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Won Mok Shim, George A. Alvarez, Yuhong V. Jiang; Maintaining multiple attentional foci: spatial separation affects behavior but not posterior parietal activity. Journal of Vision 2007;7(9):581. doi: 10.1167/7.9.581.
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© ARVO (1962-2015); The Authors (2016-present)
Humans are severely limited in their ability to maintain multiple attentional foci. In attentive tracking of moving objects, for instance, performance reduces as the number of tracked targets increases. Previous studies have interpreted such reduction in terms of a limit on the number of attentional foci. However, increasing the target number often correlates with reduced spatial separation between different targets. In this study, we isolate the role of target spatial separation from that of target number on attentive tracking. Participants view 16 bouncy balls, 4 in each visual quadrant, where the balls move randomly within their respective quadrant. Attentive tracking for a subset is required, where the subset may be a single ball (track 1), two balls within a single quadrant (track 2 near), or two balls, one in each adjacent quadrant (track 2 far). Results showed that tracking accuracy was determined primarily by spatial separation between tracked targets. Tracking two near targets was worse than tracking two far targets, despite the fact that the number of relevant distractors was fewer in the former condition. Tracking two far targets was equivalent to tracking a single target, showing little cost of splitting attention to two distant foci. In stark contrast to behavioral performance, activity in the posterior parietal cortex (as measured by fMRI) was sensitive to the number of attentional foci but not to the spatial separation between them. Parietal activity was lower in track 1 than in track 2 conditions, but it did not differ between track 2 near and track 2 far. Our results suggest that the posterior parietal cortex represents “attentional pointers” whose operation is independent of the spatial separation between the pointers. We propose that the resolution of attention is reduced by suppressive interaction between two adjacent attentional foci.
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