The ability to effectively deal with the overwhelming amount of information present in the environment at any given time requires humans to focus on some things and ignore others. For example, imagine attempting to read a book in a crowded public place. This requires the reader to focus on the words on the page and ignore the sounds of conversations going on in the background, allowing an effective focus on the task at hand. The process by which people can both select relevant and suppress irrelevant environmental information refers to a number of processes collectively referred to as “attention.” From an information-processing standpoint, attention can be conceptualized as operating not as a single, monolithic process, but rather a group of more fragmented, domain-specific processes. For instance, in the example above, attention is required to select or suppress information across more than one sensory modality. In addition, attention can select information based on its location in space, its identity, or its relevance to current goals. For this reason, research in the cognitive and brain sciences has typically focused on specific subcomponents of attentional processing. One broad distinction that has been made in the study of attention has been between the control of attention (i.e., how attention selects stimuli) and the subsequent effects of attention (i.e., what is the fate of stimuli once attended to). Within the domain of control, attentional selection can occur as the result of cognitive (top-down) or stimulus-driven (bottom-up) processes. In turn, these selection processes can bias the way in which information provided by the environment is interpreted. Since the majority of research on attention has focused on the visual system, the discussion of attention in this chapter will center primarily on the control of different aspects of visual attention. However, many of the principles discussed below hold true for the selection of information across other sensory domains. This chapter will outline relevant behavioral measures related to the control of attention, and functional theories of attention based on such measures. The major focus will be on the control of visual attention in both normal and neurologically impaired individuals, mapping functional theories of attention onto what is known about the cerebral structures subserving this process.
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