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Home , Priming (psychology), Procedural memory, Semantic memory

130 and Systems Priming and Memory Systems and Daniel L. Schucter

Rcccnt research suggcsts that thc operations of human The procedural system is an action sydem: its memory are carried out by a number of different but operations are expressed in behavior, independently of closely interacting functional systems and subsystems, any . Skillful performance of perceptual-motor subserved by corrcsponding structures and tasks and conditioning of simple -response mechanisms. A good deal of current research effort is connections are examples of tasks that depend heavily directed at identification of these memory systems, on the system. description of their properties, and the analysis of the The other four are cognitive systems. They mediate relations among them. Dichotomies of classification that changes in cognition, or conscious experience. In the ' had been proposed earlier, short-term versus long-term course of normal, everyday activity, the computational memory, episodic versus , procedural outputs of the cognitive memory systems typically guide versus declarative memory, are being replaced with more overt behavior, but such conversion of cognition into comprehensive classificatory schemes. behavior is not necessary. Indeed, in the laboratory, the One such scheme for which there is some empirical products of cognitive memory systems are analyzed in support is summarized in Table 1. It includes fivc major the form of "pure" experience or thought, with and memory systems. The five are procedural behavioral expression serving merely as an indicator of memory, perceptual priming, short-term memory, properties of cognitive processes. semantic memory, and . Each of the Short-term memory (also referred to as primary five systems is large and complex, comprising a number memory) registers and retains incoming information for of subsystems for which evidence at the present stage of a period of time measured in seconds. It makes possible our knowledge is of variable quality. a lingering impression of thc individual's present The ordering of the major systems in the overall environment beyond the duration of the physical classification scheme corresponds roughly to their presence of the stimulus information emanating from the presumed developmental sequence, with the procedural environment. system the earliest and the episodic the latest. The The function of semantic memory is to model the ordering of the systems also reflects the conjectured external world. Semantic memory represents many relations among the systems: the operations of the higher aspects of the world; it renders possible acquisition and ones depend on and are supported by the operations of retention of factual knowledge in the broadest sense. the lower ones, whereas lower systems can operate Semantic memory, for instance, mediates knowledge of essentially independently of the higher. location of objects in the space beyond the reach of The scheme in Table I does not include primitive . Another example of the capability of forms of learning, such as sensitization and habituation, semantic memory is classification of objects, events, or because little is known about their relation to other situations (or symbolic descriptions of them) into higher forms of learning and memory in humans. But, somc order conceptual categories. widely used classificatory terms are shown. Thus, since Episodic memory depends on, but transcends, the semantic and episodic memory sharc a n~~niberof range of the capabilities of semantic memory. The features, they are frequently referred to collectively as function of episodic memory is to enable the individual declarative (or propositional or factual) memory. to remember experienced events as embedded in a matrix Another frequently used distinction is that between of other personal happenings in subjective timc. One's implicit and . de- recollections of something that happened 10 minutes signates the e.~pressionof stored information without ago, or the day before, or in the more distant past depend awareness of its acquisition coordinates in space and critically on short-term memory, semantic memory, and time, that is, expression of what the individual knows especially episodic memory. without necessarily remembering how, whcn, or where Perceptual priming (henceforth just "priming") is the the knowledge was acquired. Explicit memory, on the latest addition to the developing list of major categories other hand, refers to the expression of what the person of memory. It has been studied by neuropsychologists consciously remembers as a personal experience. and cognitive only for the past 10 years or so. The idea that it expresses the operations of a Table 1. Classification of Human Memory Systems memory system other than procedural, semantic, and Major systems Other terms episodic memory is of even more recent origin. Priming does not involve explicit recollection of recent Procedural memory Implicit memory events or episodes. It refers to a facilitative efTect of Perceptual priming (PRS) Trnpl~citmemory exposure to a word or object on subsequent identifica- Short-term memory Declarative Explicit memory tion of that word or object from reduced perceptual Scmantic memory Declarative Implicit memory cues. Priming is a type of nonconscious or implicit Episodic memory Dcclarativc Explicit mcmory memory, in the sense that exposure to an item during

Material may be protected by copyright law (Title 17, U.S. Code) Priming and Memory Syslems 131 a study episode can produce priming on a subsequent relative to nonstudied words. On the perceptual test even when people do not explicity remember the identification task, people try to identify a word or object itcm or the study episode in which it was previously from a brief (e.g., 50 msec) perceptual exposure; priming encountered. is indicated by more accurate identification of recently Priming has several interesting properties. One is that studied items than of new nonstudied items. .,. its operations are unconscious: people are not aware that their facility of identification of an object has benefited in any way from earlier exposures to the same Dissociations between priming and other forms of or similar objects. A second property of priming is that cognitive memory it is unrelated to explicit memory: experiments show An early and important indication that priming is that priming effects are as large for the stimulus items subserved by different processes from those underlying that people consciously remember having encountered explicit memory for recent events was provided by earlicr as they are for the stimulus items that they do studies of' brain-damaged patients with organic . not remember. Third, priming, as measured in some tests, Amnesic patients are characterized by a severe lasts over long intervals: a single presentation of a impairment in explicit memory for recent events, despite familiar word for a few seconds in a list of 100 other relatively normal intelligence, perception, and . words may result in priming cff'ects that can be readily This memory deficit is typically produced by lesions to detected more than I year later. either medial temporal or diencephalic brain regions. A number of studies have demonstrated, however, that amnesic patients show just as much priming as do The priming paradigm normal subjects on stem completion and other implicit To study memory systematically, experimental para- memory tasks, despite their inability to remember digms have been developed that allow relatively precise explicitly the targct items or the study episodc in which estimates to be made about the nature and characteristics the items were encountered. Thus, the memory processes of various memory processcs. In most of the traditional that subserve priming are apparently supported by a explicit memory paradigms, memory is assessed with a different brain system from the processes ~lnderlying three-stage procedure: a) a sfudy phase in which people explicit memory for a recent event, because normal are exposed to a of target materials, such as a list priming can occur in the absence of explicit memory. of words or pictures, b) a rctcntion interval. which Although the dissociation between priming and typically lasts for several minutes or hours, during which explicit memory is highlighted most dramatically by people perform tasks unrelated to the study phase, and studies of amnesic patients, similar dissociations have c) a resr phuse in which people are asked to think back becn produced in experiments with normal subjects who to the study phase and either produce the target do not have any brain damage or memory deficits. One materials () or discriminate thc targets from items particularly important dissociation has becn observed in that were not presented during the study phase experimcnts that have compared the effects of semantic (recognition). Experimental studies that have,adhered to and nonsemantic study tasks on priming and cxplicit this basic procedure have provided a good deal of insight memory. It has been well established that performance into the processes involved in conscious or explicit on explicit recall and recognition tests is higher after memory for recent events. semantic that nonsemantic study of an itcm. For The typical priming experiment also comprises three cxample, when subjccts perform a semantic stages. The first two are identical with explicit memory task during the study phase of an experiment (e.g., rate paradigms. It is during the test phase that the critical the pleasantness of a word or provide a definition ol a difference between priming and explicit memory word), subsequent probability of explicitly remembering experiments occurs. Instead of being asked to try to the word is much higher than if subjects perform a remember previously studied items, people are simply nonsemantic encoding task during the study phase (e.g.. instructed to perform a task that typically involves count the number of vowels and consonants in the word). idcntification of a stimulus on the basis of partial or In contrast, however, it has bcen shown that the reduced cues; no reference is made to the prior study magnitude of priming on a stem completion task or episode. Some of the test cues represent new or perceptual identification task is unaflected by type of nonstudied items that had not bcen presented during study task: priming effects are equivalent after semantic the study phase. Priming is said to occur when the and nonsemantic encoding. probability of identifying previously studied items is The finding that priming is unaffected by semantic increased, or the time required to identify them is versus nonsemantic encoding manipulations that greatly decreased, relative to the accuracy or the latency of influence explicit memory was reported initially in idcntification of nonstudied items. experiments that used familiar words as targct materials. For example, on stem completion tasks, people are Recently, the same pattern of results has been observed asked to complete word stems or fragments (for example, in studies with visual objects. For example, in one TAB-) with the first word that comes to mind (e.g., experiment, subjects were shown line drawings of TABLE); priming is inferred from an enhanced tendency common objects during the study phase, and perrormed to complete the stems with previously studied words either a semantic encoding task (think of functions that

Material may be protected by copyright law (Title 17, U.S. Code) 132 Priming and Memory Systems

the object can be used to perform) or a nonsemantic explicit memory are based on newly created memory task (count the number of vertices in the object). Priming representations in the episodic or declarative memory was later assessed with an object complction test in system, and that experimental dissociations can be which subjects were shown perceptual fragments of attributed to the differing demands of the tests used to studied and nonstudied objects, and provided the first assess priming and explicit memory, respectively. object that came to mind in response to the perceptual Specifically, it is held that most standard-explicit memory fragment. The magnitude of priming on this test was tests require a good deal of conceptually driven identical after the semantic and nonsemantic encoding processing: semantically based, subject-initiated at- tasks, even though explicit memory for studied objects tempts to recollect the study episode. By contrast, was considerably higher after the semantic study task performance on such implicit memory tests as word than the nonsemantic study task. completion and perceptual identification is held to be Another important differencc between priming and largely dependent on data-driven processing; processing explicit memory is that priming is frequently influenced that is determined largely by the physical characteristics more by the physical form of a stimulus than is explicit of test cues. It thus follows that explicit memory, but memory. For example, priming on such visual implicit not priming, should benefit from semantic study tests such as perceptual identification, stem completion, processing (which is thought to support conceptually and fragment completion can be reduced or eliminated driven processing) more than nonsemantic study by presenting target words auditorily, thereby indicating processing, whereas priming should be more dependent that the phenomenon is modality-specific. Explicit than explicit memory on matching of surface features memory, by contrast, tends to be lcss affected by such between study and test. However, a major problem for studyjtest modality shifts. Therc is also some evidence this view is that it does not provide a satisfactory that priming can be reduced by changing the precise explanation of why priming is preserved in amnesic surface features of an item between study and test (e.g., patients. if a word is studied in handwritten form and tested in To accommodate the data on amnesic patients, a typewritten form, there may be less priming than if the number of investigators have proposed that priming word is studied and tested in the same surface form). depends on a memory system that is neuroanatomically The foregoing does not constitute an exhaustive list distinct from the limbic systcm structures (most of dissociations between priming and explicit memory. importantly, the ) that are damaged in For example, evidence also exists that priming is not amnesia and that are necessary for explicit memory. We influenced by presentation of interfering material during reccntly developed a version of this hypothesis that holds a retention interval, even though explicit memory is that priming depends on a perceptual representation highly sensitive to such inteference effects; and several system (PRS): a presemantic system comprising several studies have shown that over time can proceed interacting subsystems that handles information about differently for priming and explicit memory. Another the form and structure, but not the meaning, of words difference is that priming is unaffected by drug and objects. treatments that influence the expression of other forms Under some experimental conditions, access to PRS of cognitive memory. As would be expected, a number is hyperspecific in that small variations in cues may of similarities have also been observed between priming greatly influence accessibility of thc representations in and explicit memory, but the differences are most PRS. Another important feature is that PRS reaches its revealing theoretically and have led to a variety of' optimal functional development before other cognitive proposals concerning the nature of priming. memory systems do. Independent evidence for the existence of PRS has been provided by neuro- psychological studies of patients with certain types of reading and perceptual deficits. These studies have Theoretical accounts of priming shown that for both words and objects, access to One theoretical position holds that priming is attribut- semantic and structural knowledge can be dissociated, able to the temporary activation of preexisting units or thus suggesting that the two kinds of knowledge are nodes in memory: exposure to a word or subject represented separately. In addition, automatically activates a memory representation of it, studies of lex~cal processing using the technique of and this activation then subsides rather rapidly. positron emission tomography lead to thc same Although able to accommodate some experimental conclusion. 30th the neurophysiological and neuroima- results, this general idea has diffic~~ltyaccounting for the ging data suggest that the neural locus PRS involves finding that priming can be surprisingly long-livcd posterior cortical regions, including extrastriate occipital ---undercertain conditions, lasting for days and weeks. cortex and regions of temporal cortex. It also fails to explain the fact that priming has been In view of the previously discussed evidence that observed after exposure to unfamiliar materials, such as priming is independent of semantic study processing and nonsense words or novel patterns and objects that do dependent on structural information, it makes sense t, not have any preexisting representation in memory as hypothesize that a presemantic system such as PRS 1 a unit. critically involved in priming. In addition, there is n. A rather different view holds that both priming and reason to believe that PRS is impaired in amnesic

Material may be protected by copyright law (Title 17, U.S. Code) Priming and Memory Systems 133 patients, so the fact that amnesics generally show intact Further reading priming can be accommodated. Musen G, Treisman A (1990): Implicit and explicit memory It must be noted and emphasized, however, that the for visual patterns. J E.up Psjchol 16:127-137 notion that priming is a presemantic phenomenon Richardson-Klavehn A, Bjork RA (1988): Measures of applies only to priming effects that are observed on such memory. Annu Rev Psychol 36: 475-543 data-driven tests as word completion and perceptual Roediger HL, Wcldon MS, Challis B (1989): ~i~laining identification. Priming in both normal subjects and dissociations between implicit and explicit measures of amensic patients has also been observed on implicit retention: A processing account. In: Varieries of rnetnory memory tests that are primarily conceptually driven, and : Essay.r in honor of Endel Tulving, such as producing members of a category in response Roediger HL, Craik FIM, eds. Hillsdale, NJ: Lawrence to a category name. According to the PRS hypothesis, Erlbaum Associates Schacter DL (1987): Implicit memory: history and current priming on conceptually driven tests has a different basis status, J Exp Psychol 13: 501-5 18 than does data-driven priming, and likely depends on Schacter DL (1990): Perceptual representation systems semantic memory. and implicit memory: Toward a resolution of the A great deal remains to be learned about priming; in multiple memory debate. In: Derelopmenr and Neural particular, there is little direct evidence concerning the Bask o$ Hiqher Cognitive Function. Diamond A! ed. New neural basis of the phenomenon. Nevertheless, the York Academy of Sciences, 608:543-571 discovery and exploration of priming phenomena have Squire LR (1987): Mernory and Bruin. New York: Oxrord already opened up some new vistas for memory research, University Press and it seems likely that further empirical study and Tulving E, Schacter DL (1990): Priming and human theoretical analysis of priming will continue to pay mcmory systems, Science 247:301-306 Weiskranti! L (1987): and handsome dividends in the future. amnesia: A case for multiple memory systems. Humun Neuvobiol 6:93-105

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