A property level analysis of lexical semantic representation in Alzheimer's disease.

In order to assess the hypotheses that Alzheimer's disease (AD) results in a property level restructuring, loss, or degradation of lexical-semantic knowledge, Alzheimer's patients and normal elderly subjects were presented with a property verification task in which they were asked to judge the truth value of telegraphic statements which paired objects with their properties (e.g., "Apple is red."). Objects with either high- or low-typical exemplars of categories (e.g., "oak" is a high typical exemplar of the category "tree." while "palm" is a less typical item). Properties were varied with respect to normatively determined dominance (e.g, "fins" is a high dominant property of "trout," while "slimy" is a less dominant property) and whether they were distinctive (i.e., served to distinguish between subsets of exemplars within a category) or shared among most or all category members (e.g., "stem" for the category "fruit"). Analyses of accuracy and reaction time data suggested that AD results in neither a loss per se of representation of properties, nor a reorganization of relations between objects' properties. However, results were consistent with a property level degradation of AD patients' object concepts. While there was no evidence for a differential degradation of distinctive vs shared properties, results suggested that AD patients have degraded representations of lower dominant properties and properties of low-typical category exemplars.