Semantic dementia: implications for the neural basis of language and meaning

The clinical differentiation of progressive disorders of language is described in the context of the evolution of current terminology. The syndromes of semantic dementia and progressive non-fluent aphasia can be distinguished on clinical and neuropsychological grounds; the former is characterized by a progressive and selective disintegration of the semantic component of longterm memory. Semantic dementia is also associated with characteristic structural and functional neuroimaging findings, and may represent a form of Pick's disease (focal lobar atrophy without Alzheimer histology). Selective impairment of this fundamental component of human cognition has allowed the empirical investigation of a range of theoretical questions. We discuss ideas about the organization and representation of knowledge, the interaction of semantic and episodic memory, and the contribution of semantic memory to reading ability. Many of these ideas can be informatively modelled in the framework of connectionist theory.