This study examined the impact of irrelevant dimensional variation on
rule-based category learning in patients with Parkinson's disease
(PD), older controls (OC), and younger controls (YC). Participants were
presented with 4-dimensional, binary-valued stimuli and were asked to
categorize each into 1 of 2 categories. Category membership was based on
the value of a single dimension. Four experimental conditions were
administered in which there were zero, 1, 2, or 3 randomly varying
irrelevant dimensions. Results indicated that patients with PD were
impacted to a greater extent than both the OC and YC participants when the
number of randomly varying irrelevant dimensions increased. These results
suggest that the degree of working memory and selective attention
requirements of a categorization task will impact whether PD patients are
impaired in rule-based category learning, and help to clarify recent
discrepancies in the literature. (JINS, 2005, 11,
503–513.)