Abstract
Previously we found that Parkinson's disease (PD) patients are impaired in procedural-based category learning when category membership is defined by a nonlinear relationship between stimulus dimensions, but these same patients are normal when the rule is defined by a linear relationship (Maddox and Filoteo, 2001; Filoteo et al., 2005a,b). We suggested that PD patients' impairment was due to a deficit in recruiting “striatal units” to represent complex nonlinear rules. In the present study, we further examined the nature of PD patients' procedural-based deficit in two experiments designed to examine the impact of (1) the number of categories, and (2) category discontinuity on learning. Results indicated that PD patients were impaired only under discontinuous category conditions but were normal when the number of categories was increased from two to four. The lack of impairment in the four-category condition suggests normal integrity of striatal medium spiny cells involved in procedural-based category learning. In contrast, and consistent with our previous observation of a nonlinear deficit, the finding that PD patients were impaired in the discontinuous condition suggests that these patients are impaired when they have to associate perceptually distinct exemplars with the same category. Theoretically, this deficit might be related to dysfunctional communication among medium spiny neurons within the striatum, particularly given that these are cholinergic neurons and a cholinergic deficiency could underlie some of PD patients' cognitive impairment.
Highlights
Results of this analysis identified a main effect of group, F(1, 18) = 6.68, p < 0.05, η2p = 0.27, with Parkinson’s disease (PD) patients performing worse than normal controls (NC) participants overall, and a main effect of block, F(4, 72) = 11.06, p < 0.001, η2p =
If the category clusters from the discontinuous condition were from four different continuous categories, as opposed to two discontinuous categories, we would predict that PD patients would be normal
Note this is the first study in which we found PD patients to be impaired in learning a linear procedural-based rule, arguing against the surface-level explanation that PD patients’ deficits in category learning are due to the linearity of the rule
Summary
It is widely accepted that there are multiple category learning systems (Ashby et al, 1998, 2010; Smith et al, 1998, 2012; Ashby and Maddox, 2005, 2011) and that different neural systems play different roles in these systems (Knowlton et al, 1994, 1996; Poldrack et al, 1999; Ashby and Ell, 2001; Filoteo et al, 2001a,b, 2005a,b; Patalano et al, 2001; Keri, 2003; Reber et al, 2003; Shohamy et al, 2004a,b; Maddox et al, 2005a,b; Cincotta and Seger, 2007; Nomura et al, 2007; Price et al, 2009; Waldschmidt and Ashby, 2011). One of the more interesting, and potentially important lines of research in this area is the study of how some categories can be acquired without conscious awareness This phenomenon, often referred to as procedural-based category learning, occurs when participants learn complex categorization rules, and despite highly accurate learning, they are unable to describe explicitly why any given exemplar belongs to a specific category. The behavioral mechanisms of procedural-based category learning have received much attention in several recent studies with normal individuals (Gluck et al, 2002; Maddox and Ashby, 2004; Ashby and Maddox, 2005, 2011; Ashby and O’Brien, 2005). The number of categories to be learned does not differentially impact long-run accuracy in procedural-based category learning, but increasing the number of categories impedes the learning of explicit category rules (Maddox et al, 2004a,b)
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