Abstract
Interocular differences in orientation (orientation disparities) occur during binocular viewing of a surface slanted in depth. These orientation disparities could be exploited by the visual system to provide information about surface slant, but gradients of positional disparity provide an equally effective means to the same end. Both psychophysical and neurophysiological studies have produced conflicting results concerning a role for orientation disparities in slant perception (Cagenello and Rogers 1993, Mitchison and McKee 1990, Blakemore et al., 1972, Nelson et al., 1977). We have used two methods to distinguish psychophysically between position and orientation disparity mechanisms for detecting slant in sine wave gratings with interocular orientation differences. 1. Size. For a given slant, the magnitude of orientation disparities is independent of the stimulus size, whereas the largest positional disparities increase with increasing size. If orientation disparities limit slant detection thresholds, these should be independent of stimulus size. 2. Pedestal disparity. Thresholds for making depth discriminations are increased in the presence of a pedestal positional disparity (e.g. Badcock and Schor 1985). If an independent orientation disparity mechanism exists, slant thresholds should be unaffected by a pedestal disparity. In all 3 subjects, thresholds for detecting the direction of slant of the sine wave grating decreased with increasing stimulus size (0.5–4 degrees). Furthermore, adding a pedestal disparity to the stimulus caused a dramatic increase in slant detection thresholds. Both of these results suggest that orientation disparities are not the basis of slant detection. This psychophysical result is consistent with our earlier finding that the responses of V1 neurons do not encode binocular orientation differences.
Supported by The Christopher Welch Trust, The Royal Society and The Wellcome Trust