CONFLICT MONTTORING 627 ment,participants were trained to respond to each of three simpl ocus on this l m I sion and single- nit recording st ersionsof the exper sed and from heard to spoken ds.In ach t be organized into t e gen eral type al c elicited greater ACC activation than the the overidingof pre ent but task-irrelevant in a se cond set of experiments,Paus et al (1993)asked partic group,it as iring the par ipa sfirst to prod nt with the stimulus in d to th ommission o errors.Here we nts firs lifted ver of two finger ence of te finger.In a s ion of a lef A la dire cue In a third has op Ihe mos tly studie the in th In t digm (Stroop.1935: review see MacCleo 1991).in ”A” wi and to"L with n ch T e in fa s are the sam (red displa ed in red)or if the stim override is provided by stdies o/n o tasks.Us The yet al. sing a butt n pre the was first obs d by Pardo the p ed (PET this stud ACC the o-go condition s in c here rdguireh n by C 0(1995 ully iatio with the sponds to the h of the studi 1100 t stimuli h been fo ants of the str for the cipant to n in a requi ng the ov ing of pr sing pat (u and Kigag for this in Sir lly displaye ding mn a up of stu with th r in the to ch rom se he letter in order to ible respons pa sed aco activity th nflict tas dy ctivate ACC across tasks i volving a range of input nd noun. tifying a named by t Wh practiced stimulus-re conditio the articipan re read th ater acc ing to a no mapping.In one version of the prese ordCONFLICT MONITORING 627 the most consistent results derive from brain activation studies. In what follows, we focus on this literature; however, our conclusions can in many instances be viewed as consistent with established findings from lesion and single-unit recording studies. Although brain activation studies have reported ACC engage￾ment in a wide variety of task settings, the bulk of these studies can be organized into three general types. In one set of experiments, ACC activation has been associated with tasks calling for the overriding of prepotent but task-irrelevant responses; in a second group, it has been associated with tasks requiring the participant to choose among a set of equally permissible responses; and in a third, with tasks that lead to the commission of errors. Here we discuss these three domains in detail, suggesting how in each case ACC activation can be seen as accompanying the occurrence of conflict. Response override. A large number of studies have reported ACC activation in tasks requiring the participant to override rel￾atively automatic but task-inappropriate responses. The most fre￾quently studied of these has been the classic Stroop conflict paradigm (Stroop, 1935; for a review see MacCleod, 1991), in which the participant is asked to name the color in which a color word is displayed. Response times are greater if there is a mis￾match between the color the word refers to and the color in which the word is displayed (e.g., red displayed in green) than if the two colors are the same (red displayed in red) or if the stimulus consists of a noncolor word, a series of colored Xs, or merely a color bar. The explanation usually offered for the difficulty of the incongruent condition is that word reading, a strongly automatic process, interferes with color naming. The challenge for the par￾ticipant is to overcome the word-reading response. ACC activation on the Stroop task was first observed by Pardo, Pardo, Janer, and Raichle (1990). Using positron emission tomog￾raphy (PET), this study demonstrated increased ACC activation during performance of the incongruent condition when compared with the congruent condition. Increased ACC activation was also shown by Carter, Mintun, and Cohen (1995) in a similar compar￾ison. Several studies have also reported greater ACC activation in association with the incongruent condition when compared with the neutral condition (Bench et al., 1993; Carter et al., 1995; George et al., 1994). The finding of greater ACC activation with incongruent stimuli has been found in variants of the Stroop task as well; Bush et al. (1998) observed ACC activation in a numeric version of the task. Other tasks requiring the overriding of prepotent responses have also been shown to engage the ACC. Taylor, Kornblum, Minoshima, Oliver, and Koeppe, 1994, for example, asked partic￾ipants in one condition to name the individually displayed letters B, J, Q, and Y. In a second condition, participants were asked to respond with the name of a different letter in the group according to a simple set of rules (e.g., if J is displayed, respond with "Y"). The latter task required them to overcome the temptation to read the letter in order to recover the less stimulus-compatible response dictated by the instructions. In agreement with the Stroop studies, increased ACC activity was observed on the conflict task. A multipart PET study by Paus, Petrides, Evans, and Meyer (1993) showed that the need to override prepotent responses will activate ACC across tasks involving a range of input and output modalities. In one set of experiments, participants first performed according to extensively practiced stimulus-response pairings and later according to a novel mapping. In one version of the experi￾ment, participants were trained to respond to each of three simple visual stimuli with a direction-specific saccade. In the reversal condition, the pairing between the three stimuli and the three saccade responses was changed. Two other versions of the exper￾iment involved mappings from visual stimuli to buttons to be pressed and from heard words to spoken words. In each version, the reversal condition elicited greater ACC activation than the overlearned condition. In a second set of experiments, Paus et al. (1993) asked partic￾ipants first to produce stimulus-compatible responses, and later to produce responses less congruent with the stimulus. In one ver￾sion, participants first lifted whichever of two fingers was touched by the experimenter. Later, participants were instructed to raise the opposite finger. In a second version, participants performed a saccade in the direction of either a left-sided or right-sided visual cue, and then later were asked to respond with a saccade in the direction opposite the cue. In a third version, participants re￾sponded to the two heard letters "A" and "L" by naming the letter coming next in the alphabet. In the reversal condition, participants responded to "A" with "M" and to "L" with "B." In each version of the experiment, greater ACC activation was once again ob￾served on the task requiring the participant to overcome an in￾grained response in favor of a less familiar one. Another instance of ACC activation associated with response override is provided by studies of go/no-go tasks. Using functional magnetic resonance imaging (fMRI), Casey et al. (1997; see also Kawashima et al., 1996) had participants view a series of individ￾ually presented letters, pressing a button with each presentation but omitting this response if the presented letter was an X. The ma￾jority of trials involved non-X letters, leading the button-press response to be prepotent. In control conditions, the presented letter series contained no Xs. Greater ACC activation was observed in the go/no-go condition. As in other response override tasks, ACC activation is here associated with conditions that require the par￾ticipant to overcome a prepotent response in order to perform successfully. The finding of ACC engagement in response override tasks provides a first piece of evidence for the view that this brain area responds to the occurrence of conflict. In each of the studies we have reviewed, the strongest ACC activation was observed under conditions where it was necessary for the participant to overcome interference from prepotent but task-irrelevant responses. These circumstances can be understood as involving conflict between processing pathways leading to correct (but otherwise weaker) and incorrect (but prepotent) responses. The mechanisms responsible for this form of crosstalk are considered further in Simulation 1 A. Underdetermined responding. In a second group of studies, ACC activation occurs under conditions requiring the participant to choose from a set of responses, none of which is more obvious or compelling than the others. We describe these tasks as involving underdetermined responding, because the stimulus presented to the participant does not uniquely specify the appropriate response. The first studies to examine brain activation under such task circumstances were reported by Petersen, Fox, Posner, Mintun, and Raichle (1988, 1989). In a series of PET studies, the group asked participants to generate a verb in response to a seen or heard noun, identifying a use for the object named by the stimulus. When activation patterns for this task were compared with those for a condition in which the participant simply repeated or read the presented word, the ACC was found to be consistently engaged