[Semantic dementia--a multimodal disorder of conceptual knowledge].

Semantic dementia (SD) is a clinical syndrome characterized by progressive loss of semantic memory/ conceptual knowledge and by bilateral, but usually asymmetric, atrophy of the anterior temporal lobes (ATLS). On the basis of the neuropsychological findings of SD, the two theoretical implications for the organization of semantic memory have been suggested. First, selective impairment of semantic memory in the early stages of SD contrasts with the isolated loss of episodic memory in patients with damage to the medial temporal lobes and other Papez's circuit components. This double dissociation provides empirical evidence for fractionation of explicit memory into the two subsystems with different neural underpinnings. Second, the multimodal nature of semantic deficits in SD leads to a seminal view that semantic memory is organized as an amodal system. The ATLs play a pivotal role as a 'convergence zone' or 'semantic hub' integrating abundant verbal and perceptual attributes that are represented in the posterior temporal and temporo-occipital cortices. To develop further comprehensive theories regarding semantic memory, we should understand differential roles of the left and right ATLs and clarify the clinicoanatomical relationship between verbal, visual, and emotional aspects of semantic memory loss and the detailed anatomical localization of the lesions.