Recruitment of anterior and posterior structures in lexical-semantic processing: an fMRI study comparing implicit and explicit tasks

Previous studies examining explicit semantic processing have consistently shown activation of the left inferior frontal gyrus (IFG). In contrast, implicit semantic processing tasks have shown activation in posterior areas including the superior temporal gyrus (STG) and the middle temporal gyrus (MTG) with less consistent activation in the IFG. These results raise the question whether the functional role of the IFG is related to those processes needed to make a semantic decision or to processes involved in the extraction and analysis of meaning. This study examined neural activation patterns during a semantic judgment task requiring overt semantic analysis, and then compared these activation patterns to previously obtained results using the same semantically related and unrelated word pairs in a lexical decision task which required only implicit semantic processing (Rissman, J., Eliassen, J. C., & Blumstein, S. E. (2003). An event-related fMRI investigation of implicit semantic priming. Journal of Cognitive Neuroscience, 15, 1160-1175). The behavioral results demonstrated that the tasks were equivalent in difficulty. fMRI results indicated that the IFG and STG bilaterally showed greater activation for semantically unrelated than related word pairs across the two tasks. Comparison of the two task types across conditions revealed greater activation for the semantic judgment task only in the STG bilaterally and not in the IFG. These results suggest that the pre-frontal cortex is recruited similarly in the service of both the lexical decision and semantic judgment tasks. The increased activation in the STG in the semantic judgment task reflects the greater depth of semantic processing required in this task and indicates that the STG is not simply a passive store of lexical-semantic information but is involved in the active retrieval of this information.