Episodic memory

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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] The term Episodic Memory was coined by Endel Tulving in 1972. He was referring to the distinction between knowing and remembering. Knowing is more factual whereas remembering is a feeling that is located in the past.[2]

Events that are recorded into episodic memory may trigger episodic learning, i.e. a change in behavior that occurs as a result of an event.[3][4] For example, a fear of dogs that follows being bitten by a dog is episodic learning.

One of the main components of episodic memory is the process of recollection. Recollection is a process that elicits the retrieval of contextual information pertaining to a specific event or experience that has previously occurred.

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.[5] 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.[6]

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.[7]

Relationship to semantic memory

Endel Tulving originally described episodic memory as a record of a person’s experience that held temporally dated information and spatio-temporal relations.[8] A feature of episodic memory that Tulving later elaborates on is that it allows an agent to travel back in time.[9] A current situation may cue retrieval of a previous episode, so that context that colors the previous episode is experienced at the immediate moment. The agent is provided with a means of associating previous feelings with current situations. Semantic memory, on the other end, is a structured record of facts, concepts and skills that we have acquired. Semantic information is derived from accumulated episodic memory. Episodic memory can be thought of as a "map" that ties together items in semantic memory. For example, all encounters with how a "dog" looks and sounds like will make up the semantic representation of that word. All episodic memories concerning your dog will then reference this single semantic representation of "dog" and, likewise, all new experiences with your dog will modify your single semantic representation of your dog.

Together, semantic and episodic memory make up our declarative memory.[10] They each represent different parts of context to form a complete picture. As such, something that affects episodic memory can also affect semantic memory. For example, anterograde amnesia, from damage of the medial temporal lobe, is an impairment of declarative memory that affects both episodic and semantic memory operations.[11] Originally, Tulving proposed that episodic and semantic memory were separate systems that competed with each other in retrieval. However, this theory was rejected when Howard and Kahana completed experiments on latent semantic analysis (LSA) that supported the opposite. Instead of an increase in semantic similarity when there was a decrease in the strength of temporal associations, the two worked together so semantic cues on retrieval were strongest when episodic cues were strong as well.[12]

Age differences

Activation of specific brain areas (mostly the hippocampus) seems to be different between younger and older people upon episodic memory retrieval.[13] Older people tend to activate both left and right hippocampus, while younger 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[14] 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.[15] Furthermore, episodic memory is enhanced through AZD3480 a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept.[16] 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).[17]

Damage

In animals

Tulving (1983) proposed that to meet the criteria of episodic memory, evidence of conscious recollection must be provided. Demonstrating episodic memory in the absence of language, and thus in non-human animals, is impossible though because there are no agreed upon non-linguistic behavioral indicators of conscious experience (Griffiths et al., 1999).

This idea was first challenged by Clayton and Dickinson in their work with the Western Scrub Jay (Aphelocoma californica). They were able to demonstrate that these birds may possess an episodic-like memory system as they found that they remember where they cached different food types, and discriminately recovered them depending on the perishability of the item and time that elapsed since caching. Thus, scrub-jays appear to remember the "what-where-and-when" of specific past caching events. The authors argued that such performance meets the behavioral criteria for episodic memory, but referred to the ability as "episodic-like" memory because the study did not address the phenomenological aspects of episodic memory.

After a study done by the University of Edinburgh (2006), hummingbirds were the first animal to demonstrate two of the aspects of episodic memory— the ability to recall where certain flowers were located and how recently they were visited. Other studies have demonstrated this type of memory in different animal species, such as rats, honey bees, and primates.

To show that animals do encode and retrieve past experiences relies on the circuitry of the medial temporal lobe, a structure including the hippocampus. Animal lesion studies have provided significant findings related to the importance of particular brain structures in episodic-like memory. For example, hippocampal lesions have severely impacted all three components (what, where, and when) in animals, suggesting that the hippocampus is responsible for detecting novel events, stimuli, and places when forming new memories and on retrieving that information later on.

Despite similar neural areas and evidence from experiments, 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. However, 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 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 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

"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 can be stored in autoassociative neural networks (e.g., a Hopfield network) if the stored representation includes information on the spatiotemporal context in which an item was studied.

Notes

  1. ^ Tulving E (1984). "Precis of Elements of Episodic Memory". Behavioural and Brain Sciences 7 (2): 223–68. doi:10.1017/S0140525X0004440X. 
  2. ^ [1] from an article in "Current Biology"
  3. ^ Terry, W. S. (2006). Learning and Memory: Basic principles, processes, and procedures. Boston: Pearson Education, Inc.
  4. ^ Baars, B. J. & Gage, N. M. (2007). Cognition, Brain, and Consciousness: Introduction to cognitive neuroscience. London: Elsevier Ltd.
  5. ^ Janowsky JS, Shimamura AP, Squire LR (1989). "Source memory impairment in patients with frontal lobe lesions". Neuropsychologia 27 (8): 1043–56. doi:10.1016/0028-3932(89)90184-X. PMID 2797412. 
  6. ^ Gabrieli JD, Poldrack RA, Desmond JE (February 1998). "The role of left prefrontal cortex in language and memory". Proc. Natl. Acad. Sci. U.S.A. 95 (3): 906–13. doi:10.1073/pnas.95.3.906. PMC 33815. PMID 9448258. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=9448258. 
  7. ^ Deisseroth K, Singla S, Toda H, Monje M, Palmer TD, Malenka RC (May 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. http://linkinghub.elsevier.com/retrieve/pii/S0896627304002661. 
  8. ^ Tulving, Endel (1983). Elements of Episodic Memory. New York: Oxford University Press. 
  9. ^ Tulving, Endel (2002). "Episodic Memory: From Mind to Brain". Annual Review of Psychology 53: 1–25. doi:10.1146/annurev.psych.53.100901.135114. PMID 11752477. 
  10. ^ Tulving, Endel; Schacter, D.L. (19). "Priming and Human Memory Systems". Science. 4940 247 (4940): 301–6. doi:10.1126/science.2296719. PMID 2296719. 
  11. ^ Tulving, Endel; Hans J. Markowitsch (7). "Episodic and Declarative Memory: role of the hippocampus". Hippocampus 8 (3): 198–204. doi:10.1002/(SICI)1098-1063(1998)8:3<198::AID-HIPO2>3.0.CO;2-G. PMID 9662134. 
  12. ^ Howard, M.W.; Kahana, M.J. (2002). "When does semantic similarity help episodic retrieval". Journal of Memory and Language 46: 85–96. doi:10.1006/jmla.2001.2798. 
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  18. ^ Ben Shalom D (2003). "Memory in autism: review and synthesis". Cortex 39 (4–5): 1129–38. doi:10.1016/S0010-9452(08)70881-5. PMID 14584570. 
  19. ^ 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. 
  20. ^ Parrott AC, Lees A, Garnham NJ, Jones M, Wesnes K (1998). "Cognitive performance in recreational users of MDMA of 'ecstasy': evidence for memory deficits". Journal of Psychopharmacology (Oxford, England) 12 (1): 79–83. doi:10.1177/026988119801200110. PMID 9584971. 
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