Premotor and prefrontal correlates of category-related lexical retrieval

It has been shown that the retrieval of words denoting visually presented concrete entities engages neural systems in the left temporal lobe and that the precise pattern of activation within the temporal lobe depends in part on the conceptual category to which the entity belongs. Here, we used [15O]water positron emission tomography to test the hypothesis that the pattern of activation associated with word retrieval in left frontal lobe would also be related to conceptual category. The design entailed the performance of three tasks requiring the retrieval of words denoting animals, tools, and unique persons. The visual stimuli were presented at different rates, to produce equal performance success across categories, a feature which also had the effect of equalizing the proportion of scan time spent in mental search. All three word retrieval tasks activated the left inferior frontal gyrus, but they differed in their recruitment of two other premotor and prefrontal areas. Activity in a portion of the middle frontal gyrus, corresponding to Brodmann area 46, bore a linear relation to response latency and may index the extent of mental search. This region was most active when subjects named persons. Activity in the anterior bank of the precentral gyrus, along the inferior and middle frontal gyri, was most marked for naming tools. This region overlaps the area activated when subjects generate words for actions. We suggest that it is engaged by the retrieval of words denoting actions or objects with characteristic actions. The data presented here provide additional support for the notion that "nonclassical" language areas in extrasylvian frontal and temporal regions mediate word retrieval and that the pattern of their engagement relates to conceptual category.