episodic memory scholarpedia

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Endel Tulving and Karl K. Szpunar (2009), Scholarpedia,4(8):3332. doi:10.4249/scholarpedia.3332

revision #67580 [link to/cite thisarticle]

Curator: Dr. Endel Tulving, Rotman Research Institute of Baycrest, University of Toronto, CANADACurator: Dr. Karl K. Szpunar, Rotman Research Institute of Baycrest, University of Toronto, CANADA

Episodic memoryis the name given to the capacityto consciously remember personally experienced events andsituations. It is one of the major mental (cognitive) capacities enabled by the brain.

1 Example2 Overview3 Terminology4 Relations between episodic and semantic memory

4.1 Common features4.2 Unique features of episodic memory

5 Episodic memory: 20096 Open issues

6.1 Autobiographical memory6.2 Episodic memory in nonhuman animals6.3 Adaptive value

7 References

8 Recommended readings9 See also

In the prototypical actof exercising the capacity of episodic memory one may remember a recent trip to Paris, mentallyreliving events that happened there, in the minds eye seeing again the places visited, sights seen, sounds heard, aromassmelled, and people met.

Memory is an umbrella term that covers a variety of different forms of acquisition, retention, and use of habits, skills,knowledge, and experience. Those who study memory have found it useful to assume that different forms of learning andmemory are subserved by different memory systems--organized collections of neurocognitive components that worktogether to perform functions that other collections of components cannot perform, or cannot perform as well. An importantobjective of research has to do with the identification of these memory systems, specification of their properties, anddelineation of the nature of the relations among them.

Historically, the most basic distinction is that between procedural memory(an actionsystem that is expressed throughbehavior; e.g., when riding a bicycle) and declarative memory(a cognitivesystem that is expressed through propositional

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knowledge; e.g., when taking a classroom test). Both procedural and declarative memory are seen as consisting of anumber of subdivisions (Eichenbaum & Cohen, 2001; Schacter & Tulving, 1994; Schacter, Wagner, & Buckner, 2000;Squire, 1992; Squire & Kandel, 1999; Squire & Zola, 1998). This article describes a theory of episodic memory, one ofthe two assumed subdivisions of declarative memory. However, because the theory of episodic memory can be onlyincompletely understood in isolation of the other assumed subdivision of declarative memory, semantic memory--thesystem that enables us to acquire and retain factual knowledge about the world (e.g., knowing that Paris is a nice city tovisit in the springtime) and from which episodic memory is thought to have evolved, much of the discussion will focus onepisodic memory in relation tosemantic memory.

In this article, the term episodic memory refers to a unique memory system (or capacity) of the brain. However, that isnot the only meaning of episodic memory that one will find in the literature. For instance, the term is often used to describethe specific experience (content) that comes to mind when exercising the capacity of episodic memory and theaccompanying feeling (phenomenology) that one is currently reliving that previous experience. In the interest of clarity, thisarticle will refer to the contents of episodic memory as remembered experiences and the phenomenological experience asremembering. A similar issue exists in relation to the concept of semantic memory. Presently, the term semanticmemory also stands for a capacity of the brain. The structured contents of the semantic memory system are referred to asknowledge and the phenomenological experience as knowing (Gardiner & Richardson-Klavehn, 2000).

According to the theory of episodic memory, the assumed evolutionary sequence of episodic memory growing out ofsemantic memory is reflected in the global, monohierarchical relation between the two. That is, episodic memory shareswith semantic memory many features that distinguish both of them (i.e., all of declarative memory) from other majorsubdivisions of memory, yet it also possesses features that it does not share with any other memory system, includingsemantic memory (Mishkin, Suzuki, Gadian, & Vargha-Khadem, 1997; Tulving, 1995). The monohierarchical relation alsoimplies that episodic memory depends on semantic memory in its operations and cannot function without relevantcomponents of semantic memory, whereas semantic memory does not depend on episodic memory in its operations andcan function without episodic memory. This kind of a relation between the two memory systems mimics many other

similar relations in the living world. As a single example, consider the relation between a visual system that has no senseof color and a visual system that does: The latter has everything that the former has, plus more.

What makes episodic memory special is that it makes possible mental time travel into the past, as well as into the future,as will be seen below. No other memory system has the same capacity, at least not in the sense that episodic memorydoes.

Common features

Some of the features (or properties) that episodic memory shareswith semantic memory are:

Both systems allow the organism to know about aspects of its world that are not immediately present.

Encoding of new information [converting perceptual and cognitive input into memory traces (engrams)] is fast andmay occur on a single trial.

Encoded information (memory traces) may be multimodal (polymodal).

Storage of encoded information is transmodal: both remembered experiences and knowledge can be storedindependent of the modality through which they were acquired.

Storage of information is highly structured.

Storage of information is highly sensitive to context.



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Stored information is representational (isomorphic) with what is or could be in the world.

Access to stored information during retrieval is flexible, within limits.

Behavioral expression of what is retrieved is optional and not obligatory. Thus, it is possible to hold the retrievedinformation online, and just contemplate it.

Retrieval of information in both systems requires consciousness. It is not possible to directly retrieve informationfrom either episodic or semantic memory nonconsciously. Of course, various processes that underlie the retrieval of

remembered experiences and knowledge may take place beyond conscious awareness.

The operations of neither system depend on language, although language may greatly facilitate them.

The shared features of both systems are present in a wide range of animals; they are highly evolved in mammals andbirds.

The operations of both systems are subserved by shared, widely distributed, cerebral cortical and subcortical neuralnetworks; especially critical are those in medial temporal lobes and the diencephalon.

Any one of these properties applies equally well to both episodic and semantic memory. It is their conjunction that allowsus sometimes to classify both episodic and semantic memory together under the general label of declarative (also referred

to as cognitive or explicit) memory, without further differentiating between them. In many situations, both in the laboratoryand real life, such generalization is justifiable. In others, however, it is not, because episodic memory, in addition to theproperties listed above, also possesses unique properties that are shared by neither semantic nor any other memorysystem.

Unique features of episodic memory

Some of the features (or properties) that are uniqueto episodic memory are:

The key function of episodic memory is to allow the individual to remember personal past happenings as such;semantic memory is not capable of this function.

This remembering takes the form of mentally traveling in subjectively experienced time. Semantic memory doesnot have anything special to do with time, other than the (trivial) fact that the knowledge that is brought to mind inthe course of exercising this capacity was once learned in the past.

Episodic memory, unlike semantic memory, is self-centered. The operations of episodic memory are predicated onone's conscious awareness of oneself as an independent entity that is separate from the rest of the world. In theabsence of such awareness, episodic remembering is not possible.

Episodic remembering expresses itself phenomonally through the medium of a distinctive form of consciousawareness that is familiar to all people in the sense that they know when they are remembering and not perceiving,or imagining, or daydreaming, or having any other kind of conscious experience. The conscious awarenessaccompanying semantic knowing has a different flavor, clearly distinct from that of remembering. The two kinds of

consciousness involved in episodic remembering and semantic knowing have been named autonoetic and noetic,respectively (Tulving, 1985; Wheeler, Stuss, & Tulving, 1997).

Episodic remembering requires the activation, by way of voluntary or involuntary processes, of a special kind ofmental state that has been called episodic retrieval mode. The operational default memory state is semantic,characterized by noetic consciousness.

The ontogenetic development of episodic memory is delayed in relation to that of semantic memory: Childrenacquire a great deal of knowledge about the world they live in before they are aware of their own past personalexperiences (Nelson & Fivush, 2004).

Episodic memory tends to be more vulnerable to disease, injury, and the ravages of old age than is semantic



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memory. Brain damage is more likely to impair episodic remembering than semantic knowing, and in dementiassuch as Alzheimer's disease the impairment of episodic memory is frequently the first symptom to appear(Kitchener, Hodges, & McCarthy, 1998; Klein, Loftus, & Kihlstrom, 2002; Rosenbaum et al., 2005; Vargha-Khademet al., 1997).

Episodic memory that all healthy humans possess probably does not exist in other animals, although episodic-likememory capacities have already been identified in several species (Clayton, Bussey, & Dickinson, 2003; Griffiths,Dickinson, & Clayton, 1999; Olton, 1984).

Episodic memory is dependent on neural networks that extend beyond those that subserve the operations ofsemantic memory (Aggleton & Pearce, 2001; Nyberg et al., 2000).

Some of these features are more reasonable in light of empirical evidence than others. The whole enterprise of studyingmultiple memory systems is in an early stage. Therefore a great deal of further work and thought is required, and indeed isbeing spent, on many aspects of the problem (Dere, Easton, Nadel, & Huston, 2008; Szpunar & McDermott, 2008; Tulving2002a).

Today, a thumbnail description of episodic memory can be defined in terms of these unique features, against the backdropof the shared features.

Episodic memory is a recently evolved, late developing, and early deteriorating brain/mind (neurocognitive) memorysystem. It is oriented to the past, more vulnerable than other memory systems to neuronal dysfunction, and probablyunique to humans. It makes possible mental time travel through subjective time--past, present, and future. This mental timetravel allows one, as an owner of episodic memory (self), through the medium of autonoetic awareness, to rememberone's own previous thought about experiences, as well as to think about one's own possible future experiences. Theoperations of episodic memory require, but go beyond, the semantic memory system. Retrieving information from episodicmemory (remembering) requires the activation, by way of voluntary or involuntary processes, of a special mental set,dubbed episodic retrieval mode. The neural components of episodic memory comprise a widely distributed network ofcerebral cortical and subcortical brain regions that overlap with and extend beyond the networks subserving other memorysystems. The essence of episodic memory lies in the conjunctionof three concepts--self, autonoetic awareness, and

subjective time.

The remainder of this article summarizes some of the open issues concerning episodic memory, issues that remain in thefocus of investigators.

Autobiographical memory

Episodic memory is closely related to autobiographical memory. That term, however, most often is used in the sense ofsignificant life experiences, either remembered orknown. That is, people typically include facts such as when and where

they were born as an important part of their life story although these are necessarily acquired through the semantic ratherthan episodic system. Episodic memory, on the other hand, has to do with remembered experiences alone (with regard toone's past). The distinction between episodic and semantic autobiographical memory receives support from relevantneuroimaging studies (Levine et al., 2004; Svoboda, McKinnon, & Levine, 2006).

Episodic memory in nonhuman animals

There exists essentially universal agreement among all practitioners of the science of memory that many species other thanhumans possess highly developed semantic memory systems. These allow them to acquire complex and intricateknowledge of various sorts about their own ecological niches (as well as other aspects) of the world.



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Mishkin, M., Suzuki, W. A., Gadian, D. G., & Vargha Khadem, F. (1997). Hierarchical organization of cognitivememory. Philosophical Transactions of the Royal Society B, 352, 1461 1467.

Nelson, K., & Fivush, R. (2004). The emergence of autobiographical memory: A social-cultural developmentaltheory. Psychological Review, 111, 486-511.

Nyberg, L., Habib, R., Tulving, E., Cabeza, R., Houle, S., Persson, J., & McIntosh, A. R. (2000). Large scaleneurocognitive networks underlying episodic memory.Journal of Cognitive Neuroscience, 12, 163-173.

Olton, D. S. Comparative analysis of episodic memory.Behavioral and brain sciences, 7, 250-251.

Rosenbaum, R. S., Kohler, S., Schacter, D. L., Moscovitch, M., Westmacott, R., Black, S. E., Gao, F., & Tulving, E.(2005). The case of K.C.: Contributions of a memory-impaired person to memory theory.Neuropsychologia, 43,989-1021.

Schacter, D. L., & Addis, D. R. (2007). The cognitive neuroscience of constructive memory: Remembering the pastand imagining the future. Philosophical Transactions of the Royal Society of London B, 362, 773-786.

Schacter, D. L., & Tulving, E. (1994). What are the memory systems of 1994? In D.L. Schacter & E. Tulving(Eds.),Memory systems 1994(pp. 1 38). Cambridge, MA: MIT Press.

Schacter, D. L., Wagner, A. D., & Buckner, R. L. (2000). Memory systems of 1999. In E. Tulving & F. I. M. Craik(Eds.), The Oxford handbook of memory(pp. 627-643). New York: Oxford University Press.

Squire, L. R. (1992). Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans.Psychological Review, 99, 195-231.

Squire, L. R., & Kandel, E. R. (1999).Memory: From mind to molecules. New York: Freeman & Co.

Squire, L. R., & Zola, S. M. (1998). Episodic memory, semantic memory, and amnesia.Hippocampus, 8, 205 211.

Suddendorf, T., & Corballis, M. C. (1997). Mental time travel and the evolution of the human mind. Genetic,Social, and General Psychology Monographs, 123, 133-167.

Suddendorf, T., & Corballis, M. C. (2007). The evolution of foresight: What is mental time travel, and is it uniqueto humans?Behavioral and Brain Sciences, 30, 299-313.

Svoboda, E., McKinnon, M. C., & Levine, B. (2006). The functional neuroanatomy of autobiographical memory: Ameta-analysis.Neuropsychologia, 44, 2189-2208.

Szpunar, K. K. (in press). Episodic future thought: An emerging concept. Perspectives on Psychological Science.

Szpunar, K. K., & McDermott, K. B. (2008). Episodic memory: An evolving concept. In D. Sweat, R. Menzel, H.Eichenbaum & H. L. Roediger III (Eds.),Learning and memory: A comprehensive reference(pp. 491-510). OxfordElsevier.

Tulving, E. (1983).Elements of episodic memory. New York: Oxford University Press.

Tulving, E. (1985). Memory and consciousness. Canadian Psychology, 26, 1-12.

Tulving, E. (1995). Organization of memory: Quo vadis? In M.S. Gazzaniga (Ed.), The cognitive neurosciences(pp.839-847). Cambridge, MA: MIT Press.

Tulving, E. (2002a). Episodic memory: From mind to brain. Annual Review of Psychology, 53, 1-25.

Tulving, E. (2002b). Chronesthesia: Awareness of subjective time. In D. T. Stuss & R. C. Knight (Eds.), Principlesof Frontal Lobe Function(pp. 311-325). New York: Oxford University Press.



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