Understanding implicit memory: A cognitive neuroscience approach.

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  • Understanding Implicit MemoryA Cognitive Neuroscience Approach

    Daniel L. Schacter Harvard University

    Dissociations between implicit and explicit memory haveattracted considerable attention in recent memory re-search. A central issue concerns whether such dissociationsrequire the postulation of separate memory systems orare best understood in terms of different processes oper-ating within a single system. This article presents a cog-nitive neuroscience approach to implicit memory in gen-eral and the systems-processes debate in particular, whichdraws on evidence from research with brain-damaged pa-tients, neuroimaging techniques, and nonhuman primates.The article illustrates how a cognitive neuroscience ori-entation can help to supply a basis for postulating memorysystems, can provide useful constraints for processingviews, and can encourage the use of research strategiesthat the author refers to as cross-domain hypothesis testingand cross-domain hypothesis generation, respectively. Thecognitive neuroscience orientation suggests a comple-mentary role for multiple systems and processing ap-proaches.

    In the introduction to an excellent review of memory andamnesia research, Rozin (1976) wistfully remarked, "Ifind myself wishing that I were writing this paper a littleless than a hundred years ago, in 1890, at the close of adecade that I would consider the golden age of memory"(p. 3). Considering the lasting achievements of that de-cadeEbbinghaus's pioneering experiments, Ribot'sobservations on disorders of memory, Korsakoff's de-scription of the amnesic syndrome that now bears hisname, and William James's (1890/1983) superb chapterson memory in the epic The Principles of PsychologyRozin's characterization is highly appropriate.

    It is too early to say whether future writers willsomeday look back on the decade of the 1980s as anothergolden age of memory. Nevertheless, it already seems clearthat the 1980s will be viewed as a golden age, or at leastthe beginning of a golden age, for one issue in memoryresearch: the investigation of implicit memory (Graf &Schacter, 1985; Schacter, 1987). Implicit memory is anunintentional, nonconscious form of retention that canbe contrasted with explicit memory, which involves con-scious recollection of previous experiences. Explicitmemory is typically assessed with recall and recognitiontasks that require intentional retrieval of informationfrom a specific prior study episode, whereas implicitmemory is assessed with tasks that do not require con-scious recollection of specific episodes.

    Although the explicit-implicit distinction was in-

    troduced during the 1980s, the sort of contrast that itcaptures is not new; related distinctions between con-scious and unconscious memories, to take just one ex-ample, have been around for more than a century (forhistorical considerations, see Roediger, 1990b; Schacter,1987). The critical development during the past decadehas been the systematic demonstration, exploration, andattempted explanation of dissociations between explicitand implicit memory. Some of these dissociations havebeen provided by experiments demonstrating that brain-damaged amnesic patients with severe impairments ofexplicit memory can exhibit intact implicit memory;others come from studies showing that specific experi-mental variables produce different and even opposite ef-fects on explicit and implicit memory tasks (for reviews,see Richardson-Klavehn & Bjork, 1988; Roediger, 1990b;Schacter, 1987;Shimamura, 1986). Fueled by these strik-ing and frequently counter-intuitive dissociations, thestudy of implicit memory emerged from the decade ofthe 1980s at the forefront of memory research.

    In this article I outline a general research strategyfor attempting to understand implicit memory that I referto as a cognitive neuroscience approach. This approachis motivated by the general idea that it is useful to combinecognitive research and theory, on the one hand, with neu-ropsychological and neurobiological observations aboutbrain systems, on the other, making use of data frombrain-damaged patients, neuroimaging techniques, andeven lesion and single-cell recording studies of nonhumananimals. The cognitive neuroscience orientation has itselfundergone rapid development during the past decade and

    Editor's note. Articles based on APA award addresses that appear in theAmerican Psychologist are scholarly articles by distinguished contributorsto the field. As such, they are given special consideratioin in the AmericanPsychologist's editorial selection process.

    This article was originally presented as a Distinguished ScientificAward for an Early Career Contribution to Psychology address at the99th Annual Convention of the American Psychological Association inSan Francisco in August 1991.

    Author's note. The work described in this article has been supportedby Air Force Office of Scientific Research Grant 90-0187, National In-stitute on Aging Grant 1 RO1 AG08441-01, and a grant from theMcDonnell-Pew Cognitive Neuroscience Program.

    I am grateful to Barbara Church, Lynn Cooper, and Steve Rapcsakfor their collaborative efforts on several of the experiments summarizedin the article. I thank Douglas Nelson and Henry L. Roediger for com-ments on an earlier draft of the paper and Dana Osowiecki for help withpreparation of the manuscript.

    Correspondence concerning this article should be addressed toDaniel L. Schacter, Department of Psychology, Harvard University, 33Kirkland St., Cambridge, MA 02138.

    April 1992 American PsychologistCopyright 1992 by the American Psychological Association, Inc. 0003-066X/92/$2.00Vo\. 47, No. 4, 559-569

    559

  • is now a major force in the study of perception, attention,language, and emotion (cf. Gazzaniga, 1984; Kosslyn,Flynn, Amsterdam, & Wang, 1990; LeDoux & Hirst,1986; Weingartner & Lister, 1991). A growing numberof investigators have adopted a cognitive neuroscienceapproach to the study of human memory (for a repre-sentative sampling, see Olton, Gamzu, & Corkin, 1985;Squire & Butters, 1984; Squire, Weinberger, Lynch, &McGaugh, 1992; for a historical review, see Polster, Nadel,&Schacter, 1991).1

    I will discuss the cognitive neuroscience orientationin relation to a major issue that has arisen in implicitmemory research: the debate between memory systemsand processing accounts of implicit-explicit dissociations.The former account holds that implicit memory effectsdepend on brain systems that are distinct from the mem-ory system that supports explicit remembering (cf. Cohen,1984; Hayman & Tulving, 1989; Keane, Gabrieli, Fen-nema, Growden, & Corkin, 1991; Schacter, 1990; Squire,1987; Tulving & Schacter, 1990; Weiskrantz, 1989); thelatter account holds that postulation of multiple memorysystems is neither necessary nor justified and that relevantdissociations can be understood in terms of relations be-tween processing operations carried out during study andtest (cf. Blaxton, 1989;Jacoby, 1983;Masson, 1989; Roe-diger, Weldon, & Challis, 1989).

    I suggest that a cognitive neuroscience orientationmay help to resolve, or at least guide the investigation of,several key issues in the systems versus processes debate.More specifically, I will discuss four important featuresof a cognitive neuroscience approach in relation to thisdebate: (a) It provides an empirical basis for postulatingmemory systems that is independent of dissociations ob-served in implicit-explicit memory experiments; (b) itaids development of well-specified systems views that cansuggest helpful constraints for processing approaches; (c)it encourages the use of cross-domain hypothesis testing;and (d) it also encourages the use of cross-domain hy-pothesis generation. I will illustrate each of these featureswith relevant examples from my own and others' labo-ratories.

    Basis for Postulating Memory SystemsAs noted earlier, interest in implicit memory has beenfueled by the observation of dissociations between tasksthat tap implicit and explicit memory, respectively. Con-sider, for example, the stem completion task, in whichsubjects are given three-letter word beginnings (e.g.,TAB) and are asked to complete them with the first wordthat comes to mind; no reference is made to a prior studyepisode. Implicit memory is indicated when subjectscomplete a stem more frequently with a word that wasrecently presented on a study list (e.g., TABLE) than witha word that was not presented on the list (e.g., TABLET);this facilitation of task performance is known as director repetition priming (e.g., Tulving & Schacter, 1990). Itis well-established that priming effects on the stem com-pletion task can be dissociated from explicit memory. Forinstance, as indicated initially by the classic studies of

    Warrington and Weiskrantz (1974), patients with organicamnesia can show normal priming effects on stem com-pletion performance, despite severely impaired explicitmemory (cf. Graf, Squire, & Mandler, 1984; Warrington& Weiskrantz). Dissociations between stem completionpriming and explicit memory have also been observedwith normal, nonamnesic subjects. One of the morecompelling phenomena involves the well-known depth-of-processing effect, which was initially established duringthe 1970s in studies of explicit memory (e.g., Craik &Tulving, 1975): Semantic study processing (i.e., thinkingabout the meaning of a word) generally produces muchhigher levels of subsequent recall and recognition perfor-mance than does nonsemantic study processing (i.e.,thinking about the physical features of a word). By con-trast, the magnitude of priming effects on the stem com-pletion task are little affectedand sometimes entirelyunaffectedby differences in depth of processing thatare produced by different study tasks (cf. Bowers &Schacter, 1990; Graf & Mandler, 1984). Study-test mo-dality shifts can also yield striking implicit-explicit dis-sociations: When a word is presented in one modalityduring the study task (i.e., auditory) and presented inanother during test (i.e., visual), stem completion primingeffects are reduced significantly, whereas explicit memoryperformance is little affected (Graf, Shimamura, & Squire,1985; Schacter & Graf, 1989).

    These kinds of dissociations are now familiar tomemory researchers, and a comparable list could bereadily constructed for various other implicit and explicittasks. The critical question for the present purposes con-cerns their relation to the multiple memory systems de-bate: Do dissociations between implicit and explicitmemory tasks constitute either a necessary or sufficientcondition for postulating that different memory systemssupport performance on the two types of task? Althoughdissociations clearly constitute a necessary condition formaking such claimsit would be difficult to argue con-vincingly for multiple memory systems in the absence ofany evidence that they operate differentlyit seemsequally clear that they do not constitute a sufficient con-dition. There are several reasons why one cannot makesimple leaps from empirical dissociations to postulationof memory systems (e.g., Dunn & Kirsner, 1988; Jacoby,1983), but perhaps the most compelling argument is re-lated to the apparent ubiquity of dissociations in memoryresearch. It has been known for many years that disso-ciations can be produced between explicit memorytasksrecall and recognition are prime examplesandit has been established more recently that dissociationscan be produced between implicit memory tasks (cf.

    1 Much of what is discussed in this article could just as easily be

    described with the phrase cognitive neuropsychology as with the phrasecognitive neuroscience. However, the term cognitive neuropsychology oftenconnotes a purely functional approach to patients with cognitive deficitsthat does not make use of, or encourage interest in, evidence and ideasabout brain systems and processes. Because I believe that neural con-straints can be important for cognitive theorizing, I use the term cognitiveneuroscience instead of cognitive neuropsychology.

    560 April 1992 American Psychologist

  • Blaxton, 1989; Witherspoon & Moscovitch, 1989). Thus,if we were to accept the idea that an empirical dissociationbetween, say, Explicit Task X and Implicit Task Y is alonesufficient to claim that different memory systems supportperformance on the two tasks, theoretical chaos wouldlikely ensue: A long list of explicit memory systems, tosay nothing of implicit memory systems, would be quicklycomposed (Roediger, 1990a). On the other hand, we havealready acknowledged that empirical dissociations con-stitute a necessary condition for postulating multiplememory systems. How, then, can we extricate ourselvesfrom the apparent impasse?

    I suggest that it is crucial to have a basis for postu-lating different memory systems that is independent ofdissociations observed in implicit-explicit memory ex-periments and that a cognitive neuroscience orientationcan help to provide it. If claims about memory systemsare supported by independent evidenceand are notmade simply in response to the latest experimental dis-sociation between implicit and explicit memory tasksthen the aforementioned theoretical chaos can be greatlyreduced by applying the logic of converging operations(Garner, Hake, & Eriksen, 1956).

    To illustrate the point concretely, consider a criticismof the multiple memory systems approach offered byRoediger (1990a, 1990b) and Blaxton (1989). These in-vestigators noted that one account of dissociations be-tween word completion and word identification tasks, onthe one hand, and recall and recognition tasks, on theother, is that priming effects are mediated by a semanticmemory system, whereas explicit remembering dependson an episodic memory system (e.g., Tulving, 1983). Incontrast, Roediger and Blaxton argued that both explicitand implicit memory are mediated by different types ofprocessing in a single (episodic) memory system. Specif-ically, they invoked the principle of transfer-appropriateprocessing (Morris, Bransford, & Franks, 1977), whichholds that memory performance depends on the extentto which processing operations performed during a studytask match or overlap with processing operations per-formed during a memory test. They suggested further, inconformity with previous suggestions by Jacoby (1983),that implicit tasks such as stem completion and wordidentification depend largely on data-driven processing(i.e., bottom-up processing that is driven primarily byperceptual properties of study and test materials), whereasexplicit tasks such as recall and recognition depend largelyon conceptually driven processing (i.e., top-down pro-cessing that is driven primarily by subject-initiated activ-ities such as elaborating and organizing). This generalposition allowed Roediger and Blaxton to account for thepreviously mentioned finding that semantic-elaborativestudy processing increases explicit but not implicit mem-ory, whereas changes in modality and other physical fea-tures of target stimuli can affect impl...

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