CONFLICT MONTTORING With practice on some initially difficult tasks performance be Theoretical Background comes incre s t rateat which their top down influence can be withdrawn without causing a deterioration ly migh at is,how does a r itself within order for the recruitment.modulation.and dise ment of control to occur.control processes need a he systems that they y mod which its top-down influence is exerted.there must also exist a evaluative compon ntthat monitors proce sing,ma Control as Conflict Prevention to develop an account of this evaluative function. succinct ription of the prob The Conflict Monitoring Hypothesis toward cha which the demand for conrol might b in function we refer to This as conflict monitoring.By the overall a ount we will put forth toring ves to tra slate the other one,in that they change th e or hegtsTpovidedb haffer(197 in o ho showed that dramatic lar.we will cor ot is made to these wo tasks simul rea of the human frontal lobe.the of this an be understo ving tro anterior cingulate cortex(ACC).Our second objective is to artic at the level c ant produc es a resp onse in one 0eeo esses has been ur od as rmance in an ex article is divid into two main sections,correspo ding to the on-pro ing cap Allpo the ACC.The section begins by revie (e ent brain activatio Schneider&Detweil r,1987).h rgue I mode of a centra processor are mo riately cor results of a first co ling studv.whic osstalk i ce be n pa ulate bra 411 of er Part i as a foundation Part 2 turns to the issue of how confli sstalk as a ubiquitous pitfall of paralle g might play a ro ating cogniti Th from on but eachinon-ishift port (1980)pu ed sys ndan In a ontrolling le inte ions:of g separ :Pn leads natu rally to a unifying.mechanistic explanation for these isual search,where the top-down control of visual attention hasCONFLICT MONITORING 625 With practice on some initially difficult tasks performance be￾comes increasingly automatic (e.g., Anderson, 1982; Shiffrin & Schneider, 1977). As this happens, the need for control diminishes. How do control processes evaluate the rate at which their top￾down influence can be withdrawn without causing a deterioration in performance? Clearly, in order for the recruitment, modulation, and disengage￾ment of control to occur, control processes need access to infor￾mation about the functioning of the systems that they modulate. That is, in addition to the regulative dimension of control, by which its top-down influence is exerted, there must also exist an evaluative component that monitors information processing, mak￾ing an assessment of current demands. If one is to expunge the homunculus from theories of cognitive control, it will be necessary to develop an account of this evaluative function. The Conflict Monitoring Hypothesis In this article we take an initial step toward characterizing the evaluative side of cognitive control, proposing one mechanism by which the demand for control might be gauged. Specifically, we will argue that there exists a system that monitors for the occur￾rence of conflicts in information processing, a function we refer to as conflict monitoring. By the overall account we will put forth, conflict monitoring serves to translate the occurrence of conflict into compensatory adjustments in control: The conflict monitoring system first evaluates current levels of conflict, then passes this information on to centers responsible for control, triggering them to adjust the strength of their influence on processing. A first goal of the present work is to draw together evidence for the occurrence of conflict monitoring. In particular, we will con￾sider data suggesting that the detection of conflict may be among the functions of a particular area of the human frontal lobe, the anterior cingulate cortex (ACC). Our second objective is to artic￾ulate the hypothesis that conflict monitoring serves as a basis for the regulation of control, showing how this idea can be used to explain a set of interesting empirical phenomena. We begin, in the next section, with some theoretical consider￾ations, deriving an initial motivation for the idea of conflict mon￾itoring from current theories of cognitive control. After this, the article is divided into two main sections, corresponding to the objectives identified above. Part 1 examines the possibility that a conflict monitoring function might be reflected in the behavior of the ACC. The section begins by reviewing recent brain activation studies, which together encourage the idea that the ACC may respond specifically to the occurrence of conflict. We then present the results of a first computational modeling study, which serves to articulate our interpretation of the brain activation data and to demonstrate the sufficiency of the theory to account for them. With Part 1 as a foundation, Part 2 turns to the issue of how conflict monitoring might play a role in modulating cognitive control. The section focuses on three behavioral phenomena, quite different from one another in their details, but each involving on-line shifts in control. In a second computational modeling study, we show how linking conflict monitoring to the modulation of control leads naturally to a unifying, mechanistic explanation for these phenomena. Theoretical Background We have suggested that the systems subserving cognitive con￾trol are likely to include an evaluative system, which keeps tabs on current demands. This raises the question, what precisely might such a system measure? That is, how does a need for increased control manifest itself within the processing system? One potential answer can be derived directly from current theories of cognitive control, which portray it as serving to prevent the occurrence of conflicts in information processing. Control as Conflict Prevention Given the highly parallel and distributed character of cognitive processing, one of its inherent hazards is crosstalk interference between concurrent processes. A succinct description of the prob￾lem is provided by Mozer and Sitton (1998): One can conceive of processing ... as occurring along a certain neural pathway. If the processing pathways for two stimuli are nonoverlap￾ping, then processing can take place in parallel. But if the pathways cross—i.e., they share common resources or hardware—the stimuli will interact or interfere with one another, (p. 342) This sort of interference is perhaps easiest to illustrate in the setting of dual-task performance. According to Navon and Miller (1987), concurrently performed tasks interfere with one another when "each produces outputs, throughputs, or side effects that are harmful to the processing of the other one, in that they change the state of some variable that is relevant for the performance of the concurrent task" (p. 435). A concrete example is provided by Shaffer (1975), who showed that dramatic decrements in perfor￾mance occur in both typing to dictation and reading aloud when an attempt is made to perform these two tasks simultaneously. The difficulty of this combination can be understood as deriving from crosstalk between the processing pathways activated by auditory and visual inputs, leading to conflicting responses at the level of both speech and typing. The result is a slowing of response times and an increase in the frequency of errors, including so-called crosstalk errors where the participant produces a response in one modality that should have been delivered in the other. Conflict between concurrent processes has been understood as affecting performance in an extremely wide variety of domains. Indeed, it has been credited with placing a central limitation on human information-processing capacity: Allport (1987), in agree￾ment with a number of other researchers (e.g., Cohen et al., 1990; Duncan, 1996; Mozer, 1991; Mozer & Sitton, 1998; Navon, 1985; Navon & Miller, 1987; Schneider & Detweiler, 1987), has argued that "the behavioral phenomena attributed in the past to the limited capacity of a central processor are more appropriately conceptu￾alized ... as the expression of crosstalk interference between par￾allel processes" (p. 411). This recognition of crosstalk as a ubiquitous pitfall of parallel processing has led to a particular view of cognitive control, ac￾cording to which one of its central functions is to prevent conflicts. As Allport (1980) put it, "for any distributed system, fundamental issues are raised by the demands of conflict resolution and of controlling undesirable interactions: of keeping separate processes separate" (p. 38). The job of dealing with these problems falls to cognitive control. This view can be discerned in much work on visual search, where the top-down control of visual attention has