Abstract
A single flash accompanied with two auditory stimuli would be perceived as two flashes (Shams et al., 2000). In present study, we examined whether the number of auditory stimuli could affect the apparent motion, which is induced by multiple flash stimuli. We conducted experiments in which the visual stimuli (2.9 X 8.7 arc min) that induced the apparent motion on the front parallel plane, or in depth dimension, were accompanied with multiple auditory stimuli (105 Hz tones). The number of the visual and auditory stimuli ranged from two to four. The duration of each stimulus and inter stimulus interval were respectively 67 msec and 83 msec for both visual and auditory stimuli. There were three distance conditions, which was defined by the distance between the first and last visual stimuli: 0 (stationary), 52.0, and 104.0 arc min for the motion on the front parallel plane (rightward, or leftward), and 0, 7.2, and 17.4 arc min (crossed, or uncrossed disparity) for the motion in depth (approaching, or going away from observer). The visual stimuli were located within these distances at regular intervals. Each condition was presented 10 times in random order for each observer. In each trial, the observer reported the number of the visual stimuli (Exps 1, 2), and the distance between the first and last visual stimuli (Exp 2). When the number of the auditory stimuli was more (less) than that of the visual stimuli, observers tended to perceive more (less) visual stimuli than the veridical number of the visual stimuli for both motion on the front parallel plane and in depth dimension. When the observers perceived more (less) visual stimuli than the veridical number, the perceived distance of the apparent motion was significantly larger (smaller) than the reported distance in the trials in which they perceived the visual stimuli with veridical numbers. These results indicate that the auditory stimulation modifies the apparent motion by modulating the extent of the displacement and step numbers.