Journal of Experimental Psychology: General 1994, Vol. 123, No. 2. 161-177 Copyright 1994 by the American Psychological Association, Inc. 0096-3445/94^3.00 Shifting Visual Attention Between Objects and Locations: Evidence From Normal and Parietal Lesion Subjects Robert Egly, Jon Driver, and Robert D. Rafal Space- and object-based attention components were examined in neurologically normal and parietal-lesion subjects, who detected a luminance change at 1 of 4 ends of 2 outline rectangles. One rectangle end was precued (75% valid); on invalid-cue trials, the target appeared at the other end of the cued rectangle or at 1 end of the uncued rectangle. For normals, the cost for invalid cues was greater for targets in the uncued rectangle, indicating an object-based component. Both right- and left-hemisphere patients showed costs that were greater for contralesional targets. For right- hemisphere patients, the object cost was equivalent for contralesional and ipsilesional targets, indicating a spatial deficit, whereas the object cost for left-hemisphere patients was larger for contralesional targets, indicating an object deficit. Selective attention allows us to pick out and respond to relevant information while ignoring the myriad distracting stimuli in cluttered visual scenes. Several decades of intense research have substantially advanced our understanding of the operations and neural mechanisms of selective attention. There is now broad agreement that, although attention may play a special role in integrating elementary visual features (Treisman, 1991; Treisman & Gelade, 1980), attention can also modulate the coding of these elementary features within the visual system (Chaudhuri, 1990; Prinzmetal, Presti, & Posner, 1986). Moreover, there has been increas- ing success in relating behaviorally identified mechanisms of attention to specific neural substrates, based on evidence ranging from the effects of brain damage (e.g., Posner, 1988), to neuroimaging in neurologically normal subjects (e.g., Corbetta, Miezen, Dobmeyer, Shulman, & Petersen, 1991), and to single-cell recording from behaving primates (e.g., Moran & Desimone, 1985). Against this background of growing interdisciplinary con- sensus, one issue stands out because it has become increas- ingly rather than decreasingly controversial: the dispute between space-based and object-based models of visual attention (e.g., Kanwisher & Driver, 1992). The former model suggests that visual attention is directed to particular locations in a purely spatial representation of the visual Robert Egly and Robert D. Rafal, Department of Neurology and Center for Neuroscience, University of California, Davis; Jon Driver, Department of Experimental Psychology, University of Cambridge, Cambridge, England. This research was supported by Public Health Service (PHS) Grant F32 NS09162 to Robert Egly, PHS Grant RO1 MH41544 to Robert D. Rafal, and MRC (United Kingdom) and North Atlantic Treaty Organization grants to Jon Driver. Additional support was provided by the McDonnell-Pew Center for Cognitive Neuro- science of Attention, University of Oregon. We thank our patients for their willing participation. Correspondence concerning this article should be addressed to Robert Egly, Department of Neurology, University of California, Davis, 150 Muir Road, Martinez, California 94553. Electronic mail may be sent to marva4!egly@ucdavis.edu. field; the analogy of a spotlight is often suggested (e.g., LaBerge, 1983; Posner, 1980). In contrast, object-based models suggest that attention is directed to the candidate objects (or perceptual groups) that result from a preattentive segmentation of the visual scene in accordance with group- ing principles (e.g., Driver & Baylis, 1989; Duncan, 1984). There are at least two different ways to conceive of object- based attention (Vecera & Farah, 1992). According to one interpretation, position plays absolutely no role in the se- lected representations, which would be object centered, in the strong sense of Marr (1982). On a less extreme view, attention is object based in the sense that positions in the field are selected together specifically because they belong to the same object (e.g., Baylis & Driver, 1992; Farah, 1990). It is this latter sense of object-based attention that has usually been advocated and that we consider in this article. We begin with a review of existing data, which reveals evidence for both the purely space-based view of attention and the object-based view (as defined above). We note, however, that the evidence for these two views has come from very different paradigms. These previous findings suggest that there may be both space-based and object- based components to visual attention. In other words, the traditional opposition between space-based and object- based theories may be a false dichotomy because they are not mutually exclusive possibilities. We then develop a new technique for measuring both space-based and object- based attentional components within the same paradigm. We demonstrate in normal subjects that both components apply to covert orienting in a luminance-detection task. Finally, we examine the effects of parietal injury, known to disrupt covert visual orienting (e.g., Posner, Walker, Friedrich, & Rafal, 1984), on the space-based and object- spaced components. Previous Evidence for Space-Based Attention One of the classic paradigms for investigating visual attention uses a visual precue to indicate the likely location 161