Episodic vs. Semantic Memory

Episodic memory is a recently evolved, late developing, past-oriented memory system, probably unique to humans, that allows remembering of previous experiences as experienced. William JAMES (1890) discussed it as simply "memory." The advent of many different forms of memory since James's time has made adjectival modifications of the term necessary. Semantic memory is the closest relative of episodic memory in the family of memory systems. It allows humans and nonhuman animals to acquire and use knowledge about their world. Although humans habitually express and exchange their knowledge through language, language is not necessary for either remembering past experiences or knowing facts about the world.

Episodic and semantic memory are alike in many ways, and for a long time were thought of and classified together as an undifferentiated "declarative" memory that was distinguished from "procedural" memory. Nevertheless, rapidly accumulating evidence suggests that episodic and semantic memory are fundamentally different in a number of ways, and therefore need to be treated separately. In what follows, the similarities and differences are briefly summarized.

Episodic and semantic systems share a number of features that collectively define "declarative" (or "cognitive") memory in humans.

  1. Both are large and complex, and have unmeasurable capacity to hold information, unlike WORKING MEMORY, which has limited capacity.
  2. Cognitive operations involved in encoding of information are similar for both episodic and semantic memory. Frequently a single short-lived event is sufficient for a permanent "addition" to the memory store, unlike in many other forms of learning that require repeated experiences of a given kind.
  3. Both are open to multimodal influences and can receive information for storage through different sensory modalities, as well as from internally generated sources.
  4. The operations of both systems are governed by principles such as encoding specificity and transfer-appropriate processing.
  5. Stored information in both systems represents aspects of the world, and it has truth value, unlike many other forms of learned behavior that do not.
  6. Both are "cognitive" systems: their informational "contents" can be thought about independently of any overt action, although such action can be and frequently is taken. As cognitive systems, episodic and semantic memory differ from all forms of procedural memory in which overt behavior at input and output is obligatory.
  7. Information in both systems is flexibly accessible through a variety of retrieval queries and routes.
  8. Information retrieved from either system can be expressed and communicated to others symbolically.
  9. Information in both systems is accessible to INTROSPECTION: we can consciously "think" about things and events in the world, as we can "think" about what we did yesterday afternoon, or in the summer camp at age ten.
  10. The processes of both forms of memory depend critically on the integrity of the medial temporal lobe and diencephalic structures of the brain.

Consider now the differences.

  1. The simplest way of contrasting episodic and semantic memory is in terms of their functions: episodic memory is concerned with remembering, whereas semantic memory is concerned with knowing. Episodic remembering takes the form of "mental travel through subjective time," accompanied by a special kind of awareness ("autonoetic," or self-knowing, awareness). Semantic knowing takes the form of thinking about what there is, or was, or could be in the world; it is accompanied by another kind of awareness ("noetic," or knowing awareness). Language is frequently involved in both episodic and semantic memory, but it need not be.
  2. The relation between remembering and knowing is one of embeddedness: remembering always implies knowing, whereas knowing does not imply remembering.
  3. Episodic memory is arguably a more recent arrival on the evolutionary scene than semantic memory. Many animals other than humans, especially mammals and birds, possess well-developed knowledge-of-the-world (semantic memory) systems. But there is no evidence that they have the ability to autonoetically remember past events in the way that humans do.
  4. Episodic lags behind semantic memory in human development. Young children acquire a great deal of knowledge about their world before they become capable of adult-like episodic remembering.
  5. Episodic memory is the only form of memory that is oriented toward the past: retrieval in episodic memory necessarily involves thinking "back" to an earlier time. All other forms of memory, including semantic memory, are present-oriented: utilization (retrieval) of information usually occurs for the purpose of whatever one is doing now without any thinking "back" to the experiences of the past.
  6. Episodic remembering is characterized by a state of awareness (autonoetic) that is different from that in semantic memory (noetic). When one recollects an event autonoetically, one reexperiences aspects of a past experience; when one recalls a fact learned in the past, reexperiencing of the learning episode is not necessary.
  7. Episodic remembering has an affectively laden "tone" that is absent in semantic knowing. William James (1890) referred to is as a "feeling of warmth and intimacy."

Given the many similarities and some fundamental differences between episodic and semantic memory, it is difficult to provide a simple description of the relation between the two. According to one proposal, however, the relation is process-specific: The two systems operate serially at the time of encoding: information "enters" episodic memory "through" semantic memory. They operate in parallel in holding the stored information: a given datum may be stored in one or both systems. And the two systems can act independently at the time of retrieval: recovery of episodic information can occur separately of retrieval of semantic information (Tulving 1995).

The term "episodic memory" is sometimes used in senses that differ from the memory-systems orientation presented here. Some writers use "episodic memory" in its original sense of task orientation: Episodic memory refers to tasks in which information is encoded for storage on a particular occasion. This kind of usage is popular in work with animals. Other writers use "episodic memory" as a particular kind of memory information or "material," namely past "events," in contrast with the "facts" of semantic memory. The systems-based definition of episodic memory as described here is more comprehensive than either the task-specific and material-specific definitions. Finally, some writers still prefer the traditional view that there is only one kind of declarative memory, and they use the terms "episodic" and "semantic" for descriptive purposes only.

The evidential basis for the distinction between episodic and semantic memory has been growing steadily over the past ten or fifteen years. General reviews have been provided by Nyberg and Tulving (1996), Nyberg, McIntosh, and Tulving (1997), and Wheeler, Stuss, and Tulving (1997). Functional dissociations between autonoetic and noetic awareness in memory retrieval have been reviewed by Gardiner and Java (1993). Developmental evidence for the distinction has been presented by Mitchell (1989), Nelson (1993), Nilsson et al. (1997), and Perner and Ruffman (1995). Pertinent psychopharmacological data have been reported by Curran et al. (1993). Dissociations between episodic and semantic memory produced by known or suspected brain damage have been reported, among others, by Hayman, Macdonald, and Tulving (1993); Markowitsch (1995); Shimamura and Squire (1987); and Vargha-Khadem et al. (1997). Electrophysiological correlates of "remembering" versus "knowing" have been described by Düzel et al. (1997), and differences in EEG power spectra in episodic versus semantic retrieval by Klimesch, Schimke, and Schwaiger (1994). Finally, evidence in support of the distinction between episodic and semantic memory has been provided by a number of recent studies of functional neuroimaging, especially POSITRON EMISSION TOMOGRAPHY (Buckner and Tulving 1995). One of the most persistent findings is that episodic retrieval is accompanied by changes in neuronal activity in brain regions such as the right prefrontal cortex, medial parietal cortical regions, and the left CEREBELLUM, whereas comparable semantic retrieval processes are accompanied by changes in the left frontal and temporal regions (Buckner 1996; Cabeza and Nyberg 1997; Fletcher, Frith, and Rugg 1997; Haxby et al. 1996; Nyberg, Cabeza, and Tulving 1996, Nyberg, McIntosh, and Tulving 1997; Shallice et al. 1994; Tulving et al. 1994). Future studies will undoubtedly further clarify the emerging picture of the functional neuroanatomy of episodic and semantic memory .

It is a moot question whether episodic and semantic memory are basically similar or basically different. The question is not unlike one about basic similarities and differences between, say, vertebrates and invertebrates. As frequently happens in science, it all depends on one's interest and purpose.

See also

Additional links

-- Endel Tulving

References

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Further Readings

Dalla Barba, G., M. C. Mantovan, E. Ferruzza, and G. Denes. (1997). Remembering and knowing the past: A case study of isolated retrograde amnesia. Cortex 33:143-154.

Horner, M. D. (1990). Psychobiological evidence for the distinction between episodic and semantic memory. Neuropsychology Review 1:281-321.

Humphreys, M. S., J. D. Bain, and R. Pike. (1989). Different ways to cue a coherent memory system: A theory for episodic, semantic, and procedural tasks. Psychological Review 96:208-233.

Humphreys, M. S., J. Wiles, and S. Dennis. (1994). Toward a theory of human memory: Data structures and access processes. Behavioral and Brain Sciences 17:655-692.

Huron, C., J. M. Danion, F. Giacomoni, D. Grange, P. Robert, and L. Rizzo. (1995). Impairment of recognition memory with, but not without, conscious recollection in schizophrenia. American Journal of Psychiatry 152:1737-1742.

Kihlstrom, J. F. (1984). A fact is a fact is a fact. Behavioral and Brain Sciences 7:243-244.

Kitchner, E.G., J. R. Hodges, and R. McCarthy. (1998). Acquisition of post-morbid vocabulary and semantic facts in the absence of episodic memory. Brain 121:1313-1327.

Mandler, G. (1987). Memory: Conscious and unconscious. In P. R. Solomon, G. R. Goethals, C. M. Kelley, and B. R. Stephens, Eds., Memory -- An Interdisciplinary Approach. New York: Springer, p. 42.

McKoon, G., R. Ratcliff, and G. S. Dell. (1985). A critical evaluation of the semantic-episodic distinction. Journal of Experimental Psychology: Learning, Memory and Cognition 12:295-306.

Roediger, H. L., M. S. Weldon, and B. H. Challis. (1989). Explaining dissociations between implicit and explicit measures of retention: A processing account. In H. L. Roediger and F. I. M. Craik, Eds., Varieties of Memory and Consciousness: Essays in Honour of Endel Tulving. Hillsdale, N.J: Erlbaum, pp. 3-41.

Roediger, H. L., S. Rajaram, and K. Srinivas. (1990). Specifying criteria for postulating memory systems. Conference of the National Institute of Mental Health et al: The development and neural bases of higher cognitive functions. (1989, Philadelphia, Pennsylvania). Annals of the New York Academy of Sciences 608:572-595.

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

Squire, L. R. (1993). Memory and the hippocampus: A synthesis from findings with rats, monkeys, and humans. Psychological Review 99:195-231.

Tulving, E. (1983). Elements of Episodic Memory. Oxford: Clarendon Press.

Tulving, E. (1985). How many memory systems are there? American Psychologist 40:385-398.

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

Tulving, E. (1991). Concepts of human memory. In L. Squire, G. Lynch, N. M. Weinberger, and J. L. McGaugh, Eds., Memory: Organization and Locus of Change. New York: Oxford University Press, pp. 3-32.

Tulving, E. (1993). What is episodic memory? Current Perspectives in Psychological Science 2:67-70.

Tulving, E. (1998). Brain/mind correlates of memory. In M. Sa bourin, M. Robert, and F. I. M. Craik, Eds., Advances in Psychological Science, vol. 2: Biological and cognitive aspects. Hove, England: Psychology Press .

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