Category Learning in HD 1 In Press, Behavioral Neuroscience (3/01) A Possible Role of the Striatum in Linear and Nonlinear Category Learning: Evidence from Patients with Huntington’s Disease

Linear and nonlinear categorization rule learning was examined in patients with Huntington’s disease (HD) and controls using the perceptual categorization task. In the linear condition, HD patients displayed an early-training deficit, whereas later in training, they were not statistically different from controls. In the nonlinear rule condition, HD patients displayed both an earlyand late-training deficit. Quantitative model-based analyses revealed that the HD patients’ deficits in the linear condition were due to an impairment in learning the experimenter-defined rule and not in applying a learned rule inconsistently. In contrast, in the nonlinear condition, the HD patients’ deficits were due to an impairment both in learning the experimenter-defined rule and in applying a learned rule consistently. Overall, these results suggest that HD can result in a deficit in learning both linear and nonlinear categorization rules. The neurobiological underpinnings of memory have been the focus of intense research for the last thirty years. A major finding from this work has been that the medial temporal system and the diencephalon play a critical role in the learning of new information. It is well known that patients with damage to these brain regions are impaired in consciously recalling newly learned facts or events (Gabrieli, 1998; Scoville & Milner, 1957; Squire, Knowlton, & Musen, 1993). In contrast, the medial temporal and diencephalon do not appear to be involved in other aspects of memory, including motor learning (Brooks & Baddeley, 1976; Nissen & Bullemer, 1987), perceptual learning (Cohen & Squire, 1980), perceptual adaptation (Benzing & Squire, 1987), and some aspects of classical conditioning (Gabrieli et al., 1995). The finding that patients with damage to the medial temporal lobe and diencephalic structures are impaired in some aspects of memory but not others has given rise to the notion that there are different forms of memory (Schacter, 1987; Squire et al., 1993; Verfaellie, & Keane, 1997). A relatively recent endeavor in the study of the neurobiological bases of memory has been to identify the structures that support those memory processes that are not mediated by the medial temporal lobe or diencephalon. Studies examining patient with focal brain lesions, for example, have implicated the visual cortex in perceptual priming (Gabrieli et al., 1995; Keane, Gabrieli, Mapstone, Johnson, & Corkin, 1 J. Vincent Filoteo, Department of Psychiatry, University of California, San Diego and Veterans Affairs Medical Center, San Diego; W. Todd Maddox, Department of Psychology, University of Texas; Jennifer D. Davis, Department of Psychology, University of Utah. Special thanks go to Greg Ashby for many helpful and stimulating discussions, and to Richard Ivry and one anonymous reviewers for helpful comments on an earlier version of this