CONFLICT MONITORING of made with ch hand. Finally,Grasby et al.(1993)had participants listen to an requently re ersed er and m 2 1o fou ACC ame stud Corbetta et al (1991 study,one option is to attribu this findin in the g ion of the ERN there gain oth grew.gre A number of other the bility that ACC activation ma been rel d ERN eversas than do Is with smaller ERNs(Gehri et a provide d by similarity effect,the that pa ed y for the list if it is c sed of similar ading entrie this oh 993)3 Third in a stu where part nts were asked t king then a Z s a ulus in Given such ay,any transient cor veen pro sing in the tim e et al study i ikely to mpanied by the sort of crosstalk to cally similar words studies The close ciation bet onflict Accounting for ACC Activation:Simulation Study l of stu wed above ties that do not fall into any of t egories we laid out nficts in information processing nle D'Es al (1995)used fMRIto c the inwe des ACC activity c either arti culat On the hasis of the efold A firs on of the role of alk ir CC activation in this stud was to make the ac ed so far 0 er divided atte n study.Corbetta.Miezin.Dob 19g yer. second the hile for the nges alc ong th hird goa s to lay the groundwork for furth ling d only one of these dimen In a divided ttention d tor in any of the ations make sly an ition de mor ne of the t ains in w hich activation ssibility parallel evaluation differen 199 under Although t and err nine th th o 1990).the task most of thes models. vith ns like sals are likely to involve e the ve have onal role in trie Iving beh Lain e the t al. 1987),apoi in lateCONFLICT MONITORING 629 the strength of the response made with each hand. Participants very frequently reversed errors, and the EMG results indicated clearly that when this occurred, there was typically temporal overlap between the error and error-correcting responses. This same study provides evidence consistent with the idea that this transient re￾sponse conflict is n critical factor in the generation of the ERN; EEG data indicated that the ERN coincided with the period of response overlap on error trials. A number of other findings corroborate the connection between response conflict and the ERN. First, error trials associated with the largest ERN amplitudes more frequently involve response reversals than do trials with smaller ERNs (Gehring et al., 1993). Thus, the largest ERNs are associated with error trials where there is the strongest evidence for belated activation of the correct response. Second, an ERN appears, even in association with cor￾rect responses, if these are subsequently reversed (Gehring et al., 1993).3 Third, in a study where participants were asked to with￾hold their responses until 2 s after stimulus presentation, no ERN was observed in association with errors (Dahaene et al., 1994). Given such a delay, any transient competition between processing pathways is likely to have resolved by the time of response delivery. Thus, incorrect responding in the Dahaene et al. study is unlikely to have been accompanied by the sort of crosstalk to which we attribute the ERN. Residual studies. The close association between conflict and ACC activation in the studies we have reviewed is reinforced by the fact that conflict also appears to play a role in ACC activation studies that do not fall into any of the three categories we laid out above. For example, D'Esposito et al. (1995) used fMRI to compare ACC activity during two simple tasks performed either singly or concurrently, observing greater activation in the latter condition. On the basis of the earlier discussion of the role of crosstalk in dual-task performance, it is clear how ACC activation in this study can be explained as a response to conflict. In another divided attention study, Corbetta, Miezin, Dobmeyer, Shulman, and Petersen (1991) measured brain activity with PET while participants monitored forms in a visual display for subtle changes along the dimensions of color, shape, and direction of movement. In a focused attention condition, participants moni￾tored only one of these dimensions. In a divided attention condi￾tion, participants searched for changes in any of the three dimen￾sions. Greater ACC activation was associated with the divided attention condition. Participants made more errors in this condi￾tion, and so it may be possible to attribute ACC activation in this study to errors. However, another interesting (and closely related) possibility is that the parallel evaluation of different stimulus dimensions led on some trials to crosstalk between pathways supporting "same" and "different" responses. Although the pub￾lished data do not allow a conclusive evaluation of this possibility, it is consistent with the reported higher frequency of misses (incorrect "same" judgments) in the divided attention condition. In another study, Baker et al. (1996) found ACC activation in association with performance of the Tower of London task. Be￾cause the solution to this task is rarely immediately apparent to the unpracticed participant, competition or conflict among alternative actions seems likely to be involved. As we have already noted, certain computational models accord such competition a pivotal functional role in triggering problem-solving behavior (e.g., Laird et al., 1987), a point to which we will return in later discussion. Finally, Grasby et al. (1993) had participants listen to and immediately repeat word lists from 2 to 13 items long. Using PET, they found that ACC activation increased with list length. As in the Corbetta et al. (1991) study, one option is to attribute this finding to errors, for the frequency of errors rose along with list length. However, there are again other potential explanations that involve conflict. One is that, as list length grew, greater response compe￾tition occurred during the retrieval process. Even more intriguing is the possibility that ACC activation may have been related to interference among lexical representations being maintained in working memory. One way of examining this latter possibility is provided by the phonological similarity effect, the fact that par￾ticipants asked to repeat a list of words shows relatively poor memory for the list if it is composed of similar-sounding entries (Baddeley, 1966). If, as appears reasonable, this phenomenon can be assumed to derive from interference among representations being held in working memory, then a potentially informative experiment might be to measure ACC activity during retention of short word lists, comparing activity levels during maintenance of phonologically similar and dissimilar items. If the ACC is respon￾sive to conflict among representations in working memory, then greater activation should be seen in the condition using phonolog￾ically similar words. Accounting for ACC Activation: Simulation Study 1 In connection with each group of studies reviewed above, we briefly proposed how ACC activation can be understood as a response to the occurrence of conflicts in information processing. In the present section, we describe a set of three computer simu￾lations in which this interpretation of the ACC literature is more fully articulated. The objectives of these modeling studies were threefold. A first goal was to make the account we have presented so far more explicit, providing a precise indication of what we intend by such terms as crosstalk, conflict, top-down control, and conflict moni￾toring. A second goal was to confirm the sufficiency of these constructs, as we have used them in the conflict monitoring hy￾pothesis, to account for the results of ACC activation experiments. The third goal was to lay the groundwork for further modeling work, reported in Part 2, that examines the entire feedback loop running from conflict monitoring to cognitive control. Each of the present simulations makes use of a previously and independently implemented computational model of a single task from one of the three primary domains in which ACC activation has been reported. To examine the role of conflict in response override, we consider a model of the Stroop task (Cohen & Huston, 1994); for underdetermined responding, a model of stem completion (McClelland & Rumelhart, 1981); and to evaluate the relation between conflict and error commission, we examine a model of the Eriksen task (Servan-Schreiber, 1990), the task most frequently used in studies of the ERN. To each of these models, we apply a quantitative measure of conflict, allowing the models to be used in simulations of the 3 Although trials involving overt reversals are likely to involve the strongest coactivation of correct and incorrect responses, the account we are proposing does not require that any actual reversal occur, only that activation of the correct pathway occur while activation of the incorrect pathway is still present