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Three experiments examined anisotropies of tolerance of perceptual completion at the blind spot when a pair of line segments was presented on opposite sides of the blind spot. The tolerance of perceptual completion is defined as the maximum difference in a stimulus attribute between the line segments on opposite sides of the blind spot when perceptual completion of a line has occurred. The misalignment, orientation difference, and luminance difference between the line segments were used as the stimulus attributes in Experiments 1, 2, and 3, respectively. The results showed anisotropies of the tolerance of perceptual completion between horizontal and vertical configurations of the line segments. Vertical superiorities, which imply a greater extent of tolerance in the vertical configuration than that in the horizontal configuration, were observed for misalignment and orientation difference, while horizontal superiority, which implies a greater extent of tolerance in the horizontal configuration than that in the vertical configuration, was observed for luminance difference. We discussed possible origins of the anisotropy of the tolerance of perceptual completion at the blind spot.

We examined effects of binocular occlusion, binocular camouflage, and vergence-induced disparity cues on the perceived depth between two objects when two stimuli are presented to one eye and a single stimulus to the other (Wheatstone-Panum limiting case). The perceived order and magnitude of the depth were examined in two experimental conditions: (l) The stimulus was presented on the temporal side (occlusion condition) and (2) the nasal side (camouflage condition) of the stimulus pair on one retina so as to fuse with the single stimulus on the other retina. In both conditions, the separation between the stimulus pair presented to one eye was systematically varied. Experiment 1, with 16 observers, showed that the fused object was seen in front of the nonfused object in the occlusion condition and was seen at the same distance as the nonfused object in the camouflage condition. The perceived depth between the two objects was constant and did not depend on the separation of the stimulus pair presented to one eye. Experiment 2, with 45 observers, showed that the disparity induced by vergence mainly determined the perceived depth, and the depth magnitude increased as the separation of the stimulus pair was made wider. The results suggest that (1) occlusion provides depth-order information but not depth-magnitude information, (2) camouflage provides neither depth-order nor depth-magnitude information, and (3) vergence-induced disparity provides both order and magnitude information.One of the most interesting stimulus configurations examined in the field of binocular stereopsis is the Wheatstone-Panum limiting case: 1 two images are projected on one retina and a single image on the other retina (Panum, 1858(Panum, /1940Wheatstone, 1838). The WheatstonePanum limiting case can give rise to an impression oftwo objects at different depths and is interesting to researchers because, so far, there is no single hypothesis that can adequately account for the percept (Ono, Shimono, & Shibuta, 1992). For example, the classical double fusion hypothesis, which states that both images on one retina are fused with the single image on the other (Hering, 1865, cited in Ono & Mapp, 1995Ogle, 1962), cannot account for the magnitude and the direction of the perceived depth reThis research was supported by Grant-in-Aids for Scientific Research (7610068) provided by the Japanese Ministry of Education, Science, and Culture. K. Shimono worked on this study while he was at the Communications Research Centre, Ottawa, Canada, as a visiting researcher from the Department of Information Engineering and Logistics, Tokyo University of Mercantile Marine. The authors wish to thank Atsuki Higashiyama, two anonymous reviewers, and the editor for their critical review and helpful comments on an earlier version ofthe manuscript. Correspondence should be addressed to K. Shimono, Department ofinformation Processing Engineering and Logistics, Tokyo University of Mercantile Marine, Ettchujima 2-1-6, Koto-ku, Tokyo, Japan, 136 (e-mail: shimono@ipc.tosho-u.ac...