Electrophysiological insights into the nature of the semantic deficit in Alzheimer's disease.

Event-related brain potential (ERP) and reaction-time measures were used to determine if the specificity of a category prime differentially affects the amount of semantic priming seen in patients with dementia of the Alzheimer type (DAT) compared with normal elderly and young controls. Subjects were primed with an auditory category name followed by the visually presented name of an imageable object and indicated whether the object was a category member; the category was either superordinate to, at, or subordinate to the basic level. All groups showed similar priming effects in response to the category manipulation, as evidenced in both reaction time and the amplitude of the N400 component of the ERP. Overall, DAT subjects showed the smallest ERP priming effects and young controls the largest. The present study did not provide evidence for a strong version of a strictly "bottom-up" breakdown of the semantic networks in subjects with DAT, suggesting a role for factors such as task difficulty and memory search strategies in online categorization tasks of this type.