How Many Memory Systems Are There?

" Memory is made up of a number of interrelated systems, organized structures of operating components consisting of neural substrates and their behavioral and cognitive correlates. A ternary classificatory scheme of memory is proposed in which procedural, semantic, and episodic memory constitute a "monohierarchical" arrangement: Episodic memory is a specialized subsystem of semantic memory, and semantic memory is a specialized subsystem of procedural memory. The three memory systems differ from one another in a number of ways, including the kind of consciousness that characterizes their operations. The ternary scheme overlaps with dichotomies and trichotomies of memory proposed by others. Evidence for multiple systems is derived from many sources. Illustrative data are provided by experiments in which direct priming effects are found to be both functionally and stochastically independent of recognition memory. Solving puzzles in science has much in common with solving puzzles for amusement, but the two differ in important respects. Consider, for instance, the jigsaw puzzle that scientific activity frequently imitates. The everyday version of the puzzle is determinate: It consists of a target picture and jigsaw pieces that, when properly assembled, are guaranteed to match the picture. Scientific puzzles are indeterminate: The number of pieces required to complete a picture is unpredictable; a particular piece may fit many pictures or none; it may fit only one picture, but the picture itself may be unknown; or the hypothetical picture may be imagined, but its component pieces may remain undiscovered. This article is about a current puzzle in the science of memory. It entails an imaginary picture and a search for pieces that fit it. The picture, or the hypothesis, depicts memory as consisting of a number of systems, each system serving somewhat different purposes and operating according to somewhat different principles. Together they form the marvelous capacity that we call by the single name of memory, the capacity that permits organisms to benefit from their past experiences. Such a picture is at variance with conventional wisdom that holds memory to be essentially a single system, the idea that "memory is memory." The article consists of three main sections. In the first, 1 present some pretheoretical reasons for hypothesizing the existence of multiple memory systems and briefly discuss the concept of memory system. In the second, I describe a ternary classificatory scheme of memory--consisting of procedural, semantic, and episodic memory--and briefly compare this scheme with those proposed by others. In the third, I discuss the nature and logic of evidence for multiple systems and describe some experiments that have yielded data revealing independent effects of one and the same act of learning, effects seemingly at variance with the idea of a single system. I answer the question posed in the title of the article in the short concluding section. P r e t h e o r e t i c a l C o n s i d e r a t i o n s Why Multiple Memory Systems? It is possible to identify several a priori reasons why we should break with long tradition (Tulving, 1984a) and entertain thoughts about multiple memory systems. I mention five here. The first reason in many ways is perhaps the most compelling: No profound generalizations can be made about memory as a whole, but general statements about particular kinds of memory are perfectly possible. Thus, many questionable claims about memory in the literature, claims that give rise to needless and futile arguments, would become noncontroversial if their domain was restricted to parts of memory. Second, memory, like everything else in our world, has become what it is through a very long evolutionary process. Such a process seldom forms a continuous smooth line, but is characterized by sudden twists, jumps, shifts, and turns. One might expect, therefore, that the brain structures and mechanisms that (together with their behavioral and mental correlates) go to make up memory will also reflect such evolutionary quirks (Oakley, 1983). April 1985 • American Psychologist Copyright 1985 by the American Psychological Association, Inc. 0003-066X/85/$00.75 Vol. 40, No. 4, 385-398 385 The third reason is suggested by comparisons with other psychological functions. Consider, for instance, the interesting phenomenon of blindsight: People with damage to the visual cortex are blind in a part of their visual field in that they do not see objects in that part, yet they can accurately point to and discriminate these objects in a forced-choice situation (e.g., Weiskrantz, 1980; Weiskrantz, Warrington, Sanders, & Marshall, 1974). Such facts imply that different brain mechanisms exist for picking up information about the visual environment. Or consider the massive evidence for the existence of two separate cortical pathways involved in vision, one mediating recognition of objects, the other their location in space (e.g., Mishkin, Ungerleider, & Macko, 1983; Ungerleider & Mishkin, 1982). I f "seeing" things--something that phenomenal experience tells us is clearly uni tary-i s subserved by separable neural-cognitive systems, it is possible that learning and remembering, too, appear to be unitary only because of the absence of contrary evidence. The fourth general reason derives from what I think is an unassailable assumption that most, if not all, of our currently held ideas and theories about mental processes are wrong and that sooner or later in the future they will be replaced with more adequate concepts, concepts that fit nature better (Tulving, 1979). Our task, therefore, should be to hasten the arrival of such a future. Among other things, we should be willing to contemplate the possibility that the "memoryi s -memory" view is wrong and look for a better alternative. The fifth reason lies in a kind of failure of imagination: It is difficult to think how varieties of learning and memory that appear to be so different on inspection can reflect the workings of one and the same underlying set of structures and processes. It is difficult to imagine, for instance, that perceptualEditor's note. This article is based on a Distinguished Scientific Contribution Award address presented at the meeting of the American Psychological Association, Toronto, Canada, August 26, 1984. Award addresses, submitted by award recipients, are published as received except for minor editorial changes designed to maintain American Psychologist format. This reflects a policy of recognizing distinguished award recipients by eliminating the usual editorial review process to provide a forum consistent with that employed in delivering the award address. Author's note. This work was supported by the Natural Sciences and Engineering Research Council of Canada (Grant No. A8632) and by a Special Research Program Grant from the Connaught Fund, University of Toronto. I would like to thank Fergus-Craik and Daniel Schacter for their comments on the article and Janine Law for help with library research and the preparation of the manuscript. Requests for reprints should be sent to Endel Tulving, Department of Psychology, University of Toronto, Toronto, Canada, M5S IA1. motor adaptations to distorting lenses and their aftereffects (e.g., Kohler, 1962) are mediated by the same memory system that enables an individual to answer affirmatively when asked whether Abraham Lincoln is dead. It is equally difficult to imagine that the improved ability to make visual acuity judgments, resulting from many sessions of practice without reinforcement or feedback (e.g., Tulving, 1958), has much in common with a person's ability to remember the funeral of a close friend. If we reflect on the limits of generalizations about memory, think about the twists and turns of evolution, examine possible analogies with other biological and psychological systems, believe that most current ideas we have about the human mind are wrong, and have great difficulty apprehending sameness in different varieties of learning and memory, we might be ready to imagine the possibility that memory consists of a number of interrelated systems. But what exactly do we mean by a memory