172 MILLER■COHEN of the world(C2).However,if you are in England(C3),you should look righ (R2).This is a classic example of a circumstance requiring cognitive control which we assume depends on the PFC.How does the PFC mediate the correct behavior? We assume that cues in the environment activate internal representations within 0 facti habitual or more salient)but produces the incorrect behavior.Thus,standing at the corner(C1),your"automatic"response would be to look left(RI).However,other cues in the environment"remind"you that you are in England(C3).That is,the cues activate the corresponding PFC representation,which includes information about the appropriate action.This produ activity along the pathway leading you to look right (e.g.CI that activation of this PFC representation is necessary for you to perform the correct behavior.That is,you had to keep"in mind"the knowledge that you were in England.You might even be able to cross a few streets correctly while keeping presentation antained nhorekel to revert to themore if this activity s tha is,ifyou you are in England tual response and look left.Repeated selection can strengthen the pathway from CI to R2 and allow it to become independent of the PFC.As this happens,the behavior becomes more automatic,so you can look right without having to keep in mind that you are in En gland.An rtant question is how the PFC develops the needed to prod repre sentations ce the contextually appropriate response In an unfamiliar situation you may try various behaviors to achieve a desired goal,perhaps starting with some that have been useful in a similar circumstance (looking to the left for oncoming traffic)and,if these fail,trying others until you meet with success (e.g.by looking right).We assume that each of these is as- iated with son patter of activity within the PFC(as in Figure )When a pattern of activity by strengthening connections between the PFC neurons acti vated by that behavior.This process also strengthens connections between these neurons and those whose activity represents the situation in which the behavior was useful,establishing an association between these circumstances and the PFC eated iterations the PFC representation can be further slaborated as subter combi and rep nations of events and contingencies between them and the requisite actions are learned.As is discussed below.brainstem neuromodulatory systems may provide the relevant reinforcement signals,allowing the system to"bootstrap"in this way Obviously,many details ed to he added hef we fully unders stand the c plexity of cognitive control.But we believe that this general notio can explair many of the posited functions of the PFC.The biasing influence of PFC feedback signals on sensory systems may mediate its role in directing attention(Stuss Benson 1986:Knight 1984,1997;Banich et al 2000),signals to the motor systemP1: FXZ January 12, 2001 14:38 Annual Reviews AR121-07 172 MILLER ¥ COHEN of the world (C2). However, if you are in England (C3), you should look right (R2). This is a classic example of a circumstance requiring cognitive control, which we assume depends on the PFC. How does the PFC mediate the correct behavior? We assume that cues in the environment activate internal representations within the PFC that can select the appropriate action. This is important when the course of action is uncertain, and especially if one of the alternatives is stronger (i.e. more habitual or more salient) but produces the incorrect behavior. Thus, standing at the corner (C1), your “automatic” response would be to look left (R1). However, other cues in the environment “remind” you that you are in England (C3). That is, the cues activate the corresponding PFC representation, which includes information about the appropriate action. This produces excitatory bias signals that guide neural activity along the pathway leading you to look right (e.g. C1 →···→ R2). Note that activation of this PFC representation is necessary for you to perform the correct behavior. That is, you had to keep “in mind” the knowledge that you were in England. You might even be able to cross a few streets correctly while keeping this knowledge in mind, that is, while activity of the appropriate representation is maintained in the PFC. However, if this activity subsides—that is, if you “forget” you are in England—you are likely to revert to the more habitual response and look left. Repeated selection can strengthen the pathway from C1 to R2 and allow it to become independent of the PFC. As this happens, the behavior becomes more automatic, so you can look right without having to keep in mind that you are in England. An important question is how the PFC develops the representations needed to produce the contextually appropriate response. In an unfamiliar situation you may try various behaviors to achieve a desired goal, perhaps starting with some that have been useful in a similar circumstance (looking to the left for oncoming traffic) and, if these fail, trying others until you meet with success (e.g. by looking right). We assume that each of these is as￾sociated with some pattern of activity within the PFC (as in Figure 2). When a behavior meets with success, reinforcement signals augment the corresponding pattern of activity by strengthening connections between the PFC neurons acti￾vated by that behavior. This process also strengthens connections between these neurons and those whose activity represents the situation in which the behavior was useful, establishing an association between these circumstances and the PFC pattern that supports the correct behavior. With time (and repeated iterations of this process), the PFC representation can be further elaborated as subtler combi￾nations of events and contingencies between them and the requisite actions are learned. As is discussed below, brainstem neuromodulatory systems may provide the relevant reinforcement signals, allowing the system to “bootstrap” in this way. Obviously, many details need to be added before we fully understand the com￾plexity of cognitive control. But we believe that this general notion can explain many of the posited functions of the PFC. The biasing influence of PFC feedback signals on sensory systems may mediate its role in directing attention (Stuss & Benson 1986; Knight 1984, 1997; Banich et al 2000), signals to the motor system Annu. Rev. Neurosci. 2001.24:167-202. Downloaded from arjournals.annualreviews.org by University of California - Los Angeles on 03/27/06. For personal use only