Work from a variety of approaches suggests that the selection and representation of visual information is subject to fairly severe spatial limitations. These limitations are believed to reflect the structure and function of basic visual information-processing mechanisms, including the formation and individuation of coherent object representations (see, e.g., Cavanagh, 2001; C. W. Eriksen & Hoffman, 1973;He, Cavanagh, & Intriligator, 1996Intriligator & Cavanagh, 2001;Pelli, Palomares, & Majaj, 2004). Here we report evidence that the spatial control of attention can be modulated by nonspatial features (such as color and luminance) of the stimuli.Intriligator and Cavanagh (2001) used the term "attentional resolution" to define the smallest possible interitem distance that still allows observers to successfully shift attention from one item to another. They measured attentional resolution using a task that we will refer to as the attentional walk. In one condition, displays consisted of circular arrays of identical disks that varied in density and eccentricity. While observers fixated a central point, one of the disks briefly changed color, indicating that attention should be shifted to that disk. Following this cue, a series of verbal commands instructed the observers to move attention one disk to the left or to the right of the disk they were currently attending (e.g., "left, left, right, left," etc.). At the end of a five-to seven-step series of commands, another disk was probed by having it change colors. The observers' task was to indicate whether or not the probed disk was the end point of the attentional walk. The logic was that if the observers were able to reliably shift their attention within the array, then they would be able to follow the walk and indicate correctly where it ended. If, however, the density of the array was beyond the resolution of attentional control, then the observers would be unable to move through the array successfully and therefore unable to indicate where the walk ended. Notice that performance in the attentional walk task reflected not only the observers' ability to select an individual item, but also their ability to use that information to successfully shift attention from one item to another within arrays of items; these abilities may be separable processing limitations (Moore, Lanagan-Leitzel, & Fine, 2007). Using the attentional walk task, Intriligator and Cavanagh (2001) found that observers could easily perceive individual items within a broad range of display densities, yet they could not reliably report the end point of an attentional walk. In addition, they found that attentional resolution, as measured using the attentional walk, tended to be coarser in the upper visual field than in the lower visual field (see also He et al., 1996), and that it tended to be coarser with increasing eccentricity. These findings are consistent with the idea that spatial attention is spatially limited.In conceptualizing the limitations of attentional resolution, Intriligator and Cavanagh...
