Binocular disparity processing with opposite-contrast stimuli.

Stereoscopic perception of relative depth with reversed-contrast half images differs in several important respects from stereopsis with matched-contrast half images. Thus, reversed-contrast images show no correlated shift in visual direction, indicating that the sensory-fusion mechanism ignores opposite-sign edges; one experiment addressed this aspect of the problem. Mainly, this was a quantitative study of opposite-contrast stereopsis, in which stereoacuity was measured as a function of bar width by means of narrow-band stimuli. Acuity was about an order of magnitude worse for reversed-contrast than for matched stimuli, but the ability to see valid (disparity-dependent) depth was not altogether lost even with wide (1 cycle deg-1) reversed-contrast bars. It is generally believed that depth with opposite-contrast stimuli is mediated by interaction between binocular stimuli components that have the same sign of contrast. Perceived depth was measured as a function of disparity and thus one of the predictions of that 'same-sign hypothesis' was tested experimentally; then, the magnitude of same-sign components was manipulated within the reversed-contrast stimuli, and thus the general prediction of the same-sign hypothesis was tested. The results show conclusively that the same-sign hypothesis cannot account for opposite-contrast stereopsis; its mechanism remains unknown.