Global factors that determine the maximum disparity for seeing cyclopean surface shape

A disparity gradient limit explains why the maximum amplitude of sinusoidal disparity gratings increases with decreasing disparity spatial frequency. It also explains why the largest disparity for binocular fusion (diplopia threshold) varies directly with stimulus element separation. Does a disparity gradient limit also apply to the detection of cyclopean shape? A previous study addressed this question and concluded that it does not. We examined this question by measuring the largest disparity amplitude (dmax) at which observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to dissociate the effects of disparity gradient and disparity spatial frequency. Gabor micropatterns were used to minimize potential scale-dependent interactions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involved at high disparity spatial frequencies. Combining the gradient limit hypothesis with lowpass disparity filtering describes the pattern of dmax for both smooth and discontinuous surface shapes.