Semantic memory disorders in Alzheimer's disease: clues from semantic priming effects.

Semantic memory deficits are a common landmark in Alzheimer's disease, but the nature of these impairments remains to be clarified. The tests used to explore this memory system are not specific and involve cognitive processes often disturbed in Alzheimer's disease. A complementary way to investigate semantic memory in neuropsychology is by using the semantic priming paradigm. Here, semantic priming refers specifically to the modification of a stimulus processing behind the presentation of a related stimulus; it is a short-lived phenomenon considered an implicit measure of semantic memory integrity. However, semantic priming studies have yielded contradictory results in Alzheimer's disease, with authors reporting normal priming, less-than-normal priming, or increased priming effects (hyperpriming). The aim of this paper is to review the literature investigating semantic priming in Alzheimer's disease, and to discuss the contradictory results in the context of current models of semantic processing. For a clear comprehension of the semantic priming patterns in this pathology, we will precise the methodology used and the characteristics of the Alzheimer's disease patients examined. Besides, the surprising hyperpriming phenomenon - often observed in Alzheimer's disease at the early stage of dementia--will also be explained in detail. These results from semantic priming represent invaluable clues to widen our knowledge and conceptions about deterioration of the semantic memory in Alzheimer's disease.