When the zebra loses its stripes: Semantic priming in early Alzheimer's disease and semantic dementia

Patients suffering from Alzheimer's disease (AD) or semantic dementia (SD) both exhibit deficits on explicit tasks of semantic memory. Semantic priming (SP) paradigms provide a very pure and precise implicit measurement of semantic memory impairment, and a previous study of AD (Giffard et al., 2002) using one such paradigm revealed that AD patients in the initial stages of semantic deterioration presented an abnormally large priming effect (hyperpriming) in a category-coordinate condition, compared with controls. This astonishing phenomenon could stem from the specific loss of distinctive attributes that make it possible to distinguish between semantically close concepts, while attributes shared by different concepts belonging to a given category remain intact. To test this hypothesis and compare the degradation of semantic memory in AD and SD, we devised an SP paradigm in which word pairs had either a category-coordinate or an attribute relationship. In accordance with our hypothesis, we distinguished between shared (duck-feathers) versus distinctive attributes (zebra-stripes) and close (tiger-lion) versus distant (elephant-crocodile) category-coordinate relationships. This paradigm, together with two explicit semantic memory tasks (picture-naming and categorization), was administered to 16 AD and 8 SD patients and 30 elderly control subjects. The AD patients, at the very beginning of semantic deterioration, only displayed impaired SP effects in the distinctive attribute condition, whereas in the SD patients, who had more severe semantic deterioration, we observed an extinction of SP effects in both attribute conditions. In SD patients, we also report hyperpriming effects in both category-coordinate conditions. Our results suggest that semantic memory impairment follows the same course in both AD and SD, affecting distinctive attributes first and then shared ones. In accordance with distributed models of semantic memory, the loss of distinctive attributes leads to a confusion between close concepts and it is this which causes the transient hyperpriming phenomenon.