Anterior cingulate cortex and response conflict: Effects of frequency, inhibition and errors

Anterior cingulate cortex (ACC) may play a key role in cognitive control by monitoring for the occurrence of response conflict (i.e. simultaneous acti vation of incompatible response tendencies). Low-frequency responding might provide a minimal condition for eliciting such conflict, as a result of th e need to overcome a prepotent response tendency. We predicted that ACC wou ld be selectively engaged during low-frequency responding, irrespective of the specific task situation. To test this hypothesis, we examined ACC activ ity during the performance of simple choice-discrimination tasks, using rap id event-related functional magnetic resonance imaging. Subjects were scann ed while performing three tasks thought to tap different cognitive processe s: 'Go/No-go' (response inhibition), 'oddball' (target detection), and two- alternative forced-choice (response selection). Separate conditions manipul ated the frequency of relevant task events. Consistent with our hypothesis, the same ACC region was equally responsive to low-frequency events across all three tasks, but did not show differential responding when events occur red with equal frequency. Subregions of the ACC were also identified that s howed heightened activity during the response inhibition condition, and on trials in which errors were committed. Task-sensitive activity was also fou nd in right prefrontal and parietal cortex (response inhibition), left supe rior temporal and tempoparietal cortex (target detection), and supplementar y motor area (response selection). Taken together, the results are consiste nt with the hypothesis that the ACC serves as a generic detector of process ing conflict arising when low-frequency responses must be executed, but als o leave open the possibility that further functional specialization may occ ur within ACC subregions.