Investigating the Relationship Between Perceptual Categorization and Recognition Memory Through Induced Profound Amnesia

Are perceptual categorization and recognition memory subserved by a single memory system or by separate memory systems? A critical piece of evidence for multiple memory systems is that amnesics can categorize stimuli as well as normals but recognize those same stimuli significantly worse than normals (Knowlton & Squire, 1993). An extreme case is E.P., a profound amnesic who can categorize as well as normals but cannot recognize better than chance. This paper demonstrates that the paradigm used to test E.P. and other amnesics may be fundamentally flawed in that memory may not even be necessary to categorize the test stimuli in their paradigm. We "induced" profound amnesia in normals by telling them they had viewed subliminally presented stimuli that were never actually presented. Without any prior exposure to training stimuli, subjects' recognition performance was completely at chance, as expected, yet their categorization performance was quite good. Single Versus Multiple Memory Systems What processes are involved in judging whether an object belongs in a particular category (a categorization decision) and in judging whether an object is something that has been seen before (a recognition decision)? Formal theoretical accounts have suggested that both of these fundamental types of cognitive judgments are subserved by a single memory system. By contrast, many neuropsychological accounts have suggested that there are separate neural systems subserving categorization and recognition memory. This paper will briefly review the evidence for single memory systems and for multiple memory systems and then present recent experimental work that may challenge some of the critical evidence used to support the multiple memory systems view. Exemplar-based models, such as the Generalized Context Model (GCM; Nosofsky, 1986; see also Nosofsky & Palmeri, 1997; Palmeri, 1997), assume that both categorization and recognition rely on memory for stored instances but differ in the way that memory is probed. According to the GCM, categorization is based on the relative summed similarity of a probe item to the stored instances of the possible category responses whereas recognition is based on the absolute summed similarity of a probe item to all instFigure 1: Examples of a prototype, a low-distortion, a high-distortion, and a random pattern. ances stored in memory. In other words, categorization and recognition decisions rely on the same memories, but differ in the decision rules they use. Nosofsky (1988, 1991) has shown the GCM to provide excellent accounts of observed categorization and recognition data in a variety of experimental paradigms using normal individuals. Knowlton and Squire (1993) provided evidence for multiple memory systems by contrasting performance of amnesics and normal individuals on categorization and recognition memory tasks. They used a variant of the well known dot pattern classification and recognition paradigm (Posner & Keele, 1968). In a categorization task, amnesics and normals were initially exposed to forty high-level distortions of a prototype pattern (see Figure 1) without being told that the patterns belonged to the same category. At test, subjects were told that the patterns they had just seen were all members of the same category and were then asked to judge whether a new set of patterns were members or nonmembers of that category. Subjects were tested on the prototype, low distortions of the prototype, and high distortions of the prototype, which were all to be judged as members, and new random patterns, which were all to be judged as nonmembers. In a recognition task, amnesics and normals were initially exposed to five random patterns repeated eight times each without being told that they would later be tested on their memory for those patterns. At test, subjects were shown the five training patterns and five new random patterns and were asked to judged which were old and which were new. As shown in the top panel of Figure 2, Knowlton and Squire (1993) found that recognition memory was significantly impaired for amnesics compared to normals. However, no significant difference was observed between amne3URWRW\SH /RZý'LVWRUWLRQ 5DQGRP +LJKý'LVWRUWLRQ