Abstract
How the simultaneity experienced in visual perception depends on the location of stimuli in three-dimensional space was examined. Two vertical line stimuli (20.8 × 1.2 arc min) were presented at the same level but different depth position by the use of 5.8 arc min of binocular disparity cue; one line was in front of the other line. Pair of mirrors and displays controlled by a personal computer were arranged as a haploscope to present binocular disparity. The viewing distance to the displays was about 45 cm. A fixation point was presented on the display surface 56 arc min below the line stimuli. There were five conditions for depth location of the stimuli; the midway point of the two line stimuli was nearer or farther than the fixation point by 5.8 or 11.6 arc min of disparity, or at the same depth plane with the fixation point. There were seven SOA conditions for the presentation of the two line stimuli, ranging from −64 msec to 64 msec by 16 msec step (positive value of the SOA indicates that the presentation of the nearer stimulus was earlier than that of the farther stimulus). Each condition was presented 20 times in random order. Six observers judged which of the nearer or farther line stimuli was presented later in each trial. We found that the farther stimulus tended to be perceived as presented earlier than the presentation of the nearer stimulus, and that this tendency was more salient in the space nearer than the fixation point. The averaged SOA of the stimulus presentation, which was necessary for the two line stimuli to be perceived as presented at the same time was about 10 msec when the stimuli presented around the fixation point while it exceeded 20 msec when both of the stimuli was nearer than the fixation point. These results suggest that the temporal phenomena, such as simultaneity, in visual perception depend on the location of the stimuli in three-dimensional space.