Episodic Memory

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Psychologists have been studying memory experimentally since Hermann Ebbinghaus's (1885) groundbreaking work more than a hundred years ago, but only in the late twentieth century were questions raised about exactly what has been and is being studied in memory experiments. As a result of the pursuit of these questions it became widely if not universally accepted that there exist different kinds of memory. Episodic memory is one of these kinds.

The term episodic memory is used in several senses. One of these has to do with episodic memory as a particular class of laboratory tasks or experiments (Lockhart, 2000); another concerns episodic memory as a kind of mental capacity, or a neurocognitive system (Schacter and Tulving, 1994), that allows people to remember past experiences. Although closely related, the two senses (episodic tasks and the episodic system) should not be confused. This entry will consider the two senses in turn.

Episodic Memory Tasks

Episodic memory in the first sense manifests itself when a person remembers some information acquired on a particular occasion. Such situations occur frequently in real life where something happens at one time (Time 1) and the individual who witnessed the happening remembers it at a later time (Time 2). In the laboratory these situations are formalized as "tasks." A prototypical laboratory task of episodic memory consists of 1. an original study experience during which individual items, such as words, are encoded and stored by the learner (Time 1), and 2. a subsequent test during which some aspect of the experience is retrieved (Time 2). Episodic memory tasks are sometimes also referred to as explicit memory tasks.

Many variables affect performance on episodic memory tasks. They include ability differences among subjects, the type of information presented for study, the amount of time and effort devoted to learning, subjects' previous knowledge of the to-be-learned material, the length of the retention interval between study and test, and other such obvious factors. One important determinant of the rememberer's performance in episodic tasks includes the way he or she thinks about the material to be remembered as it is studied, the so-called encoding operations. or coding processes. Also important are the conditions under which retrieval (recovery of stored information) occurs or is attempted, and especially critical is the relation between encoding and retrieval conditions (Tulving, 1983). Performance on episodic memory tasks can be measured in a variety of ways—free or non-cued recall, cued recall, free choice or forced choice recognition, frequency judgment, and recency judgment, among others (Lockhart, 2000). Episodic memory tasks in which both study and test (encoding and retrieval) occur under fixed constant conditions are referred to as episodic memory tests and are widely used for the purpose of psychometric assessment of individuals' episodic memory abilities. The information that women outstrip men in episodic memory (Herlitz, Nilsson, and Bäckman, 1997) and that the earliest cognitive impairment in the functional development of Alzheimer's disease is episodic memory (Hodges, 2000), among other findings, is based on observations gleaned from episodic memory tests.

Episodic Memory System

The second sense of the term episodic memory is that of a hypothetical neurocognitive system that differs from the other major memory systems for which evidence exists (Schacter and Tulving, 1994). These other systems include semantic memory, procedural memory, short-term memory (also known as working memory), and the perceptual representation system that subserves perceptual priming (also known as implicit memory). Episodic memory is most closely related to semantic memory, and the two are usually regarded as subcategories of declarative memory. (Another closely related concept is autobiographical memory, which refers to recollection and knowledge of significant events from and facts about one's life.) The postulation of the existence of separable memory systems is part of the enterprise of the classification of natural phenomena of memory. Classification is a necessary prerequisite for the study of memory mechanisms and processes.

The ability of an individual to consciously recollect personally experienced past events, that is, to become aware again at Time 2 of some aspect or some part of a previous experience at Time 1, is possible only by virtue of an intact brain system specialized for that purpose, namely the episodic memory system. On the other hand, the ability to think about the world, and everything in it, which exists or is imagined as existing beyond immediate perception, depends on the integrity of the semantic memory system and does not require episodic memory (Tulving, 2001).

The defining features of episodic memory (system) are self, subjective time, and a special phenomenal awareness of remembering, familiar to all, that is referred to as autonoetic consciousness (Tulving, 2001). Episodic memory is unique among other memory systems in that it alone allows the individual to "mentally travel" through time, to remember the past and to think about the future. The evidence in support of a separable episodic system steadily increases. One source of relevant observations is the study of brain-damaged patients suffering from amnesia. Some brain-damaged patients who suffer from a severe memory disorder and are severely impaired in or completely lack episodic memory are nevertheless capable of acquiring, even if laboriously, new factual (semantic) knowledge (Hayman, MacDonald, and Tulving, 1992; Kitchener, Hodges, and McCarthy, 1998). This kind of dissociation between failure of remembering personally experienced events and success of learning new facts implies that the neural substrate of episodic and other kinds of memories are at least partially distinct. A related category of relevant evidence is source amnesia: Individuals with impaired or frail memories, such as amnesic patients and elderly people, can recall recently learned facts better than they can recollect the episode in the course of which they learned these facts (Shimamura and Squire, 1987).

A second major source of evidence is electrophysiological recording and functional neuroimaging of brain activity that is correlated with memory processes. When episodic and semantic memory retrieval are compared, the findings show not only similarities in brain activity but also differences (Dalla Barba et al., 1998; Nyberg, 1999; Tulving et al., 1994).

Relation Between Tasks and System

The two senses of episodic memory (task and system) are related but they cannot be equated. The main reason for this assertion lies in the fact that episodic tasks do not usually tap an individual's (autonoetic) awareness of self-centered experiences of the past but rather require only that the learner reproduce or otherwise indicate his or her knowledge of the semantic contents of the learning episode. Episodic memory of course can, and usually does, greatly contribute to this knowledge but it is not necessary for it. Putting it differently, episodic task performance does not only depend on the episodic system but can be supported by nonepisodic systems as well. In the extreme case, individuals without or with severely impaired episodic memory system are capable of dealing satisfactorily with many episodic memory tasks. In the laboratory, when the rememberer has been exposed to a set of study items at Time 1 and is then given a recognition test at Time 2, the rememberer's performance is influenced both by recollection of what happened at Time 1 (episodic system) and by feelings that certain test items are familiar (other systems). An extensive and rapidly expanding literature exists on the distinction between (episodic) "remembering" and (nonepisodic) "knowing" as indicants of processes involved in episodic tasks (Gardiner and Richardson-Klavehn, 2000). The feelings of familiarity enable neurological patients and other organisms whose episodic memory system is absent or severely impaired to make correct discriminations between previously encountered and previously nonencountered test items in an episodic recognition task (Vargha-Khadem et al., 1997).



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