Cognitive neuroscience studies of semantic memory in Alzheimer's disease.

Semantic memory is the component of long-term memory that stores our concepts about the world. The disruption of semantic memory as a result of brain damage may have profound negative consequences on an individual's ability to name objects and process concepts. This can be disrupted as a result of many forms of brain damage, particularly Alzheimer's disease (AD). The current paper reviews research demonstrating that semantics deteriorates early in AD, particularly on effortful semantic tasks. There is a "category effect", meaning that AD preferentially affects concepts dealing with living things and abstract concepts compared to non-living objects and verbs/actions. While this pattern of deterioration, specific for AD, may reflect a breakdown within a distributed semantic system (where living things are distinguished by a high rate of inter-correlations between concepts or by a particular mode of being learned), it is equally possible that there is a regional distribution of semantic knowledge, with living things preferentially involving left temporal regions which become damaged early on in AD. Evidence from patients with strokes and semantic dementia, as well as activation studies in normal individuals, implicates the left posterior temporal region in semantic processing for pictures, abstract words, and concrete words. AD individuals, who are impaired in a variety of semantic tasks, show functional deficits in this area, and fail to activate it normally.