Semantic memory impairment in Alzheimer's disease.

The present study investigated semantic memory and the presence of category-specific impairments in patients with dementia of the Alzheimer's type (DAT). Patients in advanced stages of Parkinson's disease (PD) and matched control subjects served as comparison groups. Semantic memory was assessed by a range of verbal and visual tasks and by direct and indirect memory tests. The DAT patients showed severe deficits on all semantic knowledge tasks. Performance was poorer on animate relative to inanimate items, on naming with and without cues, and on semantic knowledge probes, indicating a category-specific knowledge loss on these tasks. Indirect memory tests yielded significantly better preservation of knowledge compared to the traditional semantic memory tasks. Analyses of the nature of the semantic breakdown in DAT suggested combined storage and access deficits and a disturbance of the structural description and phonological output systems.