CHAPTER48AttentionINTRODUCTIONIntuition, together with cognitive and psychophysi-cal experiments, shows that the brain is limited in the amount of information that it can process at any moment in time. For instance, when people are asked to identify the objects of a briefl y presented scene, they become less accurate as the number of objects increases. Similarly, when people concentrate on one demanding task (e.g., mental arithmetic) this typically comes at the expense of performance on other simultaneous activi-ties (e.g., recalling a familiar tune). Limitations on the ability to simultaneously carry out multiple cognitive or perceptual tasks refl ect the limited capacity of some stage or stages of sensory processing, decision-making, or behavioral control. As a result of such computa-tional bottlenecks, it is necessary to have neural mech-anisms in place to ensure the selection of stimuli, or tasks, that are immediately relevant to behavior. “Attention” is a broad term denoting the mechanisms that mediate this selection.Over the past several decades, research has concen-trated on the relation between attention and sensory perception. Clearly, it is possible to focus on selected stimuli in any sensory modality—sights, sounds, smells, and touch. Most studies, however, have exam-ined vision, the dominant sensory modality for humans and nonhuman primates. These experiments have investigated visual attention at multiple levels, ranging from behavior to the single neuron. This chapter out-lines the view of attention that has emerged from these studies.A key observation is that attending to an object greatly enhances the ability to perceive and report the object’s visual attributes; conversely, withdrawing attention, either by force of the behavioral context or following certain brain lesions, can render observers practically blind to certain visual events. A second key point is that the attentional processes that underlie perceptual selection may also guide the voluntary eye movements with which foveate animals, such as monkeys and humans, scan the environment. In terms of neural organization, although some neural centers have been closely linked with the control of attention, attention appears to affect the activity of neurons at almost all levels of the visual system.VARIETIES OF ATTENTIONIn natural behavior, individuals have great fl exibil-ity as to how and when they attend and what they attend to. Attending to an object often is accompanied by overt orienting toward that object using the eyes and possibly also the head and body. When a person enters a room, it is natural to turn in that person’s direction. However, one can also attend covertly to objects that are not in the center of gaze, without looking directly at them. When driving, one can monitor a passing car while continuing to look straight ahead. Covert attention can improve peripheral visual acuity, thereby extending the functional fi eld of view. Although one can direct attention without moving the eyes, the converse does not appear to be true: experi-ments show that covert attention must be deployed to an object before that object can be targeted with an eye movement. Indeed, saccades—rapid eye movements used most commonly for scanning the environment—Fundamental Neuroscience, Third Edition 1113 © 2008, 2003, 1999 Elsevier Inc.1114 48. ATTENTION VII. BEHAVIORAL AND COGNITIVE NEUROSCIENCEmay be considered a motor manifestation of visual attention, a relationship that is discussed in more detail later.Certain external stimuli can summon attention in and of themselves. These can be physically salient objects, such as especially large, bright, or loud objects, or stimuli with special learned signifi cance, such as one’s own name or a mother’s face. This mode of atten-tional engagement, known as exogenous or stimulus driven, ensures that salient and potentially important external events do not pass unnoticed even if they are not being actively sought out. However, purposeful behavior often requires that attention be directed vol-untarily, or endogenously, based on internally defi ned goals and against potential external distractions. When reading, one purposefully directs one’s attention from one word to the next, tuning out noise and other distrac-tions. In natural behavior, endogenous and exogenous factors interact continuously to control the allocation of sensory processing (Egeth and Yantis, 1997).Another important issue is what is attended. Atten-tion can be directed to a location in space, regardless of what happens to be at that location. It can also be directed to a feature, as when one searches within a complex scene for an object of a particular shape or color. Finally, attention can select whole objects, as demonstrated by studies showing that when attention is directed to one feature of an object, the other features of the same object are selected automatically for visual processing, and by studies fi nding that when attention is directed to one of two semitransparent objects, attending to a feature of one object impairs processing of the features of the other object. In each of these cases, attention has been found to facilitate processing of the attended location, feature, or object, as assessed at the behavioral and neural levels (Reynolds and Chelazzi, 2004).Attention is thus highly fl exible and can be deployed in a manner that best serves the organism’s momen-tary behavioral goals: to locations, features, or objects, based on internal goals or the external environment, with or without accompanying orienting movements. It is important to keep in mind that although these phenomena can be isolated in laboratory experiments, all varieties of attention operate seamlessly during natural behavior.NEGLECT SYNDROME: A DEFICIT OF SPATIAL ATTENTIONStudies of patients with brain lesions have identi-fi ed regions of the brain that are involved in attention (see Fig. 48.1, which shows the major divisions of the human cortex). Unilateral lesions in the parietal lobe, FIGURE 48.1 Lateral view of a human brain. Frontal (purple), parietal (orange), temporal (blue), and occipital (yellow) lobes are outlined.VII. BEHAVIORAL AND COGNITIVE NEUROSCIENCEthe frontal lobe, and the anterior cingulate cortex (Heilman, 1979; Vallar, 1993) in humans may cause a profound inability to attend to certain spatial regions, a syndrome known as spatial neglect. At the subcorti-cal level, lesions of the