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

The clinical differentiation of progressive disorders of language is descri bed in the context of the evolution of current terminology. The syndromes o f 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 lo ngterm memory. Semantic dementia is also associated with characteristic str uctural and functional neuroimaging findings, and may represent a form of P ick'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 id eas about the organization and representation of knowledge, the interaction of semantic and episodic memory, and the contribution of semantic memory t o reading ability. Many of these ideas can be informatively modelled in the framework of connectionist theory.