Caudal anterior cingulate cortex has been found to be activated in a wide range of cognitive tasks. On the basis of their review of 29 PET studies, Peter Strick and colleagues drew a broad distinction between the most caudal area of the anterior cingulate cortex, area 24c'g, lying on and just posterior to the line of the anterior commissure and more anterior regions (areas 24' and 32'). The former region is commonly activated during simple and/or familiar movements (typically in comparison with resting conditions involving no movements), movements that are associated with activity in the SMA but not lateral prefrontal areas. By contrast, areas 24' and 32' tend to be activated during the execution of more complex response selection tasks alongside activations in the pre-SMA and lateral prefrontal areas. On these grounds, Picard and Strick identified the caudal cingulate zone with CMAd observed in primates and tentatively correlated CMAr and CMAv with the anterior and posterior sectors, respectively, of the more rostral region activated by complex tasks.

A variety of complex tasks have been found to be associated with cingulate activity. Tasks that require subjects to respond to one stimulus in the presence of distracting, irrelevant information reliably result in such activation. For example, during Stroop task performance, the anterior cingulate cortex is more activated when subjects are required to name the ink color of a nonmatching color word (e.g. "Blue" written in red ink) than when naming the ink color of a stimulus that is not a color word. The anterior cingulate cortex is also activated when subjects are required to withhold responding in the context of speeded response tasks (e.g., no go and stop signal trials as compared with go trials) and when subjects must saccade away from rather than toward a target stimulus. Activity in corresponding regions is observed during the learning of new motor sequences or new noun-verb associations (compared with activity during the performance of familiar sequences or associations), following a switch to a new task (compared with repeated task performance), during free recall of remembered lists (compared to rest), when generating words that start with certain letters (compared to rest or repeating words seen or heard), and when required to make a decision based on degraded information (compared with undegraded information). As mentioned earlier, the performance of many of the tasks that activate the caudal anterior cingulate cortex is associated with corresponding deactivations in the rostral anterior cingulate cortex. Similarly, deactiva-tions in the caudal anterior cingulate cortex have been observed during the performance of some of the tasks with affective content described earlier.

Attempts have been made to characterize the kinds of tasks that activate caudal anterior cingulate cortex to infer the function of this region. In general, the activating tasks have been categorized as difficult or complex or as eliciting multiple competing responses. Following from these observations, caudal anterior cingulate cortex has been variously attributed a role in attention or executive processes, in the decision process to select or initiate a response to "funnel" to motor areas, and in the monitoring of ongoing performance.

Whereas there is potentially significant overlap in these differing theories—e.g., performance monitoring can be considered as an important executive function in regulating response selection—there is evidence that the anterior cingulate cortex plays a direct role in performance monitoring. First, scalp electrophysiological recordings reveal a negative deflection in the event-related potential following incorrect responses during choice reaction time tasks, a finding reported independently by Falkenstein and colleagues in Dortmund and by Gehring and coworkers in Illinois. This error-related negativity (ERN or Ne) begins around the time of the response and peaks roughly 100 msec after (see Fig. 2). The ERN has a midline frontocentral distribution on the scalp, and the anterior cingulate cortex is consistently found to be its most likely neural generator. The amplitude of the ERN wave varies as a function of the force with which the error is produced and the probability that the error is corrected, suggesting that the ERN relates somehow to error processing. Crucially, the observation of an ERN following performance feedback and following failures to withhold a response when required—errors that cannot be corrected—suggests that the ERN, a presumed electrophysiological index of anterior cin-gulate function, is related to performance monitoring rather than to the process of response selection itself.

Caudal anterior cingulate cortex is not uniquely activated following errors, as fMRI studies have shown this region to be activated even on correct trials. When subjects respond correctly, moreover, the activation observed is larger when there is response conflict—e.g., when distractor information presented is associated with a different response than target information—leading to the suggestion that the anterior cingulate cortex monitors for conflict rather than errors per se. Studies of response conflict provide a second line of evidence for the role of the anterior cingulate cortex in performance monitoring. For example, in a given task condition, if attention is effectively engaged to filter out irrelevant information, conflict will be low. In contrast, if attention is low, conflict will be high. When within-condition contrasts are made for subsets of trials with high attention-low conflict and subsets with low attention-high conflict, the anterior cingulate cortex activation is found to

Figure 2 Scalp distribution and electrical tracing of the error-related negativity (ERN).

Figure 2 Scalp distribution and electrical tracing of the error-related negativity (ERN).

follow the degree of conflict rather than the level of attention, consistent with the monitoring hypothesis.

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