Word production and comprehension in Alzheimer's disease: the breakdown of semantic knowledge

Relatively mildly impaired patients with suspected Alzheimer's disease (N = 14; Verbal IQ = 96) and normal controls of similar age and education (N = 11) were administered tests requiring production (naming and fluency) and comprehension of single words. Word comprehension was assessed on a superordinate level (rating words for degree of "pleasantness") and on a more specific level (matching abstract pictorial representations with printed words denoting objects, action, emotions, and modifiers). Performance on standardized measures of semantic knowledge (Vocabulary and Similarities subtests of the WAIS) was also evaluated. The naming and fluency abilities of the Alzheimer's patients were found to be highly correlated (r = .80) and impaired. Naming errors often consisted of semantic field errors which were either hierarchically or linearly related to the target name. In comparison with normals, verbal fluency was characterized by a tendency to generate proportionally more category names concurrent with reduced production of items within a category. Single-word comprehension was also impaired, except when judgments of affective meaning were required. It was argued that these results suggest that Alzheimer's disease may lead to a specific disruption in semantic knowledge characterized by a difficulty in differentiating between items within the same semantic category concurrent with the relative preservation of broader categorical information.