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Abstract:
Differences in the categorization abilities of subjects with
Parkinson's disease (PD) and medial temporal lobe amnesia have
implications for our understanding of memory systems that underlie
various types of categorization. Both PD patients (Reber et al.,
1998) and amnesics (Knowlton & Squire, 1993) have been shown to
learn perceptual categories of distortions of dot patterns just as
well as normal individuals. However, behavioral dissociations have
been reported in probabilistic categorization tasks with stimuli
composed of individuated cues. Early in learning
probabilistically-defined categories, amnesics and normal
individuals perform similarly but PD patients perform much worse
(Knowlton et al., 1994, 1996); later in learning, both PD patients
and amnesics perform worse than normals. Our goal was to build on
this corpus of behavioral studies investigating the neural basis of
these different kinds of categorization tasks. We present a series
of fMRI studies of normal individuals in which we identified areas
of brain activation associated with perceptual categorization and
probabilistic categorization. One set of studies examined a single
type of categorization task in isolation. Another set of studies
examined both kinds of categorization tasks interleaved in the same
fMRI session, thus allowing for direct comparisons of shared and
separate brain activations in the two types of categorization
tasks. Our findings are related to other recent imaging studies of
the neural basis of categorization (e.g., Poldrack et al., 1999;
Reber et al., 1998).
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