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 you remember the party on your 6th birthday, this is an episodic memory. They allow you 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

Activation of specific brain areas (mostly the hippocampus) seems to be different between younger and older people upon episodic memory retrieval.^[16] 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^[17] 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.^[18] Furthermore, episodic memory is enhanced through AZD3480, a selective agonist at the neuronal alpha4beta2 nicotinic receptor, which is developed by the company Targacept.^[19] 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).^[20]

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.^[21]

Damage

• Based on a review of behavioral studies, it is suggested that there may be selective damage to the limbic-prefrontal episodic memory system in some people with autism.^[22] 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.
• A rare type of shellfish poisoning called amnesic shellfish poisoning or "ASP" quite effectively and irreversibly damages the hippocampus, rendering one amnesic.
• Korsakoff's syndrome is caused by thiamine (vitamin B₁) deficiency, a form of malnutrition which can be precipitated by overconsumption of alcohol compared to foods.
• An acute cortisol level (by injection) has been found to significantly inhibit the recall of autobiographical memories^[23] which may contribute to memory deficits found in depression.
• The use of MDMA ("Ecstasy") has been associated with persistent deficits in episodic memory.^[24][25]

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, 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. Similarities and differences between humans and other animals are currently much debated

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

• Specific events
  • When you first set 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
  • "Who was the Prime Minister of Italy when I was born?"
• Flashbulb memories
  • Flashbulb memories are critical autobiographical memories about a major event. Some flashbulb 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 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

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.
• Tulving E (2002). "Episodic memory: from mind to brain". Annual review of psychology 53: 1–25. doi:10.1146/annurev.psych.53.100901.135114. PMID 11752477. 
• Griffiths D, Dickinson A, Clayton N (1999). "Episodic memory: what can animals remember about their past?". Trends in cognitive sciences 3 (2): 74–80. doi:10.1016/S1364-6613(98)01272-8. 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. 
• Suddendorf T, Busby J (2003). "Mental time travel in animals?". Trends in Cognitive Sciences 7 (9): 391–396. doi:10.1016/S1364-6613(03)00187-6. PMID 12963469. 
• Suddendorf T (2006). "Foresight and evolution of the human mind". Science 312 (5776): 1006–7. doi:10.1126/science.1129217. PMID 16709773. 
• Ghetti, S. Lee, J. (2010). Children's Episodic Memory. John Wiley & Sons, Ltd. pp. 365–373.

External links

• The Works of Endel Tulving - free access to papers and book chapters
• Images in Neuroscience: Episodic Memory Retrieval. American Journal of Psychiatry.
• Episodic Memory and Referential Activity

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