Abstract
Purpose: We examined contribution of low-level motion systems to the asymmetry between the upper and lower visual fields in multiple object tracking (MOT) performances. He, Cavanagh & Intriligator (1996) suggested the asymmetry originated from attentional resolution (high-level visual processing), but it is not clear whether low-level motion systems contribute to the asymmetry. To clarify this point, we conducted MOT experiments where ISI (inter-stimulus interval) for motion stimuli was varied. The operation of spatiotemporal filters for low-level motion systems is supposedly disrupted with longer ISIs (Braddick, 1974). If the asymmetry between the upper and lower visual fields is affected by ISI, it could be argued that low-level motion systems are at least partially responsible for the asymmetry. On the other hand, if there is no effect of ISI, the asymmetry is attributed more to higher-order motion systems. Method: Nine moving green disks were presented in the area of 6 x 30 degrees visual angle on a CRT screen. The ISI for apparent motion was varied in 5 steps between 0 and 200 ms. The motion stimuli were presented either in the upper or lower visual field. The color of 2 out of the 9 disks changed to red at the beginning of a trial to designate tracking targets. Subjects were asked to track the targets while keep fixating on a marker at 10 degrees above or below the center of the area for a 7 sec period. At the end of each trial, subjects reported the disks that they acknowledged as targets. Result: As ISI increased, the MOT performances decreased in lower visual field, while the performances in upper visual field were little affected. These tendencies thus resulted in smaller magnitudes of asymmetry for longer ISIs. Conclusion: These results strongly suggest that the asymmetry comes from the advantage in MOT task for the lower visual field that is related to the characteristics of low-level motion systems.