Anatomic interconnections between the prefrontal and anterior cingulate cortices suggest that these areas may have similar functions. Here we report the effect of anterior cingulate removal on task switching, error monitoring, and working memory. Neuroimaging studies have implicated the cingulate cortex in all these processes. Six macaques were taught task switching (TS) and delayed alternation (DA) paradigms. TS required switching between two conditional response tasks with mutually incompatible response selection rules. DA required alternation between two identically covered food-well positions. In the first set of experiments, anterior cingulate lesions did not consistently impair TS or DA performance. One animal performed worst on both TS and DA and in this animal the cingulate sulcus lesion was most complete. In the second set of experiments, we confirmed that larger anterior cingulate lesions, which included the sulcus, consistently impaired TS but only led to a mild and equivocal impairment of DA. The TS error pattern, however, did not suggest an impairment of TS per se. The consequence of a cingulate lesion is, therefore, distinct to that of a prefrontal lesion. TS error distribution analyses provided some support for a cingulate role in monitoring responses for errors and subsequent correction but the pattern of reaction time change in TS was also indicative of a failure to sustain attention to the task and the responses being made.

It has been suggested that the dorsolateral prefrontal cortex (DLPFC) is involved in free selection (FS), the process by which subjects themselves decide what action to perform. Evidence for this proposal has been provided by imaging studies showing activation of the DLPFC when subjects randomly generate responses. However, these response selection tasks have a hidden working memory element and it has been widely reported that the DLPFC is activated when subjects perform tasks which involve working memory. The primary aim of this experiment was to establish if the DLPFC is genuinely involved in response selection. We used repetitive transcranial magnetic stimulation (rTMS) to investigate whether temporary interference of the DLPFC could disrupt performance of a response selection task that had no working memory component. Subjects performed tasks in which they made bimanual sequences of eight nonrepeating finger movements. In the FS task, subjects chose their movements at random while a computer monitor displayed these moves. This visual feedback obviated the need for subjects to maintain their previous moves “on-line.” No selection was required for the two control tasks as responses were cued by the visual display. The attentional demands of the control tasks varied. In the high load (HL) version, subjects had to maintain their attention throughout the sequence, but this requirement was absent in the low load (LL) task. rTMS over the DLPFC slowed response times on the FS task and at the end of the sequence on the HL task, but had no effect on the LL task. rTMS over the medial frontal cortex (MFC) slowed response times on the FS task but had no effect on the HL task. This suggests that a response selection task without a working memory load will depend on the DLPFC and the MFC. The difference appears to be that the DLPFC is important when selecting between competing responses or when concentrating if there is a high attentional demand, but that the MFC is only important during the response selection task.