Anisotropies in the perception of stereoscopic surfaces: the role of orientation disparity.

We measured stereoscopic slant detection thresholds for surfaces slanting about a horizontal or a vertical axis. For random-dot covered surfaces, 1.25 deg of slant was required to detect slant about a horizontal axis, whereas 2.1 deg of slant was required to detect slant about a vertical axis. This anisotropy could be due to the fact that orientation disparities, which contain information about surface slant, are generally smaller for surfaces slanting about a vertical axis. To test this possibility, slant thresholds were measured for surfaces whose orientation disparity content was manipulated independently of the other slant information present. When the magnitude of orientation disparity was the same for surfaces slanting about a horizontal and a vertical axis, both surface orientations required about 1.5 deg of slant to be detected; thus the anisotropy became negligible. In contrast, when the orientation disparity content of a surface slanting about a vertical axis was zero, 3-4 deg of slant was required for detection; thus the anisotropy became larger. Under the conditions of these experiments, it appears that the visual system utilizes orientation disparities.