Fifteen years ago, an influential model proposed that the human dorsal anterior cingulate cortex (dACC) detects conflict and induces adaptive control of behavior. Over the years support for this model has been mixed, in particular due to divergent findings in human versus nonhuman primates. We here review recent findings that suggest greater commonalities across species. These include equivalent behavioral consequences of conflict and similar neuronal signals in the dACC, but also a common

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... re of dACC lesions to reliably abolish conflict-driven behavior. We conclude that conflict might be one among many drivers of adjustments in executive control and that the ACC might be just one component of overlapping distributed systems involved in context-dependent learning and behavioral control. In our daily life we frequently encounter the necessity to decide between competing courses of action. Conflict between potential responses is particularly common and pronounced when current goal-directed processing has to compete with more automatic, habitual responses; for example, while trying to attend to a lecture you may have to override the natural tendency to respond to your buzzing phone. Laboratory studies in humans have shown that while such competition or conflict influences performance negatively in the current trial (where conflict leads to slowed and more error-prone responding), it can improve performance in the subsequent trial when the participants are required to resolve conflict between competing choices again [1-3]. Box 1 details some of the most prominent behavioral tasks and their indices of conflict, including 'conflict cost' (slowed response time on the current trial if the trial is incongruent/high conflict) and 'conflict-induced behavioral adaptation' (faster response time on the current incongruent/ high-conflict trial if the previous trial is also incongruent/high conflict). The highly influential conflict-monitoring model [1] of cognitive control [4] proposes that this is the case because conflict serves as a regulatory signal: conflict occurs when rival response options are similarly activated and this will trigger upregulation of top-down control, reinforcing processing of information in line with current task demands. According to this model, then, conflict-induced behavioral modulations are a consequence of trial-by-trial context-dependent tuning of executive control that aim to facilitate conflict resolution [1]. This model both drew support from and promoted an extensive body of work in human neuroimaging showing that conditions involving conflict are associated with heightened activation in a set of brain regions involving primarily the dACC (Box 2) and dorsolateral prefrontal Trends There are great similarities between monkeys and humans in conflictinduced behavioral modulations in the context of various cognitive tasks. In humans and also in monkeys, neuronal activities in the dorsal anterior cingulate cortex (dACC) convey information about the level of conflict between behavioral options. In monkeys, neuronal activities in the dorsolateral prefrontal cortex (DLPFC) and the orbitofrontal cortex (OFC) represent conflict level independent of the other task-related events. In humans and also in monkeys, the behavioral effects of conflict experienced in the current trial (conflict cost) remain intact after lesions within the dACC. In some studies with human patients, but not in monkeys, the behavioral effects of conflict experienced in previous trials (conflict adaptation) are impaired after a dACC lesion. Conflict adaptation is impaired after lesions within the DLPFC (in monkeys and humans) or within the OFC (in monkeys).