CHAPTER 52The Prefrontal Cortex and Executive Brain FunctionsINTRODUCTIONWhat controls your thoughts? How do you decide what to pay attention to? How do you act appropri-ately while dining in a restaurant or listening to a lecture? How do you plan and execute errands? How do you manage to pursue long-term goals, like obtain-ing a college degree, in the face of the many distrac-tions that can knock you “off task”? In short, how does the brain manage to orchestrate the activity of millions of neurons to produce behavior that is willful, coordi-nated, and extends over time? This is called cognitive control, the ability of our thoughts and actions to rise above mere reactions to the immediate environment and be proactive: to anticipate possible futures and coordinate and direct thought and action to them. It is a hallmark of intelligent behavior.Because cognitive control necessarily involves the coordination of many different brain processes, it no doubt depends on circuits that extend over much of the cerebral cortex. However, one cortical region, the prefrontal cortex (PFC), seems to play a central role. It is the cortical area that reaches the greatest relative size in the human brain and is thus thought to be the neural instantiation of the mental qualities that we think of as “intelligent.” Accordingly, the PFC receives converg-ing information from many brain systems processing external and internal information and it is intercon-nected with motor system structures needed for vol-untary action.Here, we discuss anatomical, neuropsychological, and neurophysiological evidence for, and theories of, the role of the PFC in cognitive control. We will begin with a discussion of the characteristics of volitional, or controlled, processes as they can provide predictions about the neural mechanisms likely to underlie them.CONTROLLED PROCESSINGBehavior varies along a continuum from mindless, automatic behaviors to willful and purposeful (i.e., controlled) behaviors (see Barsalou, 1992). Automatic processes are refl exive. If a car or a predator is bearing down on us, we leap out of the way before we even have had a chance to form the intention to do so; our mind seems to “catch up” afterward. Automatic pro-cesses thus seem to depend on relatively straightfor-ward, hardwired relationships between the brain’s input and output systems. In neural terms, it seems that they depend on well-established, neural pathways just waiting to be triggered by the right sensory cue. That is, automatic processes are driven in a “bottom-up” fashion: they are determined largely by the nature of the sensory stimuli and whatever reaction they are most strongly wired to.These relatively simple, hardwired, cue-response mappings are useful in many situations. Because they can simply be fi red off by the environment, they are quick and can be executed without taxing the limited capacity of our conscious thoughts. But a creature that is only at the mercy of its immediate environment is not well equipped to deal with the ambiguity, and exploit the opportunity, of a complex and dynamic world. Sometimes, the course of action is not obvious because cues are ambiguous (i.e., they activate more than one possible internal representation), or multiple Fundamental Neuroscience, Third Edition 1199 © 2008, 2003, 1999 Elsevier Inc.1200 52. THE PREFRONTAL CORTEX AND EXECUTIVE BRAIN FUNCTIONS VII. BEHAVIORAL AND COGNITIVE NEUROSCIENCEbehaviors might be triggered and the one that is optimal for a given situation is at a disadvantage rela-tive to better established (more automatic) alternatives. In these situations, ambiguity needs to be resolved by our internal states and intentions, by knowledge of possible and desired future outcomes (goals) and what means have been successful at achieving them in the past as well as their relative costs and benefi ts. This information is used to activate the same basic sensory, memory, and motor processes that are engaged during automatic behaviors. Only now, they are not triggered by cues from the environment. They are orchestrated in a top-down fashion by our expectations about goals and the means to achieve them. So, this mode of behav-ior is controlled in the sense that we (our knowledge and intensions) are in charge, not the environment. A tenet of modern studies of behavior is that this knowl-edge is obtained by learning mechanisms that detect and store associations between cues, internal states, and actions that predict goal attainment (reward) (see Dickinson, 1980). Insofar as primates are capable of navigating situations that involve diverse, arbitrary, relationships across a wide range of informational domains, it follows that a neural system for cognitive control must have access to information from many brain systems and the ability to encode the goal-relevant relationships between them.The cognitive control system must also have the ability to select which sensory, memory, and motor processes are activated at a given moment. Selection is central because of our very limited capacity to engage in controlled behaviors. This is evident to anyone who has tried to talk on the phone and answer an e-mail at the same time; we can think about only a limited number of things at a time. This is in contrast to our high capacity for automatic processes. Because they can be triggered by the environment, any number of them can be fi red off at once, as long as they do not come into direct confl ict with one another. It makes sense that brain mechanisms for cognitive control would evolve with this capacity limitation. There is the tradeoff between amount of information and depth of analysis; focusing processing on a narrow band of information relevant to a current goal allows a much more elaborate analysis of a situation and available options. This single-mindedness also allows us to stay on track; processing information not relevant to a current goal increases the chance for distraction. In fact, in many views of cognition, control and attention are virtual synonyms. It follows that a neural system for cognitive control must have the infrastructure and mechanisms for selecting goal-relevant, and suppress-ing goal-irrelevant, processes throughout the cerebral cortex.The cognitive control system must also have a way to deal with the gaps in time that are inevitable with goal-directed behaviors. Information about predicted goals and means must be brought online before the
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