Episodic memory

Episodic memory is the memory of autobiographical events (times, places, associated emotions, and other contextual who, what, when, where, why knowledge) that can be explicitly stated. It is the collection of past personal experiences that occurred at a particular time and place. For example, if one remembers the party on his or her 6th birthday, this is an episodic memory. They allow an individual to figuratively travel back in time to remember the event that took place at that particular time and place.[1]

Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions of memory – the other is implicit memory.[2] 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 (semantic) whereas remembering is a feeling that is located in the past (episodic).[3]

Tulving has seminally defined three key properties of episodic memory recollection. These are a subjective sense of time (or mental time travel), connection to the self, and autonoetic consciousness. Autonoetic consciousness refers to a special kind of consciousness that accompanies the act of remembering which enables an individual to be aware of the self in a subjective time. Aside from Tulving, others named the important aspects of recollection which includes visual imagery, narrative structure, retrieval of semantic information and the feelings of familiarity.[4]

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.[5][6] For example, a fear of dogs after being bitten by a dog is a result of 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 occurred.

Nine properties of episodic memory

There are essentially nine properties of episodic memory that collectively distinguish it from other types of memory. Other types of memory may exhibit a few of these properties, but only episodic memory has all nine:[7]

  1. Contain summary records of sensory-perceptual-conceptual-affective processing.
  2. Retain patterns of activation/inhibition over long periods.
  3. Often represented in the form of (visual) images.
  4. They always have a perspective (field or observer).
  5. Represent short time slices of experience.
  6. They are represented on a temporal dimension roughly in order of occurrence.
  7. They are subject to rapid forgetting.
  8. They make autobiographical remembering specific.
  9. They are recollectively experienced when accessed.

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.[8] 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.[9]

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.[10]

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.[11] A feature of episodic memory that Tulving later elaborates on is that it allows an agent to imagine traveling back in time.[12] A current situation may cue retrieval of a previous episode, so that context that colours 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 hand, 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 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.[13] 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.[14] 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.[15]

Age differences

Episodic memory emerges at approximately 3- to 4-years-of-age.[16] Activation of specific brain areas (mostly the hippocampus) seems to be different between younger and older people upon episodic memory retrieval.[17] Older people tend to activate both left and right hippocampus, while younger people activate only the left one. For more information, see memory and aging.

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. An example of this would be an experience such as a close family member dying or the Christmas that you got the exact toy you wanted as a kid. The experience holds so much emotional significance that it is encoded as an extremely vivid, almost picture-perfect memory.

Pharmacological enhancement

In healthy adults, longterm visual episodic memory can be enhanced specifically[18] 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.[19] Furthermore, episodic memory is enhanced through AZD3480, a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept.[20] 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).[21]

A 2015 meta-analysis of high quality evidence found that therapeutic doses of amphetamine and methylphenidate improve performance on working memory, episodic memory, and inhibitory control tests in normal healthy adults.[22]

Damage

In animals

Main article: Episodic-like memory

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, 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.

The ability of animals to 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 [28] 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. Similarities and differences between humans and other animals are currently much debated.[29]

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. Additionally, people recall many personal events from their previous few years. For adolescents and young adults the reminiscence bump and the recent events can 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

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. ↑ Schacter, Daniel L., Gilbert, Daniel T., and Wegner, Daniel M. "Semantic and episodic memory". Psychology; Second Edition. New York: Worth, Incorporated, 2011. 240-241. Print.
  2. ↑ Tulving E (1984). "Precis of Elements of Episodic Memory". Behavioural and Brain Sciences 7 (2): 223–68. doi:10.1017/S0140525X0004440X.
  3. ↑ from an article in "Current Biology"
  4. ↑ Hassabis D and Maguire EA. 2007. Deconstructing episodic memory with construction. Trends in Cognitive Sciences (Regul Ed ) 11(7):299-306.
  5. ↑ Terry, W. S. (2006). Learning and Memory: Basic principles, processes, and procedures. Boston: Pearson Education, Inc.
  6. ↑ Baars, B. J. & Gage, N. M. (2007). Cognition, Brain, and Consciousness: Introduction to cognitive neuroscience. London: Elsevier Ltd.
  7. ↑ Conway M. A. (2009). Episodic Memory. Neuropsychologia, 47, 2305-2306
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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.
  11. ↑ Tulving, Endel (1983). Elements of Episodic Memory. New York: Oxford University Press.
  12. ↑ 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.
  13. ↑ Tulving, Endel; Schacter, D.L. (19 January 1990). "Priming and Human Memory Systems". Science. 4940 247 (4940): 301–6. doi:10.1126/science.2296719. PMID 2296719.
  14. ↑ Tulving, Endel; Hans J. Markowitsch (7 December 1998). "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.
  15. ↑ 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.
  16. ↑ Scarf D, Gross J, Colombo M, Hayne H (2013). "To have and to hold: Episodic memory in 3- and 4-year-old children.". Developmental Psychobiology 55: 125–132. doi:10.1002/dev.21004.
  17. ↑ Maguire EA, Frith CD (July 2003). "Aging affects the engagement of the hippocampus during autobiographical memory retrieval". Brain 126 (Pt 7): 1511–23. doi:10.1093/brain/awg157. PMID 12805116.
  18. ↑ Grön G, Kirstein M, Thielscher A, Riepe MW, Spitzer M (October 2005). "Cholinergic enhancement of episodic memory in healthy young adults". Psychopharmacology (Berl.) 182 (1): 170–9. doi:10.1007/s00213-005-0043-2. PMID 16021483.
  19. ↑ Apud JA, Mattay V, Chen J, et al. (May 2007). "Tolcapone improves cognition and cortical information processing in normal human subjects". Neuropsychopharmacology 32 (5): 1011–20. doi:10.1038/sj.npp.1301227. PMID 17063156.
  20. ↑ Dunbar G, Boeijinga PH, Demazières A, et al. (May 2007). "Effects of TC-1734 (AZD3480), a selective neuronal nicotinic receptor agonist, on cognitive performance and the EEG of young healthy male volunteers". Psychopharmacology (Berl.) 191 (4): 919–29. doi:10.1007/s00213-006-0675-x. PMID 17225162.
  21. ↑ Alhaj HA, Massey AE, McAllister-Williams RH (November 2006). "Effects of DHEA administration on episodic memory, cortisol and mood in healthy young men: a double-blind, placebo-controlled study". Psychopharmacology (Berl.) 188 (4): 541–51. doi:10.1007/s00213-005-0136-y. PMID 16231168.
  22. ↑ Ilieva IP, Hook CJ, Farah MJ (January 2015). "Prescription Stimulants' Effects on Healthy Inhibitory Control, Working Memory, and Episodic Memory: A Meta-analysis". J. Cogn. Neurosci.: 1–21. doi:10.1162/jocn_a_00776. PMID 25591060. The present meta-analysis was conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive functions central to academic and occupational functioning, including inhibitory control, working memory, short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found evidence for small but significant stimulant enhancement effects on inhibitory control and short-term episodic memory. Small effects on working memory reached significance, based on one of our two analytical approaches. Effects on delayed episodic memory were medium in size. However, because the effects on long-term and working memory were qualified by evidence for publication bias, we conclude that the effect of amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In some situations, a small advantage may be valuable, although it is also possible that healthy users resort to stimulants to enhance their energy and motivation more than their cognition. ... Earlier research has failed to distinguish whether stimulants’ effects are small or whether they are nonexistent (Ilieva et al., 2013; Smith & Farah, 2011). The present findings supported generally small effects of amphetamine and methylphenidate on executive function and memory. Specifically, in a set of experiments limited to high-quality designs, we found significant enhancement of several cognitive abilities. ...

    The results of this meta-analysis cannot address the important issues of individual differences in stimulant effects or the role of motivational enhancement in helping perform academic or occupational tasks. However, they do confirm the reality of cognitive enhancing effects for normal healthy adults in general, while also indicating that these effects are modest in size.
  23. ↑ 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.
  24. ↑ Joseph et al., 2003
  25. ↑ 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.
  26. ↑ 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.
  27. ↑ Morgan MJ (January 1999). "Memory deficits associated with recreational use of "ecstasy" (MDMA)". Psychopharmacology 141 (1): 30–6. doi:10.1007/s002130050803. PMID 9952062.
  28. ↑ Correia; et al. (2007). "Western Scrub-Jays Anticipate Future Needs Independently of Their Current Motivational State". Current Biology 17: 856–861. doi:10.1016/j.cub.2007.03.063.
  29. ↑ Scarf, D., Smith, C., & Stuart, M. (2014). "A spoon full of studies helps the comparison go down: A comparative analysis of Tulving’s Spoon Test.". Frontiers in Psychology 5: 893. doi:10.3389/fpsyg.2014.00893.

References

External links

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