Task-dependent modulation of regions in the left inferior frontal cortex during semantic processing

To distinguish areas involved in the processing of word meaning (semantics) from other regions involved in lexical processing more generally, subjects were scanned with positron emission tomography (PET) while performing lexi cal tasks, three of which required varying degrees of semantic analysis and one that required phonological analysis. Three closely apposed regions in the left inferior frontal cortex and one in the right cerebellum were signi ficantly active above baseline in the semantic tasks, but not in the nonsem antic task. The activity in two of the frontal regions was modulated by the difficulty of the semantic judgment. Other regions, including some in the left temporal cortex and the cerebellum, were active across all four langua ge tasks. Thus, in addition to a number of regions known to be active durin g language processing, regions in the left inferior frontal cortex were spe cifically recruited during semantic processing in a task-dependent manner. A region in the right cerebellum may be functionally related to those in th e left inferior frontal cortex. Discussion focuses on the implications of t hese results for current views regarding neural substrates of semantic proc essing.