Implicit Memory: History and Current Status
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Journal of Experimental Psychology: Copyright 1987 by the American PsychologicalAssociation, Inc. Learning, Memory,and Cognition 0278-7393/87/$00.75 1987, Vol. 13, No. 3. 501-518 CRITICAL REVIEW Implicit Memory: History and Current Status Daniel L. Schacter University of Toronto, Ontario, Canada Memory for a recent event can be expressed explicitly, as conscious recollection, or implicitly, as a facilitation of test performance without conscious recollection. A growing number of recent studies have been concerned with implicit memory and its relation to explicit memory. This article presents an historical survey of observations concerning implicit memory, reviews the findings of contempo- rary experimental research, and delineates the strengths and weaknesses of alternative theoretical accounts of implicit memory. It is argued that dissociations between implicit and explicit memory have been documented across numerous tasks and subject populations, represent an important chal- lenge for research and theory, and should be viewed in the context of other dissociations between implicit and explicit expressions of knowledge that have been documented in recent cognitive and neuropsychologicalresearch. Psychological studies of memory have traditionally relied on onstrated a variety of striking dissociations between implicit tests such as free recall, cued recall, and recognition. A promi- and explicit memory and has shown that under certain condi- nent feature of these tests is that they make explicit reference tions, implicit and explicit memory can be entirely independent to, and require conscious recollection of, a specific learning epi- of one another. These observations have raised fundamental sode. During the past several years, however, increasing atten- questions concerning the nature and composition of memory, tion has been paid to experimental situations in which informa- questions that will have to be addressed by any satisfactory the- tion that was encoded during a particular episode is subse- ory of memory. The purposes of this article are to present an quently expressed without conscious or deliberate recollection. historical survey of observations concerning implicit memory, Instead of being asked to try to remember recently presented to review modern experimental studies and theoretical analy- information, subjects are simply required to perform a task, ses, with particular emphasis on recent work in cognitive psy- such as completing a graphemic fragment of a word, indicating chology and neuropsychology, and to suggest directions for fu- a preference for one of several stimuli, or reading mirror-in- ture research. verted script; memory is revealed by a facilitation or change in Before the historical survey is initiated, two points regarding task performance that is attributable to information acquired the terms implicit and explicit memory should be clarified. during a previous study episode. Graf and Schacter (1985, First, I use these terms in the manner suggested by Graf and 1987; Schacter & Graf, t986a, 1986b) have labeled this type Schacter (1985). Implicit memory is revealed when previous of memory implicit memory, and have used the term explicit experiences facilitate performance on a task that does not re- memory to refer to conscious recollection of recently presented quire conscious or intentional recollection of those experiences; information, as expressed on traditional tests of free recall, explicit memory is revealed when performance on a task re- cued recall, and recognition. quires conscious recollection of previous experiences. Note that Recent cognitive and neuropsychological research has dem- these are descriptive concepts that are primarily concerned with a person's psychological experience at the time of retrieval. Ac- cordingl3,, the concepts of implicit and explicit memory neither This article was supported by a Special Research Program Grant refer to, nor imply the existence of, two independent or separate from the Connaught Fund, University of Toronto, and by the Natural memory systems. The question of whether implicit and explicit Sciences and Engineering Research Council of Canada Grant U0361. memory depend on a single underlying system or on multiple I am indebted to numerous colleagues for comments on an earlier underlying systems is not yet resolved, as will be discussed later draft of the article, and I thank E. Eich, P. Gabel, E. Glisky, P. Graf, L in this article. Second, the term implicit memory resembles two Jacoby, M. Johnson, J. Kihlstrom, G. Mandler, M. P. McAndrews, more familiar terms from the psychological literature: uncon- M. J. Nissen, A. Parkin, H. Roediger, T. Shallice, A. Shimamura, E. scious memory (e.g., Freud & Breuer, 1966; Prince, 1914) and Tulving, A. Young, and the students in H. Roediger's graduate seminar unaware memory or memory without awareness (e.g., Eriksen, for their helpful suggestions. I also thank C. Macdonald for excellent translations of material in French and for valuable aid with preparation 1960; Jacoby & Witherspoon, 1982). These two terms have of the manuscript. been used to describe phenomena that will be referred to here Correspondence concerning this article should be addressed to Dan- with the term implicit memory. The main reason for adopting iel L. Schacter, who is now at the Department of Psychology, University implicit memory in favor of either unconscious memory or un- of Arizona, Tucson, Arizona 85721. aware memory has to do with the conceptual ambiguity of the 501 502 DANIEL L. SCHACTER latter two terms. The terms unconscious and unaware have a voluntary recollection, but both of these concepts were used in large number of psychological meanings and implications (e.g., reference to explicit memory phenomena. Bowers, 1984; Ellenberger, 1970; Eriksen, 1960), many of The first philosopher after Leibniz to systematically discuss which do not apply to the phenomena of interest here. Although phenomena of implicit memory was a French philosopher the term implicit is not entirely free of conceptual ambiguity, it known by the surname Maine de Biran. Though virtually un- is less saturated with multiple and possibly misleading mean- known today, he published an important treatise in 1804 enti- ings than are unconscious or unaware. tled The Influence of Habit on the Faculty of Thinking (Maine de Biran, 1929). Like others before him, Maine de Biran be- Implicit Memory: An Historical Survey lieved that the analysis of habit was central to an understanding of human thought and behavior. Unlike others, however, Maine This section considers ideas and observations concerning im- de Biran elucidated a feature of habit that had not been dis- plicit memory contributed by philosophers, psychologists, neu- cussed previously in philosophical or scientific analyses: After rologists, psychiatrists, and others from the 17th century until sufficient repetition, a habit can eventually be executed auto- the middle of the 20th century. Unless otherwise stated, these matically and unconsciously without awareness of the act itself investigators did not actually use the term implicit memory in or of the previous episodes in which the habit was learned. their writings. They did, however, describe and discuss situa- Thus, he observed that repeated actions are eventually executed tions in which memory for recent experiences was expressed in with "such promptitude and facility that we no longer perceive the absence of conscious recollection. I sometimes use the the voluntary action which directs them and we are absolutely phrase implicit memory phenomena in reference to these obser- unaware of the source that they have" (p. 73). The most striking vations. This is done purely for purposes of expositional clarity feature of Maine de Biran's system, however, was his delineation and should not be seen as an attempt to put present concepts and detailed discussion of three different types of memory: me- in the minds of past observers. chanical, sensitive, and representative. The first two types are driven by habit and are involved in the largely unconscious or Philosophical Analyses: Descartes, Leibnitz, implicit expression of repeated movements (mechanical) and and Maine de Biran feelings (sensitive); the third type (representative) is involved in conscious recollection of ideas and events (pp. 156-157). Thus, It is widely recognized that both Plato and Aristotle com- according to Maine de Biran, mented extensively about the nature of memory, but both ap- pear to have been concerned exclusively with explicit memory. If signs [in Maine de Biran's system, a sign is a motor response code] are absolutely empty of ideas or separated from every repre- During the Middle Ages, St. Augustine and St. Thomas Aquinas sentative effect, from whatever cause this isolation may arise, recall had a great deal to say about explicit retrieval and search pro- is only a simple repetition of movements. I shall call this faculty for cesses, but I have not found any discussion of implicit memory it mechanicalmemory. When the.., recall of the sign is accompa- in their writings. nied or immediately followed by the clear appearance of a well cir- cumscribed idea, I shall attribute to it representativememory, If the The first clear reference to an implicit memory phenomenon sign expresses an affective modification, a feeling or even a fantastic appears to have been made by Descartes in his 1649 The Pas- image whatsoever, a vague, uncertain concept, which cannot be sions of the Soul (cited by Perry