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Episodic memory is the memory of autobiographical events (times, places, associated emotions, and other contextual knowledge) that can be explicitly stated. 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.[1]

Contents

Cognitive neuroscience

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 of the study material or rehearsing it in working memory (Gabrieli et al., 1998).

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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).

Relationship 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.

Age differences

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.

Relationship to emotion

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

In healthy adults, longterm visual episodic memory can be enhanced specifically[2] 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.[3] Furthermore, episodic memory is enhanced through AZD3480 a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept.[4] 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. A recent placebo controlled study found that DHEA, which is a functional cortisol antagonist, improves episodic memory in healthy young men (Alhaj et al. 2006)[5].

Damage

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 Western Scrub-jays (Aphelocoma californica) 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 demonstrated 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[7]). 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,[8] 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 memory

An autobiographical memory is a personal representation of general or specific events and personal facts. Autobiographical memory also refers to memory of a person’s history. An individual does not remember exactly everything that has happened in one’s past. Memory is constructive, where previous experience affects how we remember events and what we end up recalling from memory. Autobiographical memory is constructive and reconstructed as an evolving process of past history. A person’s autobiographical memory is fairly reliable; although, the reliability of autobiographical memories is questionable because of memory distortions.

Autobiographical memories can differ for special periods of life. People recall few personal events from the first years of their lives. The loss of these first events is called childhood or infantile amnesia. People tend to recall many personal events from adolescence and early adulthood. This effect is called the reminiscence bump. Finally, people recall many personal events from the last few years. This is called the recency effect. For adolescents and young adults the reminiscence bump and the recency effect coincide.

It is known that autobiographical memories initially are stored as episodic memories, but it is currently unknown if autobiographical memories are the same as episodic memories or if the autobiographical memories become converted to semantic memories with time.

Types

  • Specific Events
  • When you first stepped foot in the ocean.
  • General Events
  • What it feels like stepping into the ocean in general. This is a memory of what a personal event is generally like. It might be based on the memories of having stepped in the ocean, many times during the years.
  • Personal Facts
  • Flash Bulb Memories
  • Flash bulb memories are critical Autobiographical Memories about a major event. Some flash bulb memories are shared within a social group:
"The assassination of John Kennedy?"
"The assassination of Martin Luther King, Jr.?"
"The Challenger explosion?"
"The verdict in the O.J. Simpson trial?"
"When you learned that Princess Diana had died?"
"When you heard about 9/11?"

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.

References

  • Deisseroth K, Singla S, Toda H, Monje M, Palmer TD, Malenka RC (2004). "Excitation-neurogenesis coupling in adult neural stem/progenitor cells". Neuron 42 (4): 535–52. doi:10.1016/S0896-6273(04)00266-1. PMID 15157417. 
  • Tulving, E. (1972). Episodic and semantic memory. In E. Tulving & W. Donaldson (Eds.), Organization of memory, (pp. 381–403). New York: Academic Press.
  • Tulving, E. (1983). Elements of Episodic Memory. Oxford: Clarendon Press.
  • Griffiths D, Dickinson A, Clayton N (1999). Episodic memory: what can animals remember about their past?. 3. pp. 74–80. PMID 10234230. 
  • Clayton NS, Dickinson A (1998). "Episodic-like memory during cache recovery by scrub jays". Nature 395 (6699): 272–4. doi:10.1038/26216. PMID 9751053. 
  • Kart-Teke E, De Souza Silva MA, Huston JP, Dere E (2006). "Wistar rats show episodic-like memory for unique experiences". Neurobiology of learning and memory 85 (2): 173–82. doi:10.1016/j.nlm.2005.10.002. PMID 16290193. 
  • Eacott MJ, Easton A, Zinkivskay A (2005). "Recollection in an episodic-like memory task in the rat". Learn. Mem. 12 (3): 221–3. doi:10.1101/lm.92505. PMID 15897259. 

External links

Notes

  1. ^ Tulving, E. (1984). Precis of Elements of Episodic Memory. Behavioural and Brain Sciences, 7, 223 – 268.
  2. ^ Cholinergic enhancement of episodic memory in healthy young adults - Psychopharmacology vol. 182, pages 170-179
  3. ^ Tolcapone improves cognition and cortical information processing in normal human subjects -Neuropsychopharmacology 2007 May
  4. ^ Effects of TC-1734 (AZD3480), a selective neuronal nicotinic receptor agonist, on cognitive performance and the EEG of young healthy male volunteers - Psychopharmacology, May 2007
  5. ^ Alhaj HA, Massey AE, McAllister-Williams RH. Effects of DHEA administration on episodic memory, cortisol and mood in healthy young men: a double-blind, placebo-controlled study. Psychopharmacology (2006) 188: 541–551 DOI 10.1007/s00213-005-0136-y
  6. ^ Buss C, Wolf OT, Witt J, Hellhammer DH (September 2004). "Autobiographic memory impairment following acute cortisol administration". Psychoneuroendocrinology 29 (8): 1093–6. doi:10.1016/j.psyneuen.2003.09.006. PMID 15219661. 
  7. ^ Bischof-Kohler D. 1985. Zur Phylogenese menschticher Motivation [On the phylogeny of human motivation]. In L. H. Eckensberger & E. D. Lantermann (Eds.), Emotion und Reflexivitat (pp. 3-47). Vienna: Urban & Schwarzenberg.
  8. ^ Correia SP, Dickinson A, Clayton NS. 2007. Western scrub-jays anticipate future needs independently of their current motivational state. Curr Biol. 17(10):856-61.

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