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This chapter describes neurophysiological research in humans that provides insights into the neural mechanisms mediating the different modes of attentional function. It addresses mechanisms of location selection, with a particular focus on the neural operations that determine the spatial profile of the focus of attention. It discusses data indicating that the latter is not fixed, but varies depending on top-down (voluntary or goal-directed) requirements on target selection of a given task. The evidence reviewed here is pertinent to resolving a longstanding debate about the nature of what has been metaphorically referred to as the spotlight of attention. The chapter then reviews signatures of feature-based selection and focuses on mechanisms and conditions underlying its operation in a spatially global way. Finally, it considers findings that have documented the operation of object-based attention and discusses mechanisms that grant priority over feature selection. In general, it is argued that attentional selection based on locations, features, or objects is not subserved by a single neural process, but that different neural mechanisms are coordinated to operate on a tight spatiotemporal scale, to enable the organism to perceive and act in a complex and rapidly changing environment.

This chapter reviews behavioral studies of figure-ground segregation in pigeons. In particular, it considers two related areas of research that address the same basic question: Do animals see a world filled with coherent objects, as we do? An object is defined here as a group of locations organized according to perceptual grouping cues (e.g., surface similarity or connectedness) at earlier stages of visual processing, instead of structural, invariant representations that are formed at higher levels of visual processing and are used for object recognition. The chapter first reviews research that concentrates on the ability of animals to segregate figures that require attention and action from backgrounds that can be ignored. It then attempts to answer the question: Once an object is segregated from the background, how does it affect attention?

This chapter presents an overview of the research on visual attention, which has been studied extensively in cognitive psychology and neuroscience. The studies discussed are limited to the major empirical findings on visual attention that have implications for the scientific understanding of consciousness. The chapter includes studies on feature-based attention, spatial attention, object-based attention, effortless attention, the mechanisms supporting the different forms of attention (e.g., neural structures and pathways), and the evolution of these mechanisms. This review is important for the book’s primary argument that consciousness and attention must be dissociated at some level, as there are functionally different forms of attention that seem to operate independently and to have evolved at different times from each other—an argument that is difficult to make for consciousness.

The processes underlying attention and consciousness, while overlapping, are not identical. Unconsciously processed stimuli can serve as cues for allocating stimulus-driven attention to selective locations (space-based attention) as well as to objects (object-based attention) or their features (feature-based attention). Moreover, stimuli to which attention is selectively directed do not always register in consciousness. This demonstrates that selective attention is not sufficient for conscious registration. Whereas attention can be directed separately to form and surface features at unconscious levels of processing, it can be directed to the entire object only at conscious levels. Some evidence indicates that the conscious registration of stimuli like highly familiar scenes or faces can occur without selective attention but other evidence indicates that some attention is required. A model is proposed of how attention, viewed in terms of cortical arousal distributed across the visual field, can be locally modulated by selective processes.