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 matc hed-contrast half images. Thus, reversed-contrast images show no corre lated shift in visual direction, indicating that the sensory-fusion me chanism ignores opposite-sign edges; one experiment addressed this asp ect 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 orde r of magnitude worse for reversed-contrast than for matched stimuli, b ut the ability to see valid (disparity-dependent) depth was not altoge ther last even with wide (1 cycle deg(-1)) reversed-contrast bars. It is generally believed that depth with opposite-contrast stimuli is med iated by interaction between binocular stimuli components that have th e same sign of contrast. Perceived depth was measured as a function of disparity and thus one of the predictions of that 'same-sign hypothes is' was tested experimentally; then, the magnitude of same-sign compon ents was manipulated within the reversed-contrast stimuli, and thus th e general prediction of the same-sign hypothesis was tested. The resul ts show conclusively that the same-sign hypothesis cannot account for opposite-contrast stereopsis; its mechanism remains unknown.