The anterior cingulate cortex (ACC), on the medial surface of the frontal lobes, has frequently been hypothesized to make critical contributions to the function of neural systems involved in the executive control of cognition. Three principal theories have been developed to account for this role. The first, 'motivated attention', emphasizes the limbic identity of the ACC and the effects of lesions to this area of the brain. The second, 'attention allocation', emphasizes the fact that during functional neuroimaging studies activation of the ACC is seen during tasks that elicit incompatible response tendencies that must be resolved for correct performance. The third theory, 'error detection', reflects the observation of a negative scalp potential occurring during incorrect responses which appears to have a medial frontal generator. The first and last theories suggest evaluative functions by the ACC in the service of control, while attention allocation suggests a strategic function. We have proposed that the data supporting all three theories can be reconciled if the ACC were defecting conflicting processes during task performance that might be associated with errors. In support of this hypothesis we describe results using event-related fMRI which confirm that the ACC does show error related activity but that the same region of the brain also shows increased response related activity during correct responses associated with response competition. This suggests a re-conceptualization of the contribution of the ACC to executive processes that support an evaluative role, specifically the on-line detection of processing conflicts that may be associated with deteriorating performance. Unresolved questions related to the contribution of this region to executive processes and potential future directions for research on the function of this region of the brain are discussed.
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