Contribution of nonattentive motion to object tracking

Object tracking has been generally discussed in relation to attention, but it is quite possible that nonattentive low-level motion components are involved. To elucidate this issue, we examined temporal aspects of object tracking by using stimuli comprised of just a single attribute and those comprised of multiple attributes. High-level motion processes supposedly can process cross-attribute motion, while nonattentive low-level motion processes cannot handle such motion. In Experiment 1, we measured the upper temporal limits for within- and cross-attribute object tracking, using stimuli defined by several different attributes (luminance, motion, binocular disparity, flicker, and contrast). It was found that the temporal limits with within-attribute stimuli (4-5 Hz) were much higher than those with cross-attribute stimuli (2-3 Hz). These results suggest that mechanisms involved in within- and cross-attribute object tracking are partially different. We conducted two additional experiments to clarify the nature of this difference. In Experiment 2, we measured the temporal limits for classical apparent motion perception using the same stimulus combinations as for Experiment 1. The temporal limits with within- and cross-attribute stimuli were both between 4 and 5 Hz. These values corresponded to those of within-attribute object tracking but were faster than those of cross-attribute object tracking. In Experiment 3, we measured the temporal limit for voluntary shifts of attention that did not involve motion. Temporal limits quite similar to those for cross-attribute object tracking (2-3 Hz) were obtained. These results suggest that nonattentive motion mechanisms are involved in within-attribute object tracking, whereas attention-based mechanisms mediate cross-attribute object tracking.