Semantic activation and implicit memory in Alzheimer disease.

Patients with Alzheimer disease (AD) invariably display pronounced deficits in verbal memory when retention is tested explicitly. The present study examined the possibility that tasks which require memory only implicitly would be performed normally. Moderately demented patients with probable AD were severely impaired in free recall of a word list. On a subsequent word association test, the AD patients were less likely than normals to give items from the recall list as their word associations. The results suggest that implicit verbal memory, as well as explicit memory, is impaired in AD. While the magnitude of the activation effect was significantly reduced in AD patients, it was uncorrelated with recall performance or a measure of global cognitive functioning. Memory activation may thus depend on neural circuitry outside the traditional (i.e. temporo-limbic) memory system.