Is there an anatomical basis for category-specificity? Semantic memory studies in PET and fMRI

Patients with semantic impairments sometimes demonstrate category-specific deficits suggesting that the anatomical substrates of semantic memory may r eflect categorical organisation, however, neuroimaging studies have failed to provide consistent data in support of a category-based account. We condu cted three functional neuroimaging experiments to investigate the neural co rrelates of semantic processing, two with positron emission tomography (PET ) and a third with functional magnetic resonance imaging (fMRI). The first experiment used a lexical decision task to search for brain regions selecti vely activated by concepts from four different categories-animals, fruit, t ools, and vehicles. The second experiment used a semantic categorisation ta sk to increase the demands on the semantic system and to look for evidence of consistent activations for the domains of natural kinds or man-made item s. The final experiment was a replication of the semantic categorisation ta sk using fMRI to increase the spatial resolution and statistical sensitivit y of the experiment. The results of these experiments reliably identified a distributed neural system common to both natural kinds and artifacts but f ailed to find robust evidence of functional segregation by domain or catego ries. Category effects were neither reliable nor consistently present acros s experiments although sonic were consistent with previous studies. We disc uss the implications of these findings, arguing that they are most consiste nt with a semantic system undifferentiated by category at the neural level. (C) 2001 Elsevier Science Ltd. All rights reserved.