170 MILLER■COHEN ofa conict stimulus (the word GREEN displayed n red),because there is a strong prepotent tendency to read the word("green").which competes with the response to the color("red").This illustrates one of the most fundamental aspects of cognitive control and goal-directed behavior:the ability to select a weaker task-relevant response(or source of information)in the face of competition from ,but task-irrelevantone.Patients with frontal h this task (eg.Perrett 19,CohenServan-Schreiber Vendrell et al 1995),especially when the instructions vary frequently(Dunbar& Sussman 1995,Cohenetal 1999),which suggests that they have difficulty adhering to the goal of the task or its rules in the face of a competing stronger (ie.more salient or habitual)response. Similar findir ng s are evident in the WCST.Subjects are instructed to sort card according to the shape,color,or number of symbols appearing on them and the sorting rule varies periodically.Thus,any given card can be associated with several possible actions,no single stimulus-response mapping will work,and the correct one changes and is dictated by whichever rule is currently in effect.Humans with PEC dar nage show stere ch di culty but ar unable to adapt their beh Mner 1)Monkeys with PFClesonsrrd n analog of this task(Dias et al 1996b,1997)and in others when they must switch between different rules(Rossi et al 1999). The Stroop task and WCST are variously described as tapping the cognitive functions of either selective atte or rule-bas workin ed or goal-directed behavio we functions depend on the representation of goals and rules in the form of patterns of activity in the PFC,which configure processing in other parts of the brain in accordance with current task demands.These top-down signals favor weak(but task-relevant)stimulus-response mappings when they are in competition with more habitual,strongero he Stroop task),especially whe needed (such as in the WCST).We believe that this can account for the wide range of other tasks found to be sensitive to PFC damage,such as A-not-B(Piaget 1954. Diamond Goldman-Rakic 1989),Tower of London(Shallice 1982,1988;Owen et al 1990).and others (Duncan 1986.Duncan et al 1996).Stuss Benson 1986). We build on the fundamental principle that processing in the brain is compet itive:Different carrying different source s of inf ormation,compete for expression in behavior,and the winners are those with the strongest sources ofsup port.Desimone Duncan(1995)have proposed a model that clearly articulates such a view with regard to visual attention.These authors assume that visual corti- cal neurons processing different aspects ofa scene compete witheachother viamu- tually inhibi inter action The neur ns that"win active reach higher levels hey share inhibitory interactions.Voluntary shifts of attention result from the influence of excitatory top-down signals representing the to-be-attended features of the scene.These bias the competition among neurons representing the scene,increasing the activity ofP1: FXZ January 12, 2001 14:38 Annual Reviews AR121-07 170 MILLER ¥ COHEN of a conflict stimulus (e.g. the word GREEN displayed in red), because there is a strong prepotent tendency to read the word (“green”), which competes with the response to the color (“red”). This illustrates one of the most fundamental aspects of cognitive control and goal-directed behavior: the ability to select a weaker, task-relevant response (or source of information) in the face of competition from an otherwise stronger, but task-irrelevant one. Patients with frontal impairment have difficulty with this task (e.g. Perrett 1974, Cohen & Servan-Schreiber 1992, Vendrell et al 1995), especially when the instructions vary frequently (Dunbar & Sussman 1995, Cohen et al 1999), which suggests that they have difficulty adhering to the goal of the task or its rules in the face of a competing stronger (i.e. more salient or habitual) response. Similar findings are evident in the WCST. Subjects are instructed to sort cards according to the shape, color, or number of symbols appearing on them and the sorting rule varies periodically. Thus, any given card can be associated with several possible actions, no single stimulus-response mapping will work, and the correct one changes and is dictated by whichever rule is currently in effect. Humans with PFC damage show stereotyped deficits in the WCST. They are able to acquire the initial mapping without much difficulty but are unable to adapt their behavior when the rule varies (Milner 1963). Monkeys with PFC lesions are impaired in an analog of this task (Dias et al 1996b, 1997) and in others when they must switch between different rules (Rossi et al 1999). The Stroop task and WCST are variously described as tapping the cognitive functions of either selective attention, behavioral inhibition, working memory, or rule-based or goal-directed behavior. In this article, we argue that all these functions depend on the representation of goals and rules in the form of patterns of activity in the PFC, which configure processing in other parts of the brain in accordance with current task demands. These top-down signals favor weak (but task-relevant) stimulus-response mappings when they are in competition with more habitual, stronger ones (such as in the Stroop task), especially when flexibility is needed (such as in the WCST). We believe that this can account for the wide range of other tasks found to be sensitive to PFC damage, such as A-not-B (Piaget 1954, Diamond & Goldman-Rakic 1989), Tower of London (Shallice 1982, 1988; Owen et al 1990), and others (Duncan 1986, Duncan et al 1996), Stuss & Benson 1986). We build on the fundamental principle that processing in the brain is compet￾itive: Different pathways, carrying different sources of information, compete for expression in behavior, and the winners are those with the strongest sources of sup￾port. Desimone & Duncan (1995) have proposed a model that clearly articulates such a view with regard to visual attention. These authors assume that visual corti￾cal neurons processing different aspects of a scene compete with each other via mu￾tually inhibitory interactions. The neurons that “win” the competition and remain active reach higher levels of activity than those with which they share inhibitory interactions. Voluntary shifts of attention result from the influence of excitatory top-down signals representing the to-be-attended features of the scene. These bias the competition among neurons representing the scene, increasing the activity of Annu. Rev. Neurosci. 2001.24:167-202. 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