Multiple object tracking and attentional processing.

How are attentional priorities set when multiple stimuli compete for access to the limited-capacity visual attention system? According to Pylyshyn (1989) and Yantis and Johnson (1990), a small number of visual objects can be preattentively indexed or tagged and thereby accessed more rapidly by a subsequent attentional process (e.g., the traditional "spotlight of attention"). In the present study, we used the multiple object tracking methodology of Pylyshyn and Storm (1988) to investigate the relation between what we call "visual indexing" and attentional processing. Participants visually tracked a subset of a set of identical, independently randomly moving objects in a display (the targets), and made a speeded identification response when they noticed a target or a nontarget (distractor) object undergo a subtle form transformation. We found that target form changes were identified more rapidly than nontarget form changes, and that the speed of responding to target form changes was unaffected by the number of nontargets in the display when the form-changing targets were successfully tracked. We also found that this enhanced processing only applied to the targets themselves and not to nearby nontarget distractors, showing that the allocation of a broadened region of visual attention (as in the zoom-lens model of attentional allocation) could not account for these findings. These results confirm that visual indexing bestows a processing priority to a number of objects in the visual field.