Anterior cingulate and the monitoring of response conflict: Evidence from an fMRI study of overt verb generation

Studies of a range of higher cognitive functions consistently activate a re gion of anterior cingulate cortex (ACC), typically posterior to the genu an d superior to the corpus collosum. In particular, this ACC region appears t o be active in task situations where there is a need to override a prepoten t response tendency, when responding is underdetermined, and when errors ar e made. We have hypothesized that the function of this ACC region is to mon itor for the presence of "crosstalk" or competition between incompatible re sponses. In prior work, we provided initial support for this hypothesis, de monstrating ACC activity in the same region both during error trials and du ring correct trials in task conditions designed to elicit greater response competition. In the present study, we extend our testing of this hypothesis to task situations involving underdetermined responding. Specifically, 14 healthy control subjects performed a verb-generation task during event-rela ted functional magnetic resonance imaging (fMRI), with the on-line acquisit ion of overt verbal responses. The results demonstrated that the ACC, and o nly the ACC, was more active in a series of task conditions that elicited c ompetition among alternative responses. These conditions included a greater ACC response to: (1) Nouns categorized as low vs. high constraint (i.e., d uring a norming study, multiple verbs were produced with equal frequency vs . a single verb that produced much more frequently than any other); (2) the production of verbs that were weak associates, rather than, strong associa tes of particular nouns; and (3) the production of verbs that were weak ass ociates for nouns categorized as high constraint. We discuss the implicatio n of these results for understanding the role that the ACC plays in human c ognition.