Orientation disparity and the perception of surface slant

It has been suggested that the perceptual impression of slant can result from the operation of mechanisms that are sensitive to orientation disparities between the retinal images. We have estimated the threshold for dichoptic rotation necessary to induce surface slant for stimuli that have been constructed from bandpass-filtered visual noise. Increasing the orientation bandwidth causes a large drop both in monocular and in binocular orientation acuity owing to increases in the stimulus orientation uncertainty. However, changes in orientation bandwidth have no impact on the orientation acuity for creating slant in depth, strongly suggesting that orientation-tuned mechanisms are not involved in this task. We found also that orientation thresholds for inducing slant rise substantially when the stimuli are superimposed on a pedestal of static disparity of position, which would not be predicted to have any impact on the operation of a mechanism that was sensitive solely to a disparity in the orientation domain. We conclude that perceived slant results from relatively conventional processing of the positional disparities that are inevitably created by an underlying disparity of orientation, and that for spatially extended stimuli, as used here, it is not necessary to invoke the existence of higher-order mechanisms.