A rule-based categorization deficit in Alzheimer's disease?

This study examined the categorization processes that Alzheimer`s disease ( AD) patients use during assessments of semantic memory. Rule-based categori zation involves the careful, analytic processing of strict criteria ro dete rmine category membership, particularly for items from graded categories wi th ambiguous category membership; similarity-based categorization requires an overall comparison of a test stimulus with a prototype or remembered exe mplar of the category and is relatively effective for the rapid categorizat ion of items with unambiguous category membership. To assess these processe s in AD. patients were asked to decide the category membership of test stim uli for categories with poorly defined or Fuzzy boundaries (e.g.. VEGETABLE ) and for categories with well-defined boundaries (e.g., FEMALE) and then r o judge the representativeness of the test stimulus for its chosen category . A subgroup of AD patients demonstrated a typical pattern of impaired sema ntic memory compared to healthy control subjects; that is, difficulty decid ing the category membership of rest items from furry categories. Among thes e patients. we Found no deficit in category membership decisions about item s taken from well-defined categories. We also found that AD patients and he althy controls do not differ in their representativeness judgments of items within a correctly judged category. These findings are most consistent wit h the hypothesis that rule-based categorization difficulty limits semantic memory in AD. (C); 2001 Academic Press.