A BIASED COMPETITION ACCOUNT OF OBJECT- BASED ATTENTION

Because the visual system cannot process all of the items present in a visual scene, some stimuli must be selected over others to prevent the visual system from becoming overloaded. Visual attention allows some stimuli or events to be processed instead of others. Most research on attentional selection has focused on spatial or location-based attention, in which the locations occupied by stimuli are selected for further processing. Recent research, however, has demonstrated the importance of objects in guiding attentional selection. Because of the long history of spatial attention research, theories of spatial attention are more mature than theories of other visual processes, such as object segregation and object attention. In the present chapter, we outline a biased competition account of object segregation and attention, following similar accounts that have been developed for visual search (Desimone & Duncan, 1995). In the biased competition account, there are two sources of visual information that allow an object to be processed over other objects: bottom-up information carried by the physical stimulus and topdown information based on an observer’s goals. We use the biased competition account to combine many diverse findings from both behavioral and neurobiological studies of object attention. 2 Until the mid-1980s, the majority of research on selective visual attention studied spatial attention and the processes by which stimuli were selected on the basis of their location. Distinctions on topics such as the movement of the spatial focus (does it move smoothly through space or does it jump from place to place? ) and the shape of the spatial window (is it a spotlight, a zoom lens, or a gradient? ) were important to the theoretical perspectives that dominated the literature. However, the tide began to change with increasing demonstrations that objects may be the recipients of visual attention. With increasing demonstrations of so-called “object-based” attention, the strong spatial (or location-based) account fell from favor. Although there was initial debate over whether attention selects objects or locations, many researchers studying visual attention would agree that both forms of selection are possible because it is unlikely that there is a single attentional “bottleneck” or limitation (e.g., Allport, 1993; Haimson & Behrmann, in preparation; Luck & Vecera, 2000; Vecera & Luck, 2000). Once the object-based versus location-based debate has been put aside, other interesting questions regarding the nature of attentional selection arise. For example, what processes form the objects that are selected? How are object-based selection and location-based selection combined to produce coordinated behavior? What are the neural mechanisms that underlie these forms of attentional selection? In this chapter we attempt to answer these questions by reviewing the recent object-based attention literature, including behavioral and neurobiological studies. Because spacebased selective attention has been studied longer and more intensively than object-based selection, theoretical accounts of spatial attention are more mature than those of object attention (for theories of spatial attention see Desimone & Duncan, 1995; Mozer & Sitton, 1998; Sperling & Weichselgartner, 1995; Treisman, 1988; Wolfe, 1994). Because there are few theoretical account of object attention, our goal here is to illustrate the types of attentional phenomena that theories of object-based attention must explain and to outline a “biased competition” account of object-based attentional selection. Following previous work (Vecera, in press), we extend the biased competition account of visual search (Desimone & Duncan, 1995) to the selection of objects by attentional processes (also see Behrmann & Haimson, 1999; Haimson & Behrmann, in preparation; O’Craven, Downing, & Kanwisher, 1999). Our account is intended to provide a framework for organizing the object-based attention literature and, hopefully, generating questions for future research (Vecera, in press). In what follows, we first discuss the visual processes relevant for object attention. We next outline generally the biased competition account of visual search presented by Desimone and Duncan (1995), and then apply the principles of biased competition to behavioral results that support object-based selection. We then discuss the cognitive neuroscience of object-based attention, focusing primarily on neuropsychological patients and neurophysiological studies. We summarize these results and discuss their relation to the biased competition account that we outline for the behavioral studies of object attention. Unit Formation and Biased Competition 3