Anterior cingulate cortex and the Stroop task: neuropsychological evidence for topographic specificity

Neuroimaging studies have implicated the anterior cingulate cortex (ACC) in many aspects of attention and cognition. Major theories of ACC function have proposed a role in conflict monitoring, executive control, response selection, and general arousal. Although the ACC is often treated as a unitary structure, extensive evidence suggests it exhibits anatomical and functional specificity. ACC activity during the Stroop color word interference task has been of particular interest. The purpose of the present study was to determine whether two different ACC subregions are necessary for intact color naming performance in the Stroop task. One experiment utilized blocked trial and mixed trial designs to emulate neuroimaging studies and to compare interference and facilitation effects, respectively. A third variant manipulated the probabilities of congruent and incongruent trials to alter levels of interference and cognitive control, or engagement of strategic processes, on a block by block basis. Two patients with focal lesions in either right mid-caudal (patient D.L.) or left rostral to mid-dorsal ACC (patient R.N.) exhibited distinctive performance profiles in these three versions of the Stroop task, providing further support for topographic specificity of function within the human ACC. Contrary to predictions from some neuroimaging experiments, damage to right mid-caudal ACC was associated with normal levels of interference and accurate performance on incongruent trials. Instead, D.L. showed reduced levels of facilitation relative to controls. Further, interference was not modulated by the probability manipulation in D.L., suggesting equivalently high levels of cognitive control in both conditions. Conversely, damage to left mid-dorsal ACC resulted in consistently lower accuracy on incongruent trials, indicating deficits in maintaining task set and inhibiting the automatic response. These results can help to constrain interpretations of ACC activations in functional imaging experiments of the Stroop task.