Neural Mechanisms of Visual Attention: Object-Based Selection of a Region in Space Catherine M. Arrington and Thomas H. Carr Michigan State University Andrew R. Mayer and Stephen M. Rao Medical College of Wisconsin Abstract & Objects play an important role in guiding spatial attention through a cluttered visual environment. We used event-related functional magnetic resonance imaging (ER-fMRI) to measure brain activity during cued discrimination tasks requiring subjects to orient attention either to a region bounded by an object (object-based spatial attention) or to an unbounded region of space (location-based spatial attention) in anticipa- tion of an upcoming target. Comparison between the two tasks revealed greater activation when attention selected a region bounded by an object. This activation was strongly lateralized to the left hemisphere and formed a widely distributed network including (a) attentional structures in parietal and temporal cortex and thalamus, (b) ventral-stream object processing structures in occipital, inferior-temporal, and parahippocampal cortex, and (c) control structures in medial- and dorsolateral-prefrontal cortex. These results suggest that object-based spatial selection is achieved by imposing addi- tional constraints over and above those processes already operating to achieve selection of an unbounded region. In addition, ER-fMRI methodology allowed a comparison of validly versus invalidly cued trials, thereby delineating brain structures involved in the reorientation of attention after its initial deployment proved incorrect. All areas of activation that differentiated between these two trial types resulted from greater activity during the invalid trials. This outcome suggests that all brain areas involved in attentional orienting and task performance in response to valid cues are also involved on invalid trials. During invalid trials, additional brain regions are recruited when a perceiver recovers from invalid cueing and reorients attention to a target appearing at an uncued location. Activated brain areas specific to attentional reorientation were strongly right-lateralized and included posterior temporal and inferior parietal regions previously implicated in visual atten- tion processes, as well as prefrontal regions that likely subserve control processes, particularly related to inhibition of inap- propriate responding. & INTRODUCTION The job of visual attention is to select information to be given priority during perception. Theories of visual attention once took for granted that visual attention selects a region of space, which is then inspected for the presence of interesting or useful information. There is considerable evidence to support such a view, much of which comes from variations on Posner's spatial cueing paradigm (Posner, 1980; Posner, Snyder, & Davidson, 1980). In this task, target stimuli appear at locations spread about the visual field. A cue to the most likely location presented in advance of a target alters the speed and accuracy of target processing. Shifting of visual attention based on the cue can be measured by comparing response times and accuracy when the target appears at the cued location versus when it appears at some other location in the visual field. The typical findings in these studies have been benefits, in terms of faster and more accurate responding at or near the cued location, and costs, in terms of slower and less accurate responding at uncued locations. Theories of visual attention built on these findings are ``location-based.'' They propose that attention is de- ployed to a location defined on environmental spatial coordinates. Attention facilitates the processing of in- formation arising at or near that location, leaving other locations unhelped or inhibited. The rate at which facilitation falls off with distance around the selected location can be varied. This gives perceivers a degree of control over the size of the facilitated region of space. The mechanism of spatial selection has been likened to a spotlight (Posner, 1980), a zoom lens (Eriksen & Yeh, 1985), or a gradient (Henderson, 1991; LaBerge & Brown, 1989) centered on the selected location. But does visual attention always work in this fashion, selecting one spatial region at the expense of others? Groundbreaking work by Duncan (1984) showed costs for switching attention between objects that occupied D 2000 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 12:Supplement 2, pp. 106±117