The spatial resolution of visual attention

Two tasks were used to evaluate the grain of visual attention, the minimum spacing at which attention can select individual items. First, observers pe rformed a tracking task at many viewing distances. When the display subtend ed less than 1 degrees in size, tracking was no longer possible even though observers could resolve the items and their motions: The items were visibl e but could not be individuated one from the other. The limiting size for s election was roughly the same whether tracking one or three targets, sugges ting that the resolution limit acts independently of the capacity limit of attention. Second, the closest spacing that still allowed individuation of single items in dense, static displays was examined. This critical spacing was about 50% coarser in the radial direction compared to the tangential di rection and was coarser in the upper as opposed to the lower visual field. The results suggest that no more than about 60 items can be arrayed in the central 30 degrees of the visual field while still allowing attentional acc ess to each individually. Our data show that selection has a coarse grain, much coarser than visual resolution. These measures of the resolution of at tention are based solely on the selection of location and are not confounde d with preattentive feature interactions that may contribute to measures fr om flanker and crowding tasks. The results suggest that the parietal area i s the most likely locus of this selection mechanism and that it acts by poi nting to the spatial coordinates (or cortical coordinates) of items of inte rest rather than by holding a representation of the items themselves. (C) 2 001 Academic Press.