer is also assumed to have links to long-term memory and semantical meaning.

The working memory model explains many practical observations, such as why it is easier to do two different tasks (one verbal and one visual) than two similar tasks (e.g., two visual), and the aforementioned word-length effect. Working memory is also the premise for what allows us to do everyday activities involving thought. It is the section of memory where we carry out thought processes and use them to learn and reason about topics.

Researchers distinguish between recognition and recall memory. Recognition memory tasks require individuals to indicate whether they have encountered a stimulus (such as a picture or a word) before. Recall memory tasks require participants to retrieve previously learned information. For example, individuals might be asked to produce a series of actions they have seen before or to say a list of words they have heard before.

Topographic memory involves the ability to orient oneself in space, to recognize and follow an itinerary, or to recognize familiar places. Getting lost when traveling alone is an example of the failure of topographic memory.

Flashbulb memories are clear episodic memories of unique and highly emotional events. People remembering where they were or what they were doing when they first heard the news of President Kennedy's assassination, the Sydney Siege or of 9/11 are examples of flashbulb memories.

Anderson (1976) divides long-term memory into "declarative (explicit)" and "procedural (implicit)" memories.

Declarative memory requires conscious recall, in that some conscious process must call back the information. It is sometimes called "explicit memory", since it consists of information that is explicitly stored and retrieved.

In contrast, procedural memory (or "implicit memory") is not based on the conscious recall of information, but on implicit learning. It can best be summarized as remembering how to do something. Procedural memory is primarily employed in learning motor skills and should be considered a subset of implicit memory. It is revealed when one does better in a given task due only to repetition – no new explicit memories have been formed, but one is unconsciously accessing aspects of those previous experiences. Procedural memory involved in motor learning depends on the cerebellum and basal ganglia.

A characteristic of procedural memory is that the things remembered are automatically translated into actions, and thus sometimes difficult to describe. Some examples of procedural memory include the ability to ride a bike or tie shoelaces.

Another major way to distinguish different memory functions is whether the content to be remembered is in the past, retrospective memory, or in the future, prospective memory. Thus, retrospective memory as a category includes semantic, episodic and autobiographical memory. In contrast, prospective memory is memory for future intentions, or "remembering to remember" (Winograd, 1988). Prospective memory can be further broken down into event- and time-based prospective remembering. Time-based prospective memories are triggered by a time-cue, such as going to the doctor (action) at 4pm (cue). Event-based prospective memories are intentions triggered by cues, such as remembering to post a letter (action) after seeing a mailbox (cue). Cues do not need to be related to the action (as the mailbox/letter example), and lists, sticky-notes, knotted handkerchiefs, or string around the finger all exemplify cues that people use as strategies to enhance prospective memory.

Infants do not have the language ability to report on their memories and so verbal reports cannot be used to assess very young children's memory. Throughout the years, however, researchers have adapted and developed a number of measures for assessing both infants' recognition memory and their recall memory. Habituation and operant conditioning techniques have been used to assess infants' recognition memory and the deferred and elicited imitation techniques have been used to assess infants' recall memory.

Techniques used to assess infants' recognition memory include the following:

Techniques used to assess infants' recall memory include the following:

Researchers use a variety of tasks to assess older children and adults' memory. Some examples are:Brain areas involved in the neuroanatomy of memory such as the hippocampus, the amygdala, the striatum, or the mammillary bodies are thought to be involved in specific types of memory. For example, the hippocampus is believed to be involved in spatial learning and declarative learning, while the amygdala is thought to be involved in emotional memory.

Damage to certain areas in patients and animal models and subsequent memory deficits is a primary source of information. However, rather than implicating a specific area, it could be that damage to adjacent areas, or to a pathway "traveling" through the area is actually responsible for the observed deficit. Further, it is not sufficient to describe memory, and its counterpart, learning, as solely dependent on specific brain regions. Learning and memory are usually attributed to changes in neuronal synapses, thought to be mediated by long-term potentiation and long-term depression. However, this has been questioned on computational as well as neurophysiological grounds by the cognitive scientist Charles R. Gallistel and others.

In general, the more emotionally charged an event or experience is, the better it is remembered; this phenomenon is known as the memory enhancement effect. Patients with amygdala damage, however, do not show a memory enhancement effect.

Cognitive neuroscientists consider memory as the retention, reactivation, and reconstruction of the experience-independent internal representation. The term of internal representation implies that such definition of memory contains two components: the expression of memory at the behavioral or conscious level, and the underpinning physical neural changes (Dudai 2007). The latter component is also called engram or memory traces (Semon 1904). Some neuroscientists and psychologists mistakenly equate the concept of engram and memory, broadly conceiving all persisting after-effects of experiences as memory; others argue against this notion that memory does not exist until it is revealed in behavior or thought (Moscovitch 2007).

Study of the genetics of human memory is in its infancy. A notable initial success was the association of APOE with memory dysfunction in Alzheimer's Disease. The search for genes associated with normally varying memory continues. One of the first candidates for normal variation in memory is the protein "KIBRA," which appears to be associated with the rate at which material is forgotten over a delay period. There has been some evidence that memories are stored in the nucleus of neurons.

Up until the mid-1980s it was assumed that infants could not encode, retain, and retrieve information. A growing body of research now indicates that infants as young as 6-months can recall information after a 24-hour delay. Furthermore, research has revealed that as infants grow older they can store information for longer periods of time; 6-month-olds can recall information after a 24-hour period, 9-month-olds after up to five weeks, and 20-month-olds after as long as twelve months. In addition, studies have shown that with age, infants can store information faster. Whereas 14-month-olds can recall a three-step sequence after being exposed to it once, 6-month-olds need approximately six exposures in order to be able to remember it.

Although 6-month-olds can recall information over the short-term, they have difficulty recalling the temporal order of information. It is only by 9 months of age that infants can recall the actions of a two-step sequence in the correct temporal order – that is, recalling step 1 and then step 2. In other words, when asked to imitate a two-step action sequence (such as putting a toy car in the base and pushing in the plunger to make the toy roll to the other end), 9-month-olds tend to imitate the actions of the sequence in the correct order (step 1 and then step 2). Younger infants (6-month-olds) can only recall one step of a two-step sequence. Researchers have suggested that these age differences are probably due to the fact that the dentate gyrus of the hippocampus and the frontal components of the neural network are not fully developed at the age of 6-months.

In fact, the term 'infantile amnesia' refers to the phenomenon of accelerated forgetting during infancy. Importantly, infantile amnesia is not unique to humans, and preclinical research (using rodent models) provides insight into the precise neurobiology of this phenomenon. A review of the literature from behavioral neuroscientist Dr Jee Hyun Kim suggests that accelerated forgetting during early life is at least partly due to rapid growth of the brain during this period.

One of the key concerns of older adults is the experience of memory loss, especially as it is one of the hallmark symptoms of Alzheimer's disease. However, memory loss is qualitatively different in normal aging from the kind of memory loss associated with a diagnosis of Alzheimer's (Budson & Price, 2005). Research has revealed that individuals' performance on memory tasks that rely on frontal regions declines with age. Older adults tend to exhibit deficits on tasks that involve knowing the temporal order in which they learned information; source memory tasks that require them to remember the specific circumstances or context in which they learned information; and prospective memory tasks that involve remembering to perform an act at a future time. Older adults can manage their problems with prospective memory by using appointment books, for example.

Gene transcription profiles were determined for the human frontal cortex of individuals from age 26 to 106 years. Numerous genes were identified with reduced expression after age 40, and especially after age 70. Genes that play central roles in memory and learning were among those showing the most significant reduction with age. There was also a marked increase in DNA damage, likely oxidative damage, in the promoters of those genes with reduced expression. It was suggested that DNA damage may reduce the expression of selectively vulnerable genes involved in memory and learning.

While not a disorder, a common "temporary" failure of word retrieval from memory is the tip-of-the-tongue phenomenon. Sufferers of Anomic aphasia (also called Nominal aphasia or Anomia), however, do experience the tip-of-the-tongue phenomenon on an ongoing basis due to damage to the frontal and parietal lobes of the brain.

Interference can hamper memorization and retrieval. There is retroactive interference, when learning new information makes it harder to recall old information and proactive interference, where prior learning disrupts recall of new information. Although interference can lead to forgetting, it is important to keep in mind that there are situations when old information can facilitate learning of new information. Knowing Latin, for instance, can help an individual learn a related language such as French – this phenomenon is known as positive transfer.

This research on the effects of stress on memory may have practical implications for education, for eyewitness testimony and for psychotherapy: students may perform better when tested in their regular classroom rather than an exam room, eyewitnesses may recall details better at the scene of an event than in a courtroom, and persons suffering from post-traumatic stress may improve when helped to situate their memories of a traumatic event in an appropriate context.

Stressful life experiences may be a cause of memory loss as a person ages. Glucocorticoids that are released during stress, damage neurons that are located in the hippocampal region of the brain. Therefore, the more stressful situations that someone encounters, the more susceptible they are to memory loss later on. The CA1 neurons found in the hippocampus are destroyed due to glucocorticoids decreasing the release of glucose and the reuptake of glutamate. This high level of extracellular glutamate allows calcium to enter NMDA receptors which in return kills neurons. Stressful life experiences can also cause repression of memories where a person moves an unbearable memory to the unconscious mind. This directly relates to traumatic events in one's past such as kidnappings, being prisoners of war or sexual abuse as a child.

The more long term the exposure to stress is, the more impact it may have. However, short term exposure to stress also causes impairment in memory by interfering with the function of the hippocampus. Research shows that subjects placed in a stressful situation for a short amount of time still have blood glucocorticoid levels that have increased drastically when measured after the exposure is completed. When subjects are asked to complete a learning task after short term exposure they often have difficulties. Prenatal stress also hinders the ability to learn and memorize by disrupting the development of the hippocampus and can lead to unestablished long term potentiation in the offspring of severely stressed parents. Although the stress is applied prenatally, the offspring show increased levels of glucocorticoids when they are subjected to stress later on in life.

Research reported in 2013 revealed that it is possible to artificially stimulate prior memories and artificially implant false memories in mice. Using optogenetics, a team of RIKEN-MIT scientists caused the mice to incorrectly associate a benign environment with a prior unpleasant experience from different surroundings. Some scientists believe that the study may have implications in studying false memory formation in humans, and in treating PTSD and schizophrenia.

A UCLA research study published in the June 2008 issue of the "American Journal of Geriatric Psychiatry" found that people can improve cognitive function and brain efficiency through simple lifestyle changes such as incorporating memory exercises, healthy eating, physical fitness and stress reduction into their daily lives. This study examined 17 subjects, (average age 53) with normal memory performance.
Eight subjects were asked to follow a "brain healthy" diet, relaxation, physical, and mental exercise (brain teasers and verbal memory training techniques). After 14 days, they showed greater word fluency (not memory) compared to their baseline performance. No long-term follow-up was conducted; it is therefore unclear if this intervention has lasting effects on memory.

There are a loosely associated group of mnemonic principles and techniques that can be used to vastly improve memory known as the art of memory.

The International Longevity Center released in 2001 a report which includes in pages 14–16 recommendations for keeping the mind in good functionality until advanced age. Some of the recommendations are to stay intellectually active through learning, training or reading, to keep physically active so to promote blood circulation to the brain, to socialize, to reduce stress, to keep sleep time regular, to avoid depression or emotional instability and to observe good nutrition.

Memorization is a method of learning that allows an individual to recall information verbatim. Rote learning is the method most often used. Methods of memorizing things have been the subject of much discussion over the years with some writers, such as Cosmos Rossellius using visual alphabets. The spacing effect shows that an individual is more likely to remember a list of items when rehearsal is spaced over an extended period of time. In contrast to this is cramming: an intensive memorization in a short period of time. Also relevant is the Zeigarnik effect which states that people remember uncompleted or interrupted tasks better than completed ones. The so-called Method of loci uses spatial memory to memorize non-spatial information.

Plants lack a specialized organ devoted to memory retention, and so plant memory has been a controversial topic in recent years. New advances in the field have identified the presence of neurotransmitters in plants, adding to the hypothesis that plants are capable of remembering. Action potentials, a physiological response characteristic of neurons, have been shown to have an influence on plants as well, including in wound responses and photosynthesis. In addition to these homologous features of memory systems in both plants and animals, plants have also been observed to encode, store and retrieve basic short-term memories.

One of the most well-studied plants to show rudimentary memory is the Venus flytrap. Native to the subtropical wetlands of the eastern United States, Venus Fly Traps have evolved the ability to obtain meat for sustenance, likely due to the lack of nitrogen in the soil. This is done by two trap-forming leaf tips that snap shut once triggered by a potential prey. On each lobe, three triggers hairs await stimulation. In order to maximize the benefit to cost ratio, the plant enables a rudimentary form of memory in which two trigger hairs must be stimulated within 30 seconds in order to result in trap closure. This system ensures that the trap only closes when potential prey is within grasp.

The field of plant neurobiology has gained a large amount of interest over the past decade, leading to an influx of research regarding plant memory. Although the Venus flytrap is one of the more highly studied, many other plants exhibit the capacity to remember, including the "Mimosa pudica" through an experiment conducted by Monica Gagliano and colleagues in 2013. As the field expands, it is likely that we will learn more about the capacity of a plant to remember.

In contrast to the short-term memory, long-term memory refers to the ability to hold information for a prolonged time and is possibly the most complex component of the human memory system. The Atkinson–Shiffrin model of memory (Atkinson 1968) suggests that the items stored in short-term memory moves to long-term memory through repeated practice and use. Long-term storage may be similar to learning—the process by which information that may be needed again is stored for recall on demand. The process of locating this information and bringing it back to working memory is called retrieval. This knowledge that is easily recalled is explicit knowledge, whereas most long-term memory is implicit knowledge and is not readily retrievable. Scientists speculate that the hippocampus is involved in the creation of long-term memory. It is unclear where long-term memory is stored, although there is evidence depicting long-term memory is stored in various parts of the nervous system. Long-term memory is permanent. Memory can be recalled, which, according to the dual-store memory search model, enhances the long-term memory. Forgetting may occur when the memory fails to be recalled on later occasions.

Storage (memory)

Memory is the process of storing and recalling information that was previously acquired. Memory is occurs through three fundamental stages: encoding, storage, and retrieval. Storing refers to the process of placing newly acquired information into memory, which is modified in the brain for easier storage. Encoding this information makes the process of retrieval easier for the brain where it can be recalled and brought into conscious thinking. Modern memory psychology differentiates between the two distinct types of memory storage: short-term memory and long-term memory. Several models of memory have been proposed over the past century, some of them suggesting different relationships between short- and long-term memory to account for different ways of storing memory.

Short-term memory is encoded in auditory, visual, spatial, and tactile forms. Short-term memory is closely related to working memory. Baddeley suggested that information stored in short-term memory continuously deteriorates, which can eventually lead to forgetting in the absence of rehearsal. George A. Miller suggested that the capacity of the short-term memory storage is about seven items plus or minus two, also known as the magic number 7, but this number has been shown to be subject to numerous variability, including the size, similarity, and other properties of the chunks. Memory span varies; it is lower for multisyllabic words than for shorter words. In general, the memory span for verbal contents i.e. letters, words, and digits, relies on the duration of time it takes to speak these contents aloud and on the degree of lexicality (relating to the words or the vocabulary of a language distinguished from its grammar and construction) of the contents. The ability to recall words. Characteristics such as the length of spoken time for each word, known as the word-length effect, or when words are similar to each other lead to fewer words being recalled.

Chunking is the process of grouping pieces of information together into “chunks”. This allows for the brain to collect more information at a given time by reducing it to more-specific groups. With the processes of chunking, the external environment is linked to the internal cognitive processes of the brain. Due to the limited capacity of the working memory, this type of storage is necessary for memory to properly function. The exact number of chunks that can be present in the working memory is not definite, but ranges from one to three chunks. The recall is not measured in terms of the items that are being remembered, but they chunks that they are put into. This type of memory storage is typically effective, as it has been found that with the appearance of the first item in a chunk, the other items can be immediately recalled. Though errors may occur, it if more common for the errors to occur at the beginning of the chunk than in the middle of the chunk. Chunks can be recalled with long-term or working memory. Simple chunks of information can be recalled without having to go through long term memory, such as the sequence ABABAB, which would use working memory for recollection. More difficult sequences, such as a phone number, would have to be split into chunks and may have to pass through long-term memory to be recalled. The spacing used in phone numbers is a common chunking method, as the grouping in the numbers allows for the digits to be remembered in clusters and not individually.

Chunking was introduced by George A. Miller who suggested that this way of organizing and processing information allows for a more effective retention of material from the environment. Miller developed the idea that chunking was a collection of similar items and when that chunk was named, it allowed for the items in that chunk to be more easily recalled. Other researchers described the items in these chunks as being strongly connected to each other, but not to the other items in other chunks. Each chunk, in their findings, would hold only the items pertaining to that topic, and not have it be relatable to any other chunk or items in that chunk. The menu for a restaurant would display this type of chucking, as the entrée category would not display anything from the dessert category, and the dessert category would not display anything form the entrée category.

Psychologist and master chess player Adriaan de Groot supported the theory of chunking through his experiment on chess positions and different levels of expertise. When presented positions of pieces from chess tournament games, the experts were more accurate at recalling the positions. However, when the groups were given random positions to remember, De Groot found that all groups performed poorly at the recalling task regardless of the participants knowledge of chess. Further research into chunking greatly impacted the studies of memory development, expertise, and immediate recall. Research into behavioral and imaging studies have also suggested that chunking can be applied to habit learning, motor skills, language processing, and visual perception.

Rehearsal is the process by which information is retained in short-term memory by conscious repetition of the word, phrase or number. If information has sufficient meaning to the person or if it is repeated enough, it can be encoded into long-term memory. There are two types of rehearsal: maintenance rehearsal and elaborate rehearsal. Maintenance rehearsal consists of constantly repeating the word or phrase of words to remember. Remembering a phone number is one of the best examples of this. Maintenance rehearsal is mainly used for the short-term ability to recall information. Elaborate rehearsal involves the association of old with new information.

Several memory models have been proposed to account for different types of recall processes, including cued recall, free recall, and serial recall. However, to explain the recall process, the memory model must identify how an encoded memory can reside in the memory storage for a prolonged period until the memory is accessed again, during the recall process; but not all models use the terminology of short-term and long-term memory to explain memory storage; the dual-store theory and a modified version of Atkinson–Shiffrin model of memory (Atkinson 1968) uses both short-and long-term memory storage, but others do not.

The multi-trace distributed memory model suggests that the memories that are being encoded are converted to vectors of values, with each scalar quantity of a vector representing a different attribute of the item to be encoded. Such notion was first suggested by early theories of Hooke (1969) and Semon (1923). A single memory is distributed to multiple attributes, or features, so that each attribute represents one aspect of the memory being encoded. Such a vector of values is then added into the memory array or a matrix, composed of different traces or vectors of memory. Therefore, every time a new memory is encoded, such memory is converted to a vector or a trace, composed of scalar quantities representing variety of attributes, which is then added to pre-existing and ever-growing memory matrix, composed of multiple traces—hence the name of the model.

Once memory traces corresponding to specific memory are stored in the matrix, to retrieve the memory for the recall process one must cue the memory matrix with a specific probe, which would be used to calculate the similarity between the test vector and the vectors stored in the memory matrix. Because the memory matrix is constantly growing with new traces being added in, one would have to perform a parallel search through all the traces present within the memory matrix to calculate the similarity, whose result can be used to perform either associative recognition, or with probabilistic choice rule, used to perform a cued recall.

While it has been claimed that human memory seems to be capable of storing a great amount of information, to the extent that some had thought an infinite amount, the presence of such ever-growing matrix within human memory sounds implausible. In addition, the model suggests that to perform the recall process, parallel-search between every single trace that resides within the ever-growing matrix is required, which also raises doubt on whether such computations can be done in a short amount of time. Such doubts, however, have been challenged by findings of Gallistel and King who present evidence on the brain’s enormous computational abilities that can be in support of such parallel support.

The multi-trace model had two key limitations: one, notion of the presence of ever-growing matrix in human memory sounds implausible; and two, computational searches for similarity against millions of traces that would be present in memory matrix to calculate similarity sounds far beyond the scope of the human recalling process. The neural network model is the ideal model in this case, as it overcomes the limitations posed by the multi-trace model and maintains the useful features of the model as well.

The neural network model assumes that neurons in a neural network form a complex network with other neurons, forming a highly interconnected network; each neuron is characterized by the activation value, and the connection between two neurons is characterized by the weight value. Interaction between each neuron is characterized by the McCullough–Pitts dynamical rule, and change of weight and connections between neurons resulting from learning is represented by the Hebbian learning rule.

Anderson shows that combination of Hebbian learning rule and McCullough–Pitts dynamical rule allow network to generate a weight matrix that can store associations between different memory patterns – such matrix is the form of memory storage for the neural network model. Major differences between the matrix of multiple traces hypothesis and the neural network model is that while new memory indicates extension of the existing matrix for the multiple traces hypothesis, weight matrix of the neural network model does not extend; rather, the weight is said to be updated with introduction of new association between neurons.

Using the weight matrix and learning/dynamic rule, neurons cued with one value can retrieve the different value that is ideally a close approximation of the desired target memory vector.

As the Anderson’s weight matrix between neurons will only retrieve the approximation of the target item when cued, modified version of the model was sought in order to be able to recall the exact target memory when cued. The Hopfield Net is currently the simplest and most popular neural network model of associative memory; the model allows the recall of clear target vector when cued with the part or the 'noisy' version of the vector.

The weight matrix of Hopfield Net, that stores the memory, closely resembles the one used in weight matrix proposed by Anderson. Again, when new association is introduced, the weight matrix is said to be ‘updated’ to accommodate the introduction of new memory; it is stored until the matrix is cued by a different vector.

First developed by Atkinson and Shiffrin (1968), and refined by others, including Raajimakers and Shiffrin, the dual-store memory search model, now referred to as SAM or search of associative memory model, remains as one of the most influential computational models of memory. The model uses both short-term memory, termed short-term store (STS), and long-term memory, termed long-term store (LTS) or episodic matrix, in its mechanism.

When an item is first encoded, it is introduced into the short-term store. While the item stays in the short-term store, vector representations in long-term store go through a variety of associations. Items introduced in short-term store go through three different types of association: (autoassociation) the self-association in long-term store, (heteroassociation) the inter-item association in long-term store, and the (context association ) which refers to association between the item and its encoded context. For each item in short-term store, the longer the duration of time an item resides within the short-term store, the greater its association with itself will be with other items that co-reside within short-term store, and with its encoded context.

The size of the short-term store is defined by a parameter, r. As an item is introduced into the short-term store, and if the short-term store has already been occupied by a maximum number of items, the item will probably drop out of the short-term storage.

As items co-reside in the short-term store, their associations are constantly being updated in the long-term store matrix. The strength of association between two items depends on the amount of time the two memory items spend together within the short-term store, known as the contiguity effect. Two items that are contiguous have greater associative strength and are often recalled together from long-term storage.

Furthermore, the primacy effect, an effect seen in memory recall paradigm, reveals that the first few items in a list have a greater chance of being recalled over others in the STS, while older items have a greater chance of dropping out of STS. The item that managed to stay in the STS for an extended amount of time would have formed a stronger autoassociation, heteroassociation and context association than others, ultimately leading to greater associative strength and a higher chance of being recalled.

The recency effect of recall experiments is when the last few items in a list are recalled exceptionally well over other items, and can be explained by the short-term store. When the study of a given list of memory has been finished, what resides in the short-term store in the end is likely to be the last few items that were introduced last. Because the short-term store is readily accessible, such items would be recalled before any item stored within long-term store. This recall accessibility also explains the fragile nature of recency effect, which is that the simplest distractors can cause a person to forget the last few items in the list, as the last items would not have had enough time to form any meaningful association within the long-term store. If the information is dropped out of the short-term store by distractors, the probability of the last items being recalled would be expected to be lower than even the pre-recency items in the middle of the list.

The dual-store SAM model also utilizes memory storage, which itself can be classified as a type of long-term storage: the semantic matrix. The long-term store in SAM represents the episodic memory, which only deals with new associations that were formed during the study of an experimental list; pre-existing associations between items of the list, then, need to be represented on different matrix, the semantic matrix. The semantic matrix remains as another source of information that is not modified by episodic associations that are formed during the exam.

Thus, the two types of memory storage, short- and long-term stores, are used in the SAM model. In the recall process, items residing in short-term memory store will be recalled first, followed by items residing in long-term store, where the probability of being recalled is proportional to the strength of the association present within the long-term store. Another memory storage, the semantic matrix, is used to explain the semantic effect associated with memory recall.

The neuroscientist Donald Hebb (1904–1985) was the first to distinguish between short-term memory and long-term memory. According to current theories in neuroscience, things that we "notice" are stored in short-term memory for up to a few minutes; this memory depends on "reverberating" electrical activity in neuronal circuits, and is very easily destroyed by interruption or interference. Memories stored for longer than this are stored in "long-term memory". Whether information is stored in long-term memory depends on its "importance"; for any animal, memories of traumatic events are potentially important for the adaptive value that they have for future avoidance behavior, and hormones that are released during stress have a role in determining what memories are preserved. In humans, traumatic stress is associated with acute secretion of epinephrine and norepinephrine (adrenaline and noradrenaline) from the adrenal medulla and cortisol from the adrenal cortex. Increases in these facilitate memory, but chronic stress associated with prolonged hypersecretion of cortisol may have the opposite effect. The limbic system is involved in memory storage and retrieval as well as giving emotional significance to sensory inputs. Within the limbic system, the hippocampus is important for explicit memory, and for memory consolidation; it is also sensitive to stress hormones, and has a role in recording the emotions of a stressful event. The amygdala may be particularly important in assigning emotional values to sensory inputs.

Repressed memory

Repressed memories are memories that have been unconsciously blocked due to the memory being associated with a high level of stress or trauma. The theory postulates that even though the individual cannot recall the memory, it may still be affecting them subconsciously, and that these memories can emerge later into the consciousness. Ideas on repressed memory hiding trauma from awareness were an important part of Sigmund Freud's early work on psychoanalysis. He later took a different view.

The existence of repressed memories is an extremely controversial topic in psychology; although some studies have concluded that it can occur in a varying but generally small percentage of victims of trauma, many other studies dispute its existence entirely. Some psychologists support the theory of repressed memories and claim that repressed memories can be recovered through therapy, but most psychologists argue that this is in fact rather a process through which false memories are created by blending actual memories and outside influences. One study concluded that repressed memories were a cultural symptom for want of written proof of their existence before the nineteenth century, but its results were disputed by some psychologists, and a work discussing a repressed memory from 1786 was eventually acknowledged, though the others stand by their hypothesis.

According to the American Psychological Association, it is not possible to distinguish repressed memories from false ones without corroborating evidence. The term repressed memory is sometimes compared to the term dissociative amnesia, which is defined in the DSM-V as an "inability to recall autobiographical information. This amnesia may be localized (i.e., an event or period of time), selective (i.e., a specific aspect of an event), or generalized (i.e., identity and life history)."

According to the Mayo Clinic, amnesia refers to any instance in which memories stored in the long-term memory are completely or partially forgotten, usually due to brain injury.
According to proponents of the existence of repressed memories, such memories can be recovered years or decades after the event, most often spontaneously, triggered by a particular smell, taste, or other identifier related to the lost memory, or via suggestion during psychotherapy.

It was initially claimed that there was no documented writing about repressed memories or dissociative amnesia (as it is sometimes referred to), before the 1800s. This finding, by Harrison G. Pope, was based on a competition in which entrants could win $1000 if they could identify "a pre-1800 literary example of traumatic memory that has been repressed by an otherwise healthy individual, and then recovered." Pope claimed that no entrant had satisfied the criteria. Ross Cheit, a political scientist at Brown University, cited Nina, a 1786 opera by the French composer Nicolas Dalayrac.

The concept of repressed memory originated with Sigmund Freud in his 1896 essay "Zur Ätiologie der Hysterie" ("On the etiology of hysteria").
One of the studies published in his essay involved a young woman by the name of Anna O. Among her many ailments, she suffered from stiff paralysis on the right side of her body. Freud stated her symptoms to be attached to psychological traumas. The painful memories had separated from her consciousness and brought harm to her body. Freud used hypnosis to treat Anna O. She is reported to have gained slight mobility on her right side. Freud's repressed memory theory joined his philosophy of psychoanalysis. Repressed memory has remained a heavily debated topic inside of Freud's psychoanalysis philosophy.

Some research indicates that memories of child sexual abuse and other traumatic incidents may be forgotten. Evidence of the spontaneous recovery of traumatic memories has been shown, and recovered memories of traumatic childhood abuse have been corroborated.

Van der Kolk and Fisler's research shows that traumatic memories are retrieved, at least at first, in the form of mental imprints that are dissociated. These imprints are of the affective and sensory elements of the traumatic experience. Clients have reported the slow emergence of a personal narrative that can be considered explicit (conscious) memory. The level of emotional significance of a memory correlates directly with the memory's veracity. Studies of subjective reports of memory show that memories of highly significant events are unusually accurate and stable over time. The imprints of traumatic experiences appear to be qualitatively different from those of nontraumatic events. Traumatic memories may be coded differently from ordinary event memories, possibly because of alterations in attentional focusing or the fact that extreme emotional arousal interferes with the memory functions of the hippocampus.

Another possibility is that traumatic events are pushed out of consciousness until a later events elicits or triggers a psychological response. Support for this idea has come from studies in which trauma was temporarily induced. For example, a high percentage of female psychiatric in-patients, and outpatients have reported experiencing histories of childhood sexual abuse. Other clinical studies have concluded that patients who experienced incestuous abuse reported higher suicide attempts and negative identity formation as well as more disturbances in interpersonal relationships.

There has also been significant questioning of the reality of repressed memories. There is considerable evidence that rather than being pushed out of consciousness, the difficulty with traumatic memories for most people are their intrusiveness and inability to forget. One case that is held up as definitive proof of the reality of repressed memories, recorded by David Corwin has been criticized by Elizabeth Loftus and Melvin Guyer for ignoring the context of the original complaint and falsely presenting the sexual abuse as unequivocal and true when in reality there was no definitive proof.

Retrospective studies (studying the extent to which participants can recall past events) depend critically on the ability of informants to recall accurate memories. The issue of reliability in participants’ introspective abilities has been questioned by modern psychologists. In other words, a participant accurately recalling and remembering their own past memories is highly criticized, because memories are undoubtedly influenced by external, environmental factors.

Psychologists Elizabeth Loftus and Katherine Ketcham are authors of the seminal work on the fallacy of repressed memory, "The Myth of Repressed Memory" (St. Martin's Press, 1994).

It is hypothesised that repression may be one method used by individuals to cope with traumatic memories, by pushing them out of awareness (perhaps as an adaptation via psychogenic amnesia) to allow a child to maintain attachment to a person on whom they are dependent for survival. Researchers have proposed that repression can operate on a social level as well.

Other theoretical causes of forgotten memories have stemmed from the idea of "Retrieval-Influenced Forgetting", which states that “false” memories will be more accurately recalled when rehearsed more, than when actual memories get rehearsed. In this scenario, the action of rehearsing a falsified memory can actually take precedence over the actual memory that a person experiences. Anderson et al. discovered that rehearsal of novel information exhibits inhibitive processes on one’s ability to remember or recall the prior (real) memory. This conclusion indicates that past memories can be easily forgotten, simply by attending to “real”, novel memories that are brought into awareness.

Memories "can" be accurate, but they are not "always" accurate. For example, eyewitness testimony even of relatively recent dramatic events is notoriously unreliable. Memories of events are a mix of fact overlaid with emotions, mingled with interpretation and "filled in" with imaginings. Skepticism regarding the validity of a memory as factual detail is warranted. For example, one study where victims of documented child abuse were reinterviewed many years later as adults, 38% of the women denied any memory of the abuse.

Arguments against the existence of "traumatic amnesia" note that various manipulations can be used to implant false memories (sometimes called "pseudomemories"). These can be quite compelling for those who develop them, and can include details that make them seem credible to others. A classic experiment in memory research, conducted by Elizabeth Loftus, became widely known as "Lost in the Mall"; in this, subjects were given a booklet containing three accounts of real childhood events written by family members and a fourth account of a wholly fictitious event of being lost in a shopping mall. A quarter of the subjects reported remembering the fictitious event, and elaborated on it with extensive circumstantial detail. This experiment inspired many others, and in one of these, Porter et al. could convince about half of his subjects that they had survived a vicious animal attack in childhood.

Such experimental studies have been criticized in particular about whether the findings are really relevant to trauma memories and psychotherapeutic situations. Nevertheless, these studies prompted public and professional concern about recovered memory therapy for past sexual abuse. When memories are "recovered" after long periods of amnesia, particularly when extraordinary means were used to secure the recovery of memory, it is now widely (but not universally) accepted that the memories are quite likely to be false, i.e. of incidents that had not occurred. It is thus recognised by professional organizations that a risk of implanting false memories is associated with some similar types of therapy. The "American Psychiatric Association" advises: "...most leaders in the field agree that although it is a rare occurrence, a memory of early childhood abuse that has been forgotten can be remembered later. However, these leaders also agree that it is possible to construct convincing pseudomemories for events that never occurred.

Nevertheless, many therapists believe in the authenticity of the recovered memories that they hear from their clients. In a non-random study by Loftus and Herzog (1991) with 16 clinicians, 13 (81%) said that they invariably believed their clients. The most common basis for this belief was the patient’s symptomology (low self-esteem, sexual dysfunction, self-destructive behaviour) or body memories (voice frozen etc.).

The mechanism(s) by which both of these phenomena happen are not well understood and, at this point it is impossible, without other corroborative evidence, to distinguish a true memory from a false one." Sheflin and Brown state that a total of 25 studies on amnesia for child sexual abuse exist and that they demonstrate amnesia in their study subpopulations. However, an editorial in the "British Medical Journal" states on the Sheflin and Brown study that "on critical examination, the scientific evidence for repression crumbles."

Obviously, not all therapists agree that false memories are a major risk of psychotherapy and they argue that this idea overstates the data and is untested. Several studies have reported high percentages of the corroboration of recovered memories, and some authors have claimed that the false memory movement has tended to conceal or omit evidence of (the) corroboration" of recovered memories.

Both true and false "memories" can be recovered using memory work techniques, but there is no evidence that reliable discriminations can be made between them. Some believe that memories "recovered" under hypnosis are particularly likely to be false.
According to The Council on Scientific Affairs for the American Medical Association, recollections obtained during hypnosis can involve confabulations and pseudomemories and appear to be less reliable than nonhypnotic recall.
Brown et al. estimate that 3 to 5% of laboratory subjects are vulnerable to post-event misinformation suggestions. They state that 5–8% of the general population is the range of high-hypnotizability. Twenty-five percent of those in this range are vulnerable to suggestion of pseudomemories for peripheral details, which can rise to 80% with a combination of other social influence factors. They conclude that the rates of memory errors run 0–5% in adult studies, 3–5% in children's studies and that the rates of false allegations of child abuse allegations run 4–8% in the general population.

Although memory distortion occurs in everyday life, the brain mechanisms involved are not easy to study in the laboratory, but neuroimaging techniques have recently been applied to this subject. In particular, there have recently been studies of false recognition, where individuals incorrectly claim to have encountered a novel object or event, and the results suggest that the hippocampus and several cortical regions may contribute to such false recognition, while the prefrontal cortex may be involved in retrieval monitoring that can limit the rate of false recognition.

Amnesia is partial or complete loss of memory that goes beyond mere forgetting. Often it is temporary and involves only part of a person's experience. Amnesia is often caused by an injury to the brain, for instance after a blow to the head, and sometimes by psychological trauma. Anterograde amnesia is a failure to remember new experiences that occur after damage to the brain; retrograde amnesia is the loss of memories of events that occurred before a trauma or injury. For a memory to become permanent (consolidated), there must be a persistent change in the strength of connections between particular neurons in the brain. Anterograde amnesia can occur because this consolidation process is disrupted; retrograde amnesia can result either from damage to the site of memory storage or from a disruption in the mechanisms by which memories can be retrieved from their stores. Many specific types of amnesia are recognized, including:
The form of amnesia that is linked with recovered memories is dissociative amnesia (formerly known as psychogenic amnesia). This results from a psychological cause, not by direct damage to the brain, and is a loss of memory of significant personal information, usually about traumatic or extremely stressful events. Usually this is seen as a gap or gaps in recall for aspects of someone's life history, but with severe acute trauma, such as during wartime, there can be a sudden acute onset of symptoms.

"Betrayal Trauma Theory" proposes that in cases of childhood abuse, dissociative amnesia is an adaptive response, and that “victims may need to remain unaware of the trauma not to reduce suffering but rather to promote survival.”
When stress interferes with memory, it is possible that some of the memory is kept by a system that records emotional experience, but there is no symbolic placement of it in time or space.
Traumatic memories are retrieved, at least at first, in the form of dissociated mental imprints of the affective and sensory elements of the traumatic experience. Clients have reported the slow emergence of a personal narrative that can be considered explicit (conscious) memory.

Psychiatrist Bessel van der Kolk divided the effects of traumas on memory functions into four sets:

According to van der Kolk, memories of highly significant events are usually accurate and stable over time; aspects of traumatic experiences appear to get stuck in the mind, unaltered by time passing or experiences that may follow. The imprints of traumatic experiences appear to be different from those of nontraumatic events, perhaps because of alterations in attentional focusing or the fact that extreme emotional arousal interferes with memory. van der Kolk and Fisler's hypothesis is that under extreme stress, the memory categorization system based in the hippocampus fails, with these memories kept as emotional and sensory states. When these traces are remembered and put into a personal narrative, they are subject to being condensed, contaminated and embellished upon.

When there is inadequate recovery time between stressful situations, alterations may occur to the stress response system, some of which may be irreversible, and cause pathological responses, which may include memory loss, learning deficits and other maladaptive symptoms. In animal studies, high levels of cortisol can cause hippocampal damage, which may cause short-term memory deficits; in humans, MRI studies have shown reduced hippocampal volumes in combat veterans with PTSD, adults with posttraumatic symptoms and survivors of repeated childhood sexual or physical abuse. Trauma may also interfere with implicit memory, where periods of avoidance may be interrupted by intrusive emotional occurrences with no story to guide them. A difficult issue is whether those presumably abused accurately recall their experiences.

The existence of repressed memory recovery has not been accepted by mainstream psychology, nor unequivocally proven to exist, and some experts in the field of human memory feel that no credible scientific support exists for the notions of repressed/recovered memories. A survey revealed that whilst memory and cognition experts tend to be skeptical of repressed memory, clinicians are much more apt to believe that traumatic memory is often repressed. One research report states that a distinction should be made between spontaneously recovered memories and memories recovered during suggestions in therapy. A criticism from Loftus is that recovered memories can be tainted by the process of recovery, the suggestions used in that process, or even cultural and environmental influences.

The Working Group on Investigation of Memories of Child Abuse of the American Psychological Association presented findings mirroring those of the other professional organizations. The Working Group made five key conclusions:Many critics believe that memories may be distorted and false. Psychologist Elizabeth Loftus questions the concept of repressed memories and the possibility of them being accurate. Loftus focuses on techniques that therapists use in order to help the patients recover their memory. Such techniques include age regression, guided visualization, trance writing, dream work, body work, and hypnosis.
Loftus' research indicates that repressed memory faces problems, such as memory alteration. In one case a teenage boy was able to “conjure a memory of an event that never occurred.” According to Loftus, if a stable person could be influenced to remember an event that never occurred, an emotionally stressed person would be even more susceptible.
Writer Mark Pendergrast has denounced the theory of repressed memories and its applications in sex abuse cases, including in particular the Jerry Sandusky case.

Serious issues arise when recovered but false memories result in public allegations; false complaints carry serious consequences for the accused. Many of those who make false claims sincerely believe the truth of what they report. A special type of false allegation, the false memory syndrome, arises typically within therapy, when people report the "recovery" of childhood memories of previously unknown abuse. The influence of practitioners' beliefs and practices in the eliciting of false "memories" and of false complaints has come under particular criticism.

It is generally accepted that people sometimes are unable to recall traumatic experiences. An old version of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV), published by the "American Psychiatric Association", states that "Dissociative amnesia is characterized by an inability to recall important personal information, usually of a traumatic or stressful nature, that is too extensive to be explained by ordinary forgetfulness."
The term "recovered memory", however, is not listed in DSM-IV or used by any mainstream formal psychotherapy modality.

Some criminal cases have been based on a witness's testimony of recovered repressed memories, often of alleged childhood sexual abuse. In some jurisdictions, the statute of limitations for child abuse cases has been extended to accommodate the phenomena of repressed memories as well as other factors. The repressed memory concept came into wider public awareness in the 1980s and 1990s followed by a reduction of public attention after a series of scandals, lawsuits, and license revocations.

A U.S. District Court accepted repressed memories as admissible evidence in a specific case. Dalenberg argues that the evidence shows that recovered memory cases should be allowed to be prosecuted in court.

The apparent willingness of courts to credit the recovered memories of complainants but not the absence of memories by defendants has been commented on: "It seems apparent that the courts need better guidelines around the issue of dissociative amnesia in both populations."

In 1995, the Ninth Circuit Court of Appeals ruled, in Franklin v. Duncan and Franklin v. Fox, Murray "et al". (312 F3d. 423, see also 884 FSupp 1435, N.D. Calif.), that repressed memory is not admissible as evidence in a legal action because of its unreliability, inconsistency, unscientific nature, tendency to be therapeutically induced evidence, and subject to influence by hearsay and suggestibility. The court overturned the conviction of a man accused of murdering a nine-year-old girl purely based upon the evidence of a 21-year-old repressed memory by a lone witness, who also held a complex personal grudge against the defendant.

In a 1996 ruling, a U.S. District Court allowed repressed memories entered into evidence in court cases. Jennifer Freyd writes that Ross Cheit's case of suddenly remembered sexual abuse is one of the most well-documented cases available for the public to see. Cheit prevailed in two lawsuits, located five additional victims and tape-recorded a confession.

On December 16, 2005, the Irish Court of Criminal Appeal issued a certificate confirming a Miscarriage of Justice to a former nun, Nora Wall whose 1999 conviction for child rape was partly based on repressed-memory evidence. The judgement stated that:

There was no scientific evidence of any sort adduced to explain the phenomenon of "flashbacks" and/or "retrieved memory", nor was the applicant in any position to meet such a case in the absence of prior notification thereof.

On August 16, 2010 the United States Second Circuit Court of Appeals in a case reversed the conviction that relied on claimed victim memories of childhood abuse stating that "The record here suggests a "reasonable likelihood" that Jesse Friedman was wrongfully convicted. The "new and material evidence” in this case is the post-conviction consensus within the social science community that suggestive memory recovery tactics can create false memories" (pg 27 FRIEDMAN v. REHAL Docket No. 08-0297). The ruling goes on to order all previous convictions and plea bargains relying in repressed memories using common memory recovered techniques be reviewed.

Recovered memory therapy is a range of psychotherapy methods based on recalling memories of abuse that had previously been forgotten by the patient. The term "recovered memory therapy" is not listed in DSM-IV or used by mainstream formal psychotherapy modality. Opponents of the therapy advance the explanation that therapy can create false memories through suggestion techniques; this has not been corroborated, though some research has shown supportive evidence. Nevertheless, the evidence is questioned by some researchers. It is possible for patients who retract their claims—after deciding their recovered memories are false—to suffer post-traumatic stress disorder due to the trauma of illusory memories.

Anders Ericsson and Walter Kintsch have introduced the notion of "long-term working memory", which they define as a set of "retrieval structures" in long-term memory that enable seamless access to the information relevant for everyday tasks. In this way, parts of long-term memory effectively function as working memory. In a similar vein, Cowan does not regard working memory as a separate system from long-term memory. Representations in working memory are a subset of representations in long-term memory. Working memory is organized into two embedded levels. The first consists of long-term memory representations that are activated. There can be many of these—there is theoretically no limit to the activation of representations in long-term memory. The second level is called the focus of attention. The focus is regarded as having a limited capacity and holds up to four of the activated representations.

Working memory

Working memory is a cognitive system with a limited capacity that is responsible for temporarily holding information available for processing. Working memory is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term memory, but some theorists consider the two forms of memory distinct, assuming that working memory allows for the manipulation of stored information, whereas short-term memory only refers to the short-term storage of information. Working memory is a theoretical concept central to cognitive psychology, neuropsychology, and neuroscience.

The term "working memory" was coined by Miller, Galanter, and Pribram, and was used in the 1960s in the context of theories that likened the mind to a computer. In 1968, Atkinson and Shiffrin used the term to describe their "short-term store". What we now call working memory was formerly referred to variously as a "short-term store" or short-term memory, primary memory, immediate memory, operant memory, and provisional memory. Short-term memory is the ability to remember information over a brief period (in the order of seconds). Most theorists today use the concept of working memory to replace or include the older concept of short-term memory, marking a stronger emphasis on the notion of manipulating information rather than mere maintenance.

The earliest mention of experiments on the neural basis of working memory can be traced back to more than 100 years ago, when Hitzig and Ferrier described ablation experiments of the prefrontal cortex (PFC); they concluded that the frontal cortex was important for cognitive rather than sensory processes. In 1935 and 1936, Carlyle Jacobsen and colleagues were the first to show the deleterious effect of prefrontal ablation on delayed response.

Numerous models have been proposed for how working memory functions, both anatomically and cognitively. Of those, the two that have been most influential are summarized below.

In 1974, Baddeley and Hitch introduced the multicomponent model of working memory. The theory proposed a model containing three components: the central executive, the phonological loop, and the visuospatial sketchpad with the central executive functioning as a control center of sorts, directing info between the phonological and visuospatial components. The central executive is responsible "inter alia" for directing attention to relevant information, suppressing irrelevant information and inappropriate actions, and coordinating cognitive processes when more than one task is simultaneously performed. A "central executive" is responsible for supervising the integration of information and for coordinating "slave systems" that are responsible for the short-term maintenance of information. One slave system, the phonological loop (PL), stores phonological information (that is, the sound of language) and prevents its decay by continuously refreshing it in a rehearsal loop. It can, for example, maintain a seven-digit telephone number for as long as one repeats the number to oneself again and again. The other slave system, the visuospatial sketchpad, stores visual and spatial information. It can be used, for example, for constructing and manipulating visual images and for representing mental maps. The sketchpad can be further broken down into a visual subsystem (dealing with such phenomena as shape, colour, and texture), and a spatial subsystem (dealing with location).

In 2000, Baddeley extended the model by adding a fourth component, the episodic buffer, which holds representations that integrate phonological, visual, and spatial information, and possibly information not covered by the slave systems (e.g., semantic information, musical information). The episodic buffer is also the link between working memory and long-term memory. The component is episodic because it is assumed to bind information into a unitary episodic representation. The episodic buffer resembles Tulving's concept of episodic memory, but it differs in that the episodic buffer is a temporary store.

Oberauer has extended Cowan's model by adding a third component, a more narrow focus of attention that holds only one chunk at a time. The one-element focus is embedded in the four-element focus and serves to select a single chunk for processing. For example, four digits can be held in mind at the same time in Cowan's "focus of attention". When the individual wishes to perform a process on each of these digits—for example, adding the number two to each digit—separate processing is required for each digit since most individuals cannot perform several mathematical processes in parallel. Oberauer's attentional component selects one of the digits for processing and then shifts the attentional focus to the next digit, continuing until all digits have been processed.

Working memory is generally considered to have limited capacity. An early quantification of the capacity limit associated with short-term memory was the "magical number seven" suggested by Miller in 1956. He claimed that the information-processing capacity of young adults is around seven elements, which he called "chunks", regardless of whether the elements are digits, letters, words, or other units. Later research revealed this number depends on the category of chunks used (e.g., span may be around seven for digits, six for letters, and five for words), and even on features of the chunks within a category. For instance, span is lower for long than short words. In general, memory span for verbal contents (digits, letters, words, etc.) depends on the phonological complexity of the content (i.e., the number of phonemes, the number of syllables), and on the lexical status of the contents (whether the contents are words known to the person or not). Several other factors affect a person's measured span, and therefore it is difficult to pin down the capacity of short-term or working memory to a number of chunks. Nonetheless, Cowan proposed that working memory has a capacity of about four chunks in young adults (and fewer in children and old adults).

Whereas most adults can repeat about seven digits in correct order, some individuals have shown impressive enlargements of their digit span—up to 80 digits. This feat is possible by extensive training on an encoding strategy by which the digits in a list are grouped (usually in groups of three to five) and these groups are encoded as a single unit (a chunk). For this to succeed, participants must be able to recognize the groups as some known string of digits. One person studied by Ericsson and his colleagues, for example, used an extensive knowledge of racing times from the history of sports in the process of coding chunks: several such chunks could then be combined into a higher-order chunk, forming a hierarchy of chunks. In this way, only some chunks at the highest level of the hierarchy must be retained in working memory, and for retrieval the chunks are unpacked. That is, the chunks in working memory act as retrieval cues that point to the digits they contain. Practicing memory skills such as these does not expand working memory capacity proper: it is the capacity to transfer (and retrieve) information from long-term memory that is improved, according to Ericsson and Kintsch (1995; see also Gobet & Simon, 2000).

Working memory capacity can be tested by a variety of tasks. A commonly used measure is a dual-task paradigm, combining a memory span measure with a concurrent processing task, sometimes referred to as "complex span". Daneman and Carpenter invented the first version of this kind of task, the "reading span", in 1980. Subjects read a number of sentences (usually between two and six) and tried to remember the last word of each sentence. At the end of the list of sentences, they repeated back the words in their correct order. Other tasks that do not have this dual-task nature have also been shown to be good measures of working memory capacity. Whereas Daneman and Carpenter believed that the combination of "storage" (maintenance) and processing is needed to measure working memory capacity, we know now that the capacity of working memory can be measured with short-term memory tasks that have no additional processing component. Conversely, working memory capacity can also be measured with certain processing tasks that don't involve maintenance of information. The question of what features a task must have to qualify as a good measure of working memory capacity is a topic of ongoing research.

Measures of working-memory capacity are strongly related to performance in other complex cognitive tasks, such as reading comprehension, problem solving, and with measures of intelligence quotient.

Some researchers have argued that working-memory capacity reflects the efficiency of executive functions, most notably the ability to maintain multiple task-relevant representations in the face of distracting irrelevant information; and that such tasks seem to reflect individual differences in the ability to focus and maintain attention, particularly when other events are serving to capture attention. Both working memory and executive functions rely strongly, though not exclusively, on frontal brain areas.

Other researchers have argued that the capacity of working memory is better characterized as the ability to mentally form relations between elements, or to grasp relations in given information. This idea has been advanced, among others, by Graeme Halford, who illustrated it by our limited ability to understand statistical interactions between variables. These authors asked people to compare written statements about the relations between several variables to graphs illustrating the same or a different relation, as in the following sentence: "If the cake is from France, then it has more sugar if it is made with chocolate than if it is made with cream, but if the cake is from Italy, then it has more sugar if it is made with cream than if it is made of chocolate". This statement describes a relation between three variables (country, ingredient, and amount of sugar), which is the maximum most individuals can understand. The capacity limit apparent here is obviously not a memory limit (all relevant information can be seen continuously) but a limit to how many relationships are discerned simultaneously.

There are several hypotheses about the nature of the capacity limit. One is that a limited pool of cognitive resources needed to keep representations active and thereby available for processing, and for carrying out processes. Another hypothesis is that memory traces in working memory decay within a few seconds, unless refreshed through rehearsal, and because the speed of rehearsal is limited, we can maintain only a limited amount of information. Yet another idea is that representations held in working memory interfere with each other.

The assumption that the contents of short-term or working memory decay over time, unless decay is prevented by rehearsal, goes back to the early days of experimental research on short-term memory. It is also an important assumption in the multi-component theory of working memory. The most elaborate decay-based theory of working memory to date is the "time-based resource sharing model". This theory assumes that representations in working memory decay unless they are refreshed. Refreshing them requires an attentional mechanism that is also needed for any concurrent processing task. When there are small time intervals in which the processing task does not require attention, this time can be used to refresh memory traces. The theory therefore predicts that the amount of forgetting depends on the temporal density of attentional demands of the processing task—this density is called "cognitive load". The cognitive load depends on two variables, the rate at which the processing task requires individual steps to be carried out, and the duration of each step. For example, if the processing task consists of adding digits, then having to add another digit every half second places a higher cognitive load on the system than having to add another digit every two seconds. In a series of experiments, Barrouillet and colleagues have shown that memory for lists of letters depends neither on the number of processing steps nor the total time of processing but on cognitive load.

Resource theories assume that the capacity of working memory is a limited resource that must be shared between all representations that need to be maintained in working memory simultaneously. Some resource theorists also assume that maintenance and concurrent processing share the same resource; this can explain why maintenance is typically impaired by a concurrent processing demand. Resource theories have been very successful in explaining data from tests of working memory for simple visual features, such as colors or orientations of bars. An ongoing debate is whether the resource is a continuous quantity that can be subdivided among any number of items in working memory, or whether it consists of a small number of discrete "slots", each of which can be assigned to one memory item, so that only a limited number of about 3 items can be maintained in working memory at all.

Several forms of interference have been discussed by theorists. One of the oldest ideas is that new items simply replace older ones in working memory. Another form of interference is retrieval competition. For example, when the task is to remember a list of 7 words in their order, we need to start recall with the first word. While trying to retrieve the first word, the second word, which is represented in proximity, is accidentally retrieved as well, and the two compete for being recalled. Errors in serial recall tasks are often confusions of neighboring items on a memory list (so-called transpositions), showing that retrieval competition plays a role in limiting our ability to recall lists in order, and probably also in other working memory tasks. A third form of interference is the distortion of representations by superposition: When multiple representations are added on top of each other, each of them is blurred by the presence of all the others. A fourth form of interference assumed by some authors is feature overwriting. The idea is that each word, digit, or other item in working memory is represented as a bundle of features, and when two items share some features, one of them steals the features from the other. The more items are held in working memory, and the more their features overlap, the more each of them will be degraded by the loss of some features.

None of these hypotheses can explain the experimental data entirely. The resource hypothesis, for example, was meant to explain the trade-off between maintenance and processing: The more information must be maintained in working memory, the slower and more error prone concurrent processes become, and with a higher demand on concurrent processing memory suffers. This trade-off has been investigated by tasks like the reading-span task described above. It has been found that the amount of trade-off depends on the similarity of the information to be remembered and the information to be processed. For example, remembering numbers while processing spatial information, or remembering spatial information while processing numbers, impair each other much less than when material of the same kind must be remembered and processed. Also, remembering words and processing digits, or remembering digits and processing words, is easier than remembering and processing materials of the same category. These findings are also difficult to explain for the decay hypothesis, because decay of memory representations should depend only on how long the processing task delays rehearsal or recall, not on the content of the processing task. A further problem for the decay hypothesis comes from experiments in which the recall of a list of letters was delayed, either by instructing participants to recall at a slower pace, or by instructing them to say an irrelevant word once or three times in between recall of each letter. Delaying recall had virtually no effect on recall accuracy. The interference theory seems to fare best with explaining why the similarity between memory contents and the contents of concurrent processing tasks affects how much they impair each other. More similar materials are more likely to be confused, leading to retrieval competition.

The capacity of working memory increases gradually over childhood and declines gradually in old age.

Measures of performance on tests of working memory increase continuously between early childhood and adolescence, while the structure of correlations between different tests remains largely constant. Starting with work in the Neo-Piagetian tradition, theorists have argued that the growth of working-memory capacity is a major driving force of cognitive development. This hypothesis has received substantial empirical support from studies showing that the capacity of working memory is a strong predictor of cognitive abilities in childhood. Particularly strong evidence for a role of working memory for development comes from a longitudinal study showing that working-memory capacity at one age predicts reasoning ability at a later age. Studies in the Neo-Piagetian tradition have added to this picture by analyzing the complexity of cognitive tasks in terms of the number of items or relations that have to be considered simultaneously for a solution. Across a broad range of tasks, children manage task versions of the same level of complexity at about the same age, consistent with the view that working memory capacity limits the complexity they can handle at a given age.

Working memory is among the cognitive functions most sensitive to decline in old age. Several explanations have been offered for this decline in psychology. One is the processing speed theory of cognitive aging by Tim Salthouse. Drawing on the finding of general slowing of cognitive processes as people grow older, Salthouse argues that slower processing leaves more time for working-memory contents to decay, thus reducing effective capacity. However, the decline of working-memory capacity cannot be entirely attributed to slowing because capacity declines more in old age than speed. Another proposal is the inhibition hypothesis advanced by Lynn Hasher and Rose Zacks. This theory assumes a general deficit in old age in the ability to inhibit irrelevant, or no-longer relevant, information. Therefore, working memory tends to be cluttered with irrelevant contents that reduce the effective capacity for relevant content. The assumption of an inhibition deficit in old age has received much empirical support but, so far, it is not clear whether the decline in inhibitory ability fully explains the decline of working-memory capacity. An explanation on the neural level of the decline of working memory and other cognitive functions in old age has been proposed by West. He argued that working memory depends to a large degree on the pre-frontal cortex, which deteriorates more than other brain regions as we grow old.

Torkel Klingberg was the first to investigate whether intensive training of working memory has beneficial effects on other cognitive functions. His pioneering study suggested that working memory can be improved by training in ADHD patients through computerized programs. This study has found that a period of working memory training increases a range of cognitive abilities and increases IQ test scores. Another study of the same group has shown that, after training, measured brain activity related to working memory increased in the prefrontal cortex, an area that many researchers have associated with working memory functions. It has been shown in one study that working memory training increases the density of prefrontal and parietal dopamine receptors (specifically, DRD1) in test persons. However, subsequent work with the same training program has failed to replicate the beneficial effects of training on cognitive performance. A meta-analytic summary of research with Klingberg's training program up to 2011 shows that this training has at best a negligible effect on tests of intelligence and of attention

In another influential study, training with a working memory task (the dual n-back task) has improved performance on a fluid intelligence test in healthy young adults. The improvement of fluid intelligence by training with the n-back task was replicated in 2010, but two studies published in 2012 failed to reproduce the effect. The combined evidence from about 30 experimental studies on the effectiveness of working-memory training has been evaluated by several meta-analyses. The authors of these meta-analyses disagree in their conclusions as to whether or not working-memory training improves intelligence. Yet, these meta-analyses agree in their estimate of the size of the effect of working-memory training: If there is such an effect, it is likely to be small.

The first insights into the neuronal and neurotransmitter basis of working memory came from animal research. The work of Jacobsen and Fulton in the 1930s first showed that lesions to the PFC impaired spatial working memory performance in monkeys. The later work of Joaquin Fuster recorded the electrical activity of neurons in the PFC of monkeys while they were doing a delayed matching task. In that task, the monkey sees how the experimenter places a bit of food under one of two identical-looking cups. A shutter is then lowered for a variable delay period, screening off the cups from the monkey's view. After the delay, the shutter opens and the monkey is allowed to retrieve the food from under the cups. Successful retrieval in the first attempt – something the animal can achieve after some training on the task – requires holding the location of the food in memory over the delay period. Fuster found neurons in the PFC that fired mostly during the delay period, suggesting that they were involved in representing the food location while it was invisible. Later research has shown similar delay-active neurons also in the posterior parietal cortex, the thalamus, the caudate, and the globus pallidus. The work of Goldman-Rakic and others showed that principal sulcal, dorsolateral PFC interconnects with all of these brain regions, and that neuronal microcircuits within PFC are able to maintain information in working memory through recurrent excitatory glutamate networks of pyramidal cells that continue to fire throughout the delay period. These circuits are tuned by lateral inhibition from GABAergic interneurons. The neuromodulatory arousal systems markedly alter PFC working memory function; for example, either too little or too much dopamine or norepinephrine impairs PFC network firing and working memory performance.

The research described above on persistent firing of certain neurons in the delay period of working memory tasks shows that the brain has a mechanism of keeping representations active without external input. Keeping representations active, however, is not enough if the task demands maintaining more than one chunk of information. In addition, the components and features of each chunk must be bound together to prevent them from being mixed up. For example, if a red triangle and a green square must be remembered at the same time, one must make sure that "red" is bound to "triangle" and "green" is bound to "square". One way of establishing such bindings is by having the neurons that represent features of the same chunk fire in synchrony, and those that represent features belonging to different chunks fire out of sync. In the example, neurons representing redness would fire in synchrony with neurons representing the triangular shape, but out of sync with those representing the square shape. So far, there is no direct evidence that working memory uses this binding mechanism, and other mechanisms have been proposed as well. It has been speculated that synchronous firing of neurons involved in working memory oscillate with frequencies in the theta band (4 to 8 Hz). Indeed, the power of theta frequency in the EEG increases with working memory load, and oscillations in the theta band measured over different parts of the skull become more coordinated when the person tries to remember the binding between two components of information.

Localization of brain functions in humans has become much easier with the advent of brain imaging methods (PET and fMRI). This research has confirmed that areas in the PFC are involved in working memory functions. During the 1990s much debate has centered on the different functions of the ventrolateral (i.e., lower areas) and the dorsolateral (higher) areas of the PFC. A human lesion study provides additional evidence for the role of the dorsolateral prefrontal cortex in working memory. One view was that the dorsolateral areas are responsible for spatial working memory and the ventrolateral areas for non-spatial working memory. Another view proposed a functional distinction, arguing that ventrolateral areas are mostly involved in pure maintenance of information, whereas dorsolateral areas are more involved in tasks requiring some processing of the memorized material. The debate is not entirely resolved but most of the evidence supports the functional distinction.

Brain imaging has also revealed that working memory functions are not limited to the PFC. A review of numerous studies shows areas of activation during working memory tasks scattered over a large part of the cortex. There is a tendency for spatial tasks to recruit more right-hemisphere areas, and for verbal and object working memory to recruit more left-hemisphere areas. The activation during verbal working memory tasks can be broken down into one component reflecting maintenance, in the left posterior parietal cortex, and a component reflecting subvocal rehearsal, in the left frontal cortex (Broca's area, known to be involved in speech production).

There is an emerging consensus that most working memory tasks recruit a network of PFC and parietal areas. A study has shown that during a working memory task the connectivity between these areas increases. Another study has demonstrated that these areas are necessary for working memory, and not simply activated accidentally during working memory tasks, by temporarily blocking them through transcranial magnetic stimulation (TMS), thereby producing an impairment in task performance.

A current debate concerns the function of these brain areas. The PFC has been found to be active in a variety of tasks that require executive functions. This has led some researchers to argue that the role of PFC in working memory is in controlling attention, selecting strategies, and manipulating information in working memory, but not in maintenance of information. The maintenance function is attributed to more posterior areas of the brain, including the parietal cortex. Other authors interpret the activity in parietal cortex as reflecting executive functions, because the same area is also activated in other tasks requiring attention but not memory.

A 2003 meta-analysis of 60 neuroimaging studies found left frontal cortex was involved in low-task demand verbal working memory and right frontal cortex for spatial working memory. Brodmann's areas (BAs) 6, 8, and 9, in the superior frontal cortex was involved when working memory must be continuously updated and when memory for temporal order had to be maintained. Right Brodmann 10 and 47 in the ventral frontal cortex were involved more frequently with demand for manipulation such as dual-task requirements or mental operations, and Brodmann 7 in the posterior parietal cortex was also involved in all types of executive function.

Working memory has been suggested to involve two processes with different neuroanatomical locations in the frontal and parietal lobes. First, a selection operation that retrieves the most relevant item, and second an updating operation that changes the focus of attention made upon it. Updating the attentional focus has been found to involve the transient activation in the caudal superior frontal sulcus and posterior parietal cortex, while increasing demands on selection selectively changes activation in the rostral superior frontal sulcus and posterior cingulate/precuneus.

Articulating the differential function of brain regions involved in working memory is dependent on tasks able to distinguish these functions. Most brain imaging studies of working memory have used recognition tasks such as delayed recognition of one or several stimuli, or the n-back task, in which each new stimulus in a long series must be compared to the one presented n steps back in the series. The advantage of recognition tasks is that they require minimal movement (just pressing one of two keys), making fixation of the head in the scanner easier. Experimental research and research on individual differences in working memory, however, has used largely recall tasks (e.g., the reading span task, see below). It is not clear to what degree recognition and recall tasks reflect the same processes and the same capacity limitations.

Brain imaging studies have been conducted with the reading span task or related tasks. Increased activation during these tasks was found in the PFC and, in several studies, also in the anterior cingulate cortex (ACC). People performing better on the task showed larger increase of activation in these areas, and their activation was correlated more over time, suggesting that their neural activity in these two areas was better coordinated, possibly due to stronger connectivity.

One approach to model the neurophysiology and the functioning of working memory is the prefrontal cortex basal ganglia working memory (PBWM).

Working memory is impaired by acute and chronic psychological stress. This phenomenon was first discovered in animal studies by Arnsten and colleagues, who have shown that stress-induced catecholamine release in PFC rapidly decreases PFC neuronal firing and impairs working memory performance through feedforward, intracellular signaling pathways. Exposure to chronic stress leads to more profound working memory deficits and additional architectural changes in PFC, including dendritic atrophy and spine loss, which can be prevented by inhibition of protein kinase C signaling. fMRI research has extended this research to humans, and confirms that reduced working memory caused by acute stress links to reduced activation of the PFC, and stress increased levels of catecholamines. Imaging studies of medical students undergoing stressful exams have also shown weakened PFC functional connectivity, consistent with the animal studies. The marked effects of stress on PFC structure and function may help to explain how stress can cause or exacerbate mental illness.
The more stress in one's life, the lower the efficiency of working memory in performing simple cognitive tasks. Students who performed exercises that reduced the intrusion of negative thoughts showed an increase in their working memory capacity. Mood states (positive or negative) can have an influence on the neurotransmitter dopamine, which in turn can affect problem solving.

Alcohol abuse can result in brain damage which impairs working memory. Alcohol has an effect on the blood-oxygen-level-dependent (BOLD) response. The BOLD response correlates increased blood oxygenation with brain activity, which makes this response a useful tool for measuring neuronal activity. The BOLD response affects regions of the brain such as the basal ganglia and thalamus when performing a working memory task. Adolescents who start drinking at a young age show a decreased BOLD response in these brain regions. Alcohol dependent young women in particular exhibit less of a BOLD response in parietal and frontal cortices when performing a spatial working memory task. Binge drinking, specifically, can also affect one's performance on working memory tasks, particularly visual working memory. Additionally, there seems to be a gender difference in regards to how alcohol affects working memory. While women perform better on verbal working memory tasks after consuming alcohol compared to men, they appear to perform worse on spatial working memory tasks as indicated by less brain activity. Finally, age seems to be an additional factor. Older adults are more susceptible than others to the effects of alcohol on working memory.

Individual differences in working-memory capacity are to some extent heritable; that is, about half of the variation between individuals is related to differences in their genes. The genetic component of variability of working-memory capacity is largely shared with that of fluid intelligence.

Little is known about which genes are related to the functioning of working memory. Within the theoretical framework of the multi-component model, one candidate gene has been proposed, namely ROBO1 for the hypothetical phonological loop component of working memory.

Working memory capacity is correlated with learning outcomes in literacy and numeracy. Initial evidence for this relation comes from the correlation between working-memory capacity and reading comprehension, as first observed by Daneman and Carpenter (1980) and confirmed in a later meta-analytic review of several studies. Subsequent work found that working memory performance in primary school children accurately predicted performance in mathematical problem solving. One longitudinal study showed that a child's working memory at 5 years old is a better predictor of academic success than IQ.

In a large-scale screening study, one in ten children in mainstream classrooms were identified with working memory deficits. The majority of them performed very poorly in academic achievements, independent of their IQ. Similarly, working memory deficits have been identified in national curriculum low-achievers as young as seven years of age. Without appropriate intervention, these children lag behind their peers. A recent study of 37 school-age children with significant learning disabilities has shown that working memory capacity at baseline measurement, but not IQ, predicts learning outcomes two years later. This suggests that working memory impairments are associated with low learning outcomes and constitute a high risk factor for educational underachievement for children. In children with learning disabilities such as dyslexia, ADHD, and developmental coordination disorder, a similar pattern is evident.

There is some evidence that optimal working memory performance links to the neural ability to focus attention on task-relevant information and to ignore distractions, and that practice-related improvement in working memory is due to increasing these abilities. One line of research suggests a link between the working memory capacities of a person and their ability to control the orientation of attention to stimuli in the environment. Such control enables people to attend to information important for their current goals, and to ignore goal-irrelevant stimuli that tend to capture their attention due to their sensory saliency (such as an ambulance siren). The direction of attention according to one's goals is assumed to rely on "top-down" signals from the pre-frontal cortex (PFC) that biases processing in posterior cortical areas. Capture of attention by salient stimuli is assumed to be driven by "bottom-up" signals from subcortical structures and the primary sensory cortices. The ability to override "bottom-up" capture of attention differs between individuals, and this difference has been found to correlate with their performance in a working-memory test for visual information. Another study, however, found no correlation between the ability to override attentional capture and measures of more general working-memory capacity.

An impairment of working memory functioning is normally seen in several neural disorders:

ADHD: Several authors have proposed that symptoms of ADHD arise from a primary deficit in a specific executive function (EF) domain such as working memory, response inhibition or a more general weakness in executive control. A meta-analytical review cites several studies that found significant lower group results for ADHD in spatial and verbal working memory tasks, and in several other EF tasks. However, the authors concluded that EF weaknesses neither are necessary nor sufficient to cause all cases of ADHD.

Several neurotransmitters, such as dopamine and glutamate may be both involved in ADHD and working memory. Both are associated with the frontal brain, self-direction and self-regulation, but cause–effect have not been confirmed, so it is unclear whether working memory dysfunction leads to ADHD, or ADHD distractibility leads to poor functionality of working memory, or if there is some other connection.

Parkinson's disease: Patients with Parkinson's show signs of a reduced verbal function of working memory. They wanted to find if the reduction is due to a lack of ability to focus on relevant tasks, or a low amount of memory capacity. Twenty-one patients with Parkinson's were tested in comparison to the control group of 28 participants of the same age.The researchers found that both hypotheses were the reason working memory function is reduced which did not fully agree with their hypothesis that it is either one or the other.

Alzheimer's disease: As Alzheimer's disease becomes more serious, less working memory functions. There is one study that focuses on the neural connections and fluidity of working memory in mice brains. Half of the mice were given an injection that is similar to Alzheimer's effects, and the other half were not. Then they were expected to go through a maze that is a task to test working memory. The study help answer questions about how Alzheimer's can deteriorate the working memory and ultimately obliterate memory functions.

Huntington's disease: A group of researchers hosted a study that researched the function and connectivity of working memory over a 30-month longitudinal experiment. It found that there were certain places in the brain where most connectivity was decreased in pre-Huntington diseased patients, in comparison to the control group that remained consistently functional.

Memory is divided into voluntary (conscious) and involuntary (unconscious) processes that function independently of each other. Theories and research on memory dates back to Hermann Ebbinghaus, who began studying nonsense syllables. Ebbinghaus classified three distinct classes of memory: sensory, short term, and long-term memory. Sensory memory is made up of a brief storage of information within a specific medium (the line you see after waving a sparkler in your field of vision is created by sensory memory). Short term memory is made up of the information currently in use to complete the task at hand. Long term memory is composed of the systems used to store memory over long periods. It enables one to remember what happened two days ago at noon, or who called last night.

Flashback (psychology)

A flashback, or involuntary recurrent memory, is a psychological phenomenon in which an individual has a sudden, usually powerful, re-experiencing of a past experience or elements of a past experience. These experiences can be happy, sad, exciting, or any other emotion one can consider. The term is used particularly when the memory is recalled involuntarily, and/or when it is so intense that the person "relives" the experience, unable to fully recognize it as memory and not something that is happening in "real time".

Flashbacks are the "personal experiences that pop into your awareness, without any conscious, premeditated attempt to search and retrieve this memory". These experiences occasionally have little to no relation to the situation at hand. Flashbacks to those suffering posttraumatic stress disorder can seriously disrupt everyday life.

Miller (1962–1974) declared that studying such fragile things as involuntary memories should not be done. This appears to have been followed since very little research has been done on flashbacks in the cognitive psychology discipline. Flashbacks have been studied within a clinical discipline however, and they have been identified as symptoms for many disorders, including post traumatic stress disorder.

Due to the elusive nature of involuntary recurrent memories, very little is known about the subjective experience of flashbacks. However, theorists agree that this phenomenon is in part due to the manner in which memories of specific events are initially encoded (or entered) into memory, the way in which the memory is organized, and also the way in which the individual later recalls the event. Overall, theories that attempt to explain the flashback phenomenon can be categorized into one of two viewpoints. The special mechanism view is clinically oriented in that it holds that involuntary memories are due to traumatic events, and the memories for these events can be attributed to a special memory mechanism. On the other hand, the basic mechanism view is more experimentally oriented in that it is based on memory research. This view holds that traumatic memories are bound by the same parameters as all other every-day memories. Both viewpoints agree that involuntary recurrent memories result from rare events that would not normally occur.

These rare events elicit strong emotional reactions from the individual since it violates normal expectations. According to the special mechanisms view, the event would lead to fragmented voluntary encoding into memory (meaning that only certain isolated parts of the event would be encoded), thus making the conscious subsequent retrieval of the memory much more difficult. On the other hand, involuntary recurrent memories are likely to become more available, and these are more likely to be triggered by external cues. In contrast to this, the basic mechanism view holds that the traumatic event would lead to enhanced and cohesive encoding of the event in memory, and this would make both voluntary and involuntary memories more available for subsequent recall.

What is currently an issue of controversy is the nature of the defining criteria that makes up an involuntary memory. Up until recently, researchers believed that involuntary memories were a result of traumatic incidents that the individual experienced at a specific time and place, but the temporal and spatial features of the event are lost during an involuntary recollection episode. In other words, people who suffer from flashbacks lose all sense of time and place, and they feel as if they are re-experiencing the event instead of just recalling a memory. This is consistent with the special mechanism viewpoint in that the involuntary (unintended) memory is based on a different memory mechanism than its voluntary (intended) counterpart. Furthermore, the initial emotions experienced at the time of encoding are also re-experienced during a flashback episode, and this can be especially distressing when the memory is of a traumatic event. It has also been demonstrated that the nature of the flashbacks experienced by an individual are static in that they retain an identical form upon each intrusion. This occurs even when the individual has learned new information that directly contradicts the information retained in the intrusive memory.

Upon further investigation, it was found that involuntary memories are usually derived from either stimuli (i.e. anything that causes a change in behaviour) that indicated the onset of a traumatic event, or from stimuli that hold intense emotional significance to the individual simply because these stimuli were closely associated with the trauma in terms of timing. These stimuli then become warning signals that if encountered again, serve to trigger a flashback. This has been termed the warning signal hypothesis. For example, a man experiences a flashback upon seeing sun spots on his lawn. This happens because he associates the sun spots with the headlights of the vehicle that he collided with, causing a horrific car accident. According to Ehlers and Clark, traumatic memories are more apt to induce flashbacks simply because of faulty encoding in that the individual fails to take contextual information into account, as well as time and place information that would usually be associated with every-day memories. These individuals become more sensitized to stimuli that they associate with the traumatic event which then serve as triggers for a flashback (even though the context surrounding the stimulus may be unrelated; such as sun spots being unrelated to headlights). These triggers may have elicited an adaptive response during the time of the traumatic experience, but they soon become maladaptive if the person continues to respond in the same way in situations in which no danger may be present.

The special mechanism viewpoint would add to this further by suggesting that these triggers activate the fragmented memory of the trauma, but protective cognitive mechanisms function to inhibit the recall of the original memory of the traumatic event. Dual representation theory enhances this idea by suggesting two separate mechanisms that account for voluntary and involuntary memories; the first of which is called the verbally accessible memory system and the latter is referred to the situationally accessible memory system.

In contrast to this, theories belonging to the basic mechanism viewpoint hold that there are no separate mechanisms that account for voluntary and involuntary memories. The recall of memories for stressful events do not differ under involuntary and voluntary recall. Instead, it is the retrieval mechanism that is different for each type of recall. In involuntary recall, the external trigger creates an uncontrolled spreading of activation in memory, whereas in voluntary recall, this activation is strictly controlled and is goal-oriented.

Several brain regions have been implicated in the neurological basis of flashbacks. The medial temporal lobes, the precuneus, the posterior cingulate gyrus and the prefrontal cortex are the most typically referenced with regards to involuntary memories.

The medial temporal lobes are commonly associated with memory. More specifically, the lobes have been linked to episodic/declarative memory and thus damage to these areas of the brain result in disruptions to declarative memory system. The hippocampus, located within the medial temporal regions, has also been highly related to memory processes. There are numerous functions in the hippocampus; these functions also include aspects of memory consolidation. Brain imaging studies have shown flashbacks activate areas associated with memory retrieval. The precuneus, located in the superior parietal lobe and the posterior cingulate gyrus have also been implicated in memory retrieval. In addition, studies have shown activity in areas of the prefrontal cortex to be involved in memory retrieval.

Thus, the medial temporal lobe, precuneus, superior parietal lobe and posterior cingulate gyrus have all been implicated in flashbacks in accordance to their roles on memory retrieval.

Memory has typically been divided into sensory, short term, and long term processes. According to Rasmuseen & Berntsen, 2009, "long-term memory processes may form the core of spontaneous thought". Thus the memory process most related to flashbacks is long term memory. As well, studies by Rasmuseen & Berntsen, 2009, have shown that long term memory is also susceptible to extraneous factors such as recency effect, arousal and rehearsal as it pertains to accessibility. Compared to voluntary memories, involuntary memories show shorter retrieval times and little cognitive effort. Finally, involuntary memories arise due to automatic processing, which does not rely on higher-order cognitive monitoring, or executive control processing. Voluntary memory is normally associated with contextual information, which is what allows for correspondence between time and place, this is not true of flashbacks. According to Brewin, Lanius et, al, 2009, flashbacks, are disconnected from contextual information, and as a result are disconnected from time and place.

To date, the specific causes of flashbacks have not yet been confirmed. Several studies have proposed various potential factors. Gunasekaran et al., 2009, indicate there may be a link between food deprivation and stress on the occurrence of flashbacks. Psychiatrists suggest temporal lobe seizures may also have some relation.

On the reverse side, several ideas have been discounted in terms of their causing flashbacks. Tym et al., 2009, suggest this list includes medication or other substances, Charles Bonnet syndrome, delayed palinopsia, hallucinations, dissociative phenomena, and depersonalization syndrome.

A study of the persistence of traumatic memories in World War II prisoners of war investigates through the administration of surveys the extent and severity of flashbacks that occur in prisoners of war. This study concluded that the persistence of severely traumatic autobiographical memories can last upwards of 65 years. Until recently, the study of flashbacks has been limited to participants who already experience flashbacks, such as those suffering from posttraumatic stress disorder, restricting researchers to observational/exploratory rather than experimental studies.

Neuroimaging techniques have been applied to the investigation of flashbacks. Using these techniques, researchers attempt to discover the structural and functional differences in the anatomy of the brain in individuals who suffer from flashbacks compared to those who do not. Neuroimaging involves a cluster of techniques, including computerized tomography, positron emission tomography, magnetic resonance imaging (including functional), as well as magnetoencephalography. Neuroimaging studies investigating flashbacks are based on current psychological theories that are used as the foundation for the research, and one such theory that is consistently investigated is the difference between explicit and implicit memory. This distinction dictates the manner in which memories are later recalled, namely either consciously (voluntarily) or unconsciously (involuntarily).

These methods have largely relied on subtractive reasoning in which the participant voluntarily recalls a memory and then the memory is again recalled, but this time through involuntary means. Involuntary memories (or flashbacks) are elicited in the participant by reading an emotionally charged script to them that is designed to trigger a flashback in individuals who suffer from post-traumatic stress disorder. The investigators record the regions of the brain that are active during each of these conditions, and then subtract the activity. Whatever is left is assumed to underpin the neurological differences between the conditions.

Imaging studies looking at patients with post-traumatic stress disorder as they undergo flashback experiences have identified elevated activation in regions of the dorsal stream including the mid-occipital lobe, primary motor cortex and supplementary motor area. The dorsal stream is involved in sensory processing and therefore these activations might underlie the vivid visual experiences associated with flashbacks. The study also found reduced activation in regions such as the inferior temporal cortex and parahippocampus which are involved in processing allocentric relations. These deactivations might contribute to feelings of dissociation from reality during flashback experiences.

Flashbacks are often associated with mental illness as they are a symptom and a feature in diagnostic criteria for posttraumatic stress disorder (PTSD), acute stress disorder, and obsessive-compulsive disorder (OCD). Flashbacks have also been observed in people suffering from manic depression, depression, homesickness, near-death experiences, epileptic seizures, and drug abuse. Some researchers have suggested that the use of some drugs can cause a person to experience flashbacks; users of lysergic acid diethylamide sometimes report "acid flashbacks". While other studies show that the use of drugs, specifically cannabis, can help reduce the occurrence of flashbacks in people with PTSD.

The psychological phenomenon has frequently been portrayed in film and television. Some of the most accurate media portrayals of flashbacks have been those related to wartime, and the association of flashbacks to post-traumatic stress disorder caused by the traumas and stresses of war. One of the earliest screen portrayals of this is in the 1945 film "Mildred Pierce".

Flashback (psychology)

A flashback, or involuntary recurrent memory, is a psychological phenomenon in which an individual has a sudden, usually powerful, re-experiencing of a past experience or elements of a past experience. These experiences can be happy, sad, exciting, or any other emotion one can consider. The term is used particularly when the memory is recalled involuntarily, and/or when it is so intense that the person "relives" the experience, unable to fully recognize it as memory and not something that is happening in "real time".

Flashbacks are the "personal experiences that pop into your awareness, without any conscious, premeditated attempt to search and retrieve this memory". These experiences occasionally have little to no relation to the situation at hand. Flashbacks to those suffering posttraumatic stress disorder can seriously disrupt everyday life.

Memory is divided into voluntary (conscious) and involuntary (unconscious) processes that function independently of each other. Theories and research on memory dates back to Hermann Ebbinghaus, who began studying nonsense syllables. Ebbinghaus classified three distinct classes of memory: sensory, short term, and long-term memory. Sensory memory is made up of a brief storage of information within a specific medium (the line you see after waving a sparkler in your field of vision is created by sensory memory). Short term memory is made up of the information currently in use to complete the task at hand. Long term memory is composed of the systems used to store memory over long periods. It enables one to remember what happened two days ago at noon, or who called last night.

Miller (1962–1974) declared that studying such fragile things as involuntary memories should not be done. This appears to have been followed since very little research has been done on flashbacks in the cognitive psychology discipline. Flashbacks have been studied within a clinical discipline however, and they have been identified as symptoms for many disorders, including post traumatic stress disorder.

These rare events elicit strong emotional reactions from the individual since it violates normal expectations. According to the special mechanisms view, the event would lead to fragmented voluntary encoding into memory (meaning that only certain isolated parts of the event would be encoded), thus making the conscious subsequent retrieval of the memory much more difficult. On the other hand, involuntary recurrent memories are likely to become more available, and these are more likely to be triggered by external cues. In contrast to this, the basic mechanism view holds that the traumatic event would lead to enhanced and cohesive encoding of the event in memory, and this would make both voluntary and involuntary memories more available for subsequent recall.

The special mechanism viewpoint would add to this further by suggesting that these triggers activate the fragmented memory of the trauma, but protective cognitive mechanisms function to inhibit the recall of the original memory of the traumatic event. Dual representation theory enhances this idea by suggesting two separate mechanisms that account for voluntary and involuntary memories; the first of which is called the verbally accessible memory system and the latter is referred to the situationally accessible memory system.

In contrast to this, theories belonging to the basic mechanism viewpoint hold that there are no separate mechanisms that account for voluntary and involuntary memories. The recall of memories for stressful events do not differ under involuntary and voluntary recall. Instead, it is the retrieval mechanism that is different for each type of recall. In involuntary recall, the external trigger creates an uncontrolled spreading of activation in memory, whereas in voluntary recall, this activation is strictly controlled and is goal-oriented.

Several brain regions have been implicated in the neurological basis of flashbacks. The medial temporal lobes, the precuneus, the posterior cingulate gyrus and the prefrontal cortex are the most typically referenced with regards to involuntary memories.

The medial temporal lobes are commonly associated with memory. More specifically, the lobes have been linked to episodic/declarative memory and thus damage to these areas of the brain result in disruptions to declarative memory system. The hippocampus, located within the medial temporal regions, has also been highly related to memory processes. There are numerous functions in the hippocampus; these functions also include aspects of memory consolidation. Brain imaging studies have shown flashbacks activate areas associated with memory retrieval. The precuneus, located in the superior parietal lobe and the posterior cingulate gyrus have also been implicated in memory retrieval. In addition, studies have shown activity in areas of the prefrontal cortex to be involved in memory retrieval.

Thus, the medial temporal lobe, precuneus, superior parietal lobe and posterior cingulate gyrus have all been implicated in flashbacks in accordance to their roles on memory retrieval.

To date, the specific causes of flashbacks have not yet been confirmed. Several studies have proposed various potential factors. Gunasekaran et al., 2009, indicate there may be a link between food deprivation and stress on the occurrence of flashbacks. Psychiatrists suggest temporal lobe seizures may also have some relation.

On the reverse side, several ideas have been discounted in terms of their causing flashbacks. Tym et al., 2009, suggest this list includes medication or other substances, Charles Bonnet syndrome, delayed palinopsia, hallucinations, dissociative phenomena, and depersonalization syndrome.

A study of the persistence of traumatic memories in World War II prisoners of war investigates through the administration of surveys the extent and severity of flashbacks that occur in prisoners of war. This study concluded that the persistence of severely traumatic autobiographical memories can last upwards of 65 years. Until recently, the study of flashbacks has been limited to participants who already experience flashbacks, such as those suffering from posttraumatic stress disorder, restricting researchers to observational/exploratory rather than experimental studies.

Neuroimaging techniques have been applied to the investigation of flashbacks. Using these techniques, researchers attempt to discover the structural and functional differences in the anatomy of the brain in individuals who suffer from flashbacks compared to those who do not. Neuroimaging involves a cluster of techniques, including computerized tomography, positron emission tomography, magnetic resonance imaging (including functional), as well as magnetoencephalography. Neuroimaging studies investigating flashbacks are based on current psychological theories that are used as the foundation for the research, and one such theory that is consistently investigated is the difference between explicit and implicit memory. This distinction dictates the manner in which memories are later recalled, namely either consciously (voluntarily) or unconsciously (involuntarily).

These methods have largely relied on subtractive reasoning in which the participant voluntarily recalls a memory and then the memory is again recalled, but this time through involuntary means. Involuntary memories (or flashbacks) are elicited in the participant by reading an emotionally charged script to them that is designed to trigger a flashback in individuals who suffer from post-traumatic stress disorder. The investigators record the regions of the brain that are active during each of these conditions, and then subtract the activity. Whatever is left is assumed to underpin the neurological differences between the conditions.

Imaging studies looking at patients with post-traumatic stress disorder as they undergo flashback experiences have identified elevated activation in regions of the dorsal stream including the mid-occipital lobe, primary motor cortex and supplementary motor area. The dorsal stream is involved in sensory processing and therefore these activations might underlie the vivid visual experiences associated with flashbacks. The study also found reduced activation in regions such as the inferior temporal cortex and parahippocampus which are involved in processing allocentric relations. These deactivations might contribute to feelings of dissociation from reality during flashback experiences.

Flashbacks are often associated with mental illness as they are a symptom and a feature in diagnostic criteria for posttraumatic stress disorder (PTSD), acute stress disorder, and obsessive-compulsive disorder (OCD). Flashbacks have also been observed in people suffering from manic depression, depression, homesickness, near-death experiences, epileptic seizures, and drug abuse. Some researchers have suggested that the use of some drugs can cause a person to experience flashbacks; users of lysergic acid diethylamide sometimes report "acid flashbacks". While other studies show that the use of drugs, specifically cannabis, can help reduce the occurrence of flashbacks in people with PTSD.

The psychological phenomenon has frequently been portrayed in film and television. Some of the most accurate media portrayals of flashbacks have been those related to wartime, and the association of flashbacks to post-traumatic stress disorder caused by the traumas and stresses of war. One of the earliest screen portrayals of this is in the 1945 film "Mildred Pierce".