The Effect of Surface Orientation on the Perception of Stereoscopic Corrugations

Abstract

For most observers there is a pronounced orientational anisotropy in the perception of three-dimensional corrugated surfaces (Rogers and Graham, 1983 Science221 1409 – 1411; Bradshaw and Rogers, 1993 Perception22 Supplement, 117). Low-frequency corrugations which are oriented vertically have been found to have higher disparity modulation thresholds, the amount of perceived depth at suprathreshold levels is smaller, and typically they take longer to see than horizontally oriented corrugations. In the present experiments, the orientation of the corrugations was manipulated (from horizontal to vertical in 22.5 deg increments) to investigate the effect of surface orientation on both (i) a threshold detection task and (ii) a suprathreshold depth-matching task. The stimuli were 10 deg in diameter and were presented on two 12 inch monochrome monitors arranged to form a Wheatstone stereoscope. The surfaces were modulated in depth at four different corrugation frequencies (from 0.1 to 0.8 cycle deg−1 in octave steps). Thresholds were found to increase monotonically with increasing surface orientation from the horizontal: ∼2.5 arc s for horizontal corrugations to ∼10 arc s for vertical corrugations. The increase in thresholds was less marked for surfaces with higher corrugation frequencies. The rate of increase of threshold was greatest for surface orientations beyond 45°. A different pattern of results was found in the suprathreshold depth-matching task. Although the perceived depth in vertically oriented corrugations was significantly smaller (>50%) than for horizontally oriented corrugations, the largest amount of perceived depth was found for corrugated surfaces oriented at 45°. These results suggest that the disparity information used to process stereoscopic corrugations at threshold may be different from that used to process suprathreshold surfaces.