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Episodic memory refers to the memory of events, times, places, associated emotions, and other conception-based knowledge in relation to an experience. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory. The counterpart to declarative, or explicit memory, is procedural memory, or implicit memory.
The cognitive neuroscience of episodic memory
The formation of new episodic memories requires the medial temporal lobe, a structure that includes the hippocampus. Without the medial temporal lobe, one is able to form new procedural memories (such as playing the piano) but cannot remember the events during which they happened. See the hippocampus and memory.
The prefrontal cortex (and in particular the left hemisphere) is also involved in the formation of new episodic memories (also known as episodic encoding). Patients with damage to the prefrontal cortex can learn new information, but tend to do so in a disordered fashion. For example, they might show normal recognition of an object they had seen in the past, but fail to recollect when or where it had been viewed (Janowsky et al., 1989). Some researchers believe that the prefrontal cortex helps organize information for more efficient storage, drawing upon its role in executive function. Others believe that the prefrontal cortex underlies semantic strategies which enhance encoding, such as thinking about the meaning the study material or rehearsing it in working memory (Gabrieli et al., 1998).
The hippocampus's role in memory storage
Researchers do not agree about how long episodic memories are stored in the hippocampus. Some researchers believe that episodic memories always rely on the hippocampus. Others believe the hippocampus only stores episodic memories for a short time, after which the memories are consolidated to the neocortex. The latter view is strengthened by recent evidence that neurogenesis in the adult hippocampus may ease the removal of old memories and increase the efficiency of forming new memories (Deisseroth et al 2004).
The relationship of episodic memory to semantic memory
Episodic memory is thought of as being a "one-shot" learning mechanism. You only need one exposure to an episode to remember it. Semantic memory, on the other hand, can take into consideration multiple exposures to each referent - the semantic representation is updated on each exposure.
Episodic memory can be thought of as a "map" that ties together items in semantic memory. For example, semantic memory will tell you what a "dog" looks and sounds like. All episodic memories concerning your dog will reference this single semantic representation of "dog" and, likewise, all new experiences with your dog will modify your single semantic representation of your dog.
Some researchers believe that episodic memories are converted from episodic into semantic memories over time. In this process, most of the episodic information about a particular event is generalized and the context of the specific events is lost. One modification of this view is that episodic memories which are recalled often are remembered as a kind of monologue. If you tell and re-tell a story repeatedly, you may feel that you no longer remember the event, but that what you're recalling is a kind of pre-written story.
Others believe that you always remember episodic memories as episodic memories. Of course, episodic memories do inform semantic knowledge and episodic memories are reliant upon semantic knowledge. The point is that some people do not believe that all episodic memories will inevitably distill away into semantic memory.
Gender differences in episodic memory performance
According to Brain activation during episodic memory retrieval: sex differences, women tend to outperform men on episodic memory tasks.
Age differences in episodic memory performance
Activation of specific brain areas (mostly the hippocampus) seems to be different between young and older people upon episodic memory retrieval, as shown by Maguire and Frith 2003. Older people tend to activate both left and right hippocampus, while young people activate only the left one. For more information, see aging and memory.
Emotion and episodic memory
The relationship between emotion and memory is complex, but generally, emotion tends to increase the likelihood that an event will be remembered later and that it will be remembered vividly. Flashbulb memory is one example of this.
Pharmacological enhancement of episodic memory
In healthy adults, longterm visual episodic memory can be enhanced specifically through administration of the Acetylcholine esterase inhibitor Donepezil, whereas verbal episodic memory can be improved in persons with the val/val genotype of the val158met polymorphism through administration of the CNS penetrant specific catecholamine-O-methyltransferase inhibitor Tolcapone. Furthermore, episodic memory is enhanced through AZD3480 a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept. Currently, there are several other products developed by several companies - including new catecholamine-O-methyltransferase inhibitors with fewer side effects - that aim for improving episodic memory.
Episodic memory damage
- One study suggests that there may be selective damage to the limbic-prefrontal episodic memory system in some people with autism (Ben Shalom, 1993). Another study points to evidence of autistic deficits in the episodic or self-conscious memory of personally experienced events (Joseph et al., 2003).
- The label "Amnesia" is most often given to patients with deficits in episodic memory.
- Alzheimer's Disease tends to damage the hippocampus before other brain areas. This means that AD patients are often classed as amnesiacs.
- A rare type of shell-fish poisoning called Amnesic shellfish poisoning or "ASP" quite effectively and irreversibly damages the hippocampus, rendering one amnesiac.
- Korsakoff's syndrome is brought on by many years' worth of excessive drinking. The syndrome is not the result of the alcohol, per se; rather, it is caused by the malnutrition that occurs when someone gets a large amount of his calories from alcohol.
Episodic memory in animals
In 1997, there was little evidence for episodic memory outside of humans. This is probably due to the difficulty in testing for it in animals. To meet the criteria of episodic memory, as espoused by Tulving (1983), evidence of conscious recollection must be provided. But demonstrating episodic memory in the absence of language, and therefore in non-human animals, is impossible because there are no agreed non-linguistic behavioural indicators of conscious experience (Griffiths et al., 1999).
Clayton & Dickinson (1998) were the first to provide evidence that animals may possess episodic memory. They demonstrated that Florida scrub-jays (Aphelocoma coerulescens) remember where they cached different food types and discriminately recovered them, depending on the perishability of the item and the amount of time that elapsed since caching. Thus, scrub-jays appear to remember the ‘what-where-and-when’ of specific caching events in the past. Clayton & Dickinson (1998) argued that such performance met the behavioural criteria for episodic memory. However, because the study did not address the phenomenological aspects of episodic memory, the authors referred to this ability as “episodic-like” memory.
According to a study done by the University of Edinburgh in 2006 hummingbirds are the first animal to demonstrate two aspects of episodic memory - the ability to recall where certain flowers were located and how recently they were visited. Scientists tracked how often hummingbirds visited eight artificial flowers filled with a sucrose solution in the birds' feeding grounds. They refilled half the flowers at 10 minute intervals and the other half 20 minutes after they had been emptied. The birds' return to the flowers matched the refill schedules: flowers refilled at 10-minute intervals were visited sooner. "To our knowledge, this is the first demonstration that animals in the wild can remember both the locations of food sources and when they visited them," said Susan Healy, of the University of Edinburgh.
Other studies have now started to demonstrate this episodic-like memory in other animal species, which have brains more similar to humans. For example, Kart-Teke and colleagues have demonstrated rats' preference for items it sees which is dependent on what it saw, where it saw it and when it saw it (Kart-Teke et al, 2006). In addition, studies by Eacott and colleagues (Eacott et al, 2005) have shown rats can recall (remember without any cueing influences) what they saw and where depending on which past situation they are being asked to remember.
Nonetheless, some scholars remain cautious about comparisons to human episodic memory (Suddendorf & Busby, 2003). Purported episodic-like memory often seems fixed to a particular domain or could be explained in terms of procedural or semantic memory. The problem may be better tractable by studying episodic memory's adaptive counterpart: the capacity to flexibly imagine future events. Suddendorf (2006) argues that the emergence of the human capacity to travel mentally to past and future events may have been a prime mover in hominin evolution.
A recent experiment addressed one of Suddendorf and Busby (2003)'s specific criticisms (the Bischof-Köhler hypothesis, which states that nonhuman animals can only take actions based on immediate needs, as opposed to future needs). Correia and colleagues demonstrated that Western scrub-jays can selectively cache different types of foods depending on which type of food they will desire at a future time, offering strong evidence against the Bischof-Köhler hypothesis by demonstrating that scrub-jays can flexibly adjust their behavior based on past experience of desiring a particular food.
Autobiographical vs episodic memory
Autobiographical memories are memories of events that happened in the past. It is currently unkown if autobiographical memories are the same as episodic memories or if the autobiographical memories become converted to semantic memories with time.
Neural network models
Episodic memories are stored in autoassociative neural networks. An early model for episodic memory is the Hopfield network developed by John Hopfield in 1982. More refined models have later been developed.
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