Memory (General)

Introduction

By a simple definition, memory is the capability of animals to acquire, retain, and make use of knowledge and skills. Since the early 1980s, the way that cognitive scientists think about memory has dramatically changed. Today, memory is more often viewed not as a unitary entity but as comprising different components or systems. Neurocognitive research has indicated that it is more appropriate to consider the human memory as a collection of multiple but closely interacting systems than as a single and indivisible complex entity (e.g. Tulving, 1985a; Squire, 1992; see also Schacter & Tulving, 1994a, for current perspectives). Different memory systems differ from one another in terms of the nature of representations they handle, the rules of their operations, and their neural substrates (e.g. Tulving, 1984; Weiskrantz, 1990; Tulving & Schacter, 1992; Schacter & Tulving, 1994b; Willingham, 1997).

Various classificatory schemes of human memory have been proposed so far. Undoubtedly, the two most influential and extended classifications are those postulated by Squire (1992) and Schacter and Tulving (1994a). Squire distinguishes two long-term memory systems: declarative and non-declarative (or procedural) memory; whereas Schacter and Tulving identify five major systems: procedural memory, perceptual representation system, semantic memory, short-term working memory and episodic memory. Related distinctions include explicit versus implicit memory, direct versus indirect memory, and memory with awareness versus memory without awareness. However, these latter dichotomies may not be memory systems, but rather forms of expression of memory. According to the Schacter and Tulving classification, retrieval operations in the procedural, perceptual representation and semantic systems are implicit, whereas in the working memory and episodic memory they are explicit. On the other hand, Squire considers declarative memory as an explicit system, whereas non-declarative memory is viewed as a heterogeneous collection of implicit abilities (Squire & Knowlton, 2000).

Explicit and Implicit Memory

Compelling evidence for the existence of multiple memory systems is provided by experimental findings of numerous convergent dissociations (functional, developmental, pharmacological, neuropsychological, neuroanatomical) between tasks of explicit and implicit memory (for reviews, see Schacter, 1987; Ruiz-Vargas, 1993; Nyberg & Tulving, 1996; Schacter, Wagner & Buckner, 2000). The original distinction between explicit and implicit memory was made by Graf and Schacter (1985). Explicit memory is revealed by intentional or conscious recollection of specific previous information, as expressed on traditional tests of free recall, cued recall and recognition. Implicit memory is revealed by a facilitation or change of performance on tests that do not require [Page 570]intentional or conscious recollection, such as perceptual identification, word stem completion, lexical decision, identification of fragmented pictures, mirror drawing, and so on.

Consider these two experimental situations: (1) A list of 20 familiar words is presented to subjects who are instructed to pay attention to each word because, after the presentation, they will be asked to reproduce as many of the presented words as possible. (2) A list of 20 familiar words is also presented to subjects who are instructed to perform an orienting task (e.g. pleasantness ratings). After this study phase, the subjects will be asked to say the first word that comes to mind in response to a series of three-letter word stems. Obviously, some word stems can be completed with presented words, and some cannot. The first experimental situation reflects one of the ways in which psychologists have traditionally measured human memory: by assessing deliberate or explicit memory of subjects for items studied in a specific learning episode with a recall test. In the second situation, it is often observed that subjects show an enhanced tendency to complete word stems corresponding to studied words in comparison to ‘new’ word stems. This phenomenon is known as repetition priming or perceptual priming.1 Priming does not involve intentional or explicit recollection of the study episode, and thus it is assumed to reflect implicit memory for previously acquired information.

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