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 t hreshold) varies directly with stimulus element separation. Does a disparit y 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 (d(max)) at whic h observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to d issociate the effects of disparity gradient and disparity spatial frequency . Gabor micropatterns were used to minimize potential scale-dependent inter actions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involv ed at high disparity spatial frequencies. Combining the gradient limit hypo thesis with lowpass disparity filtering describes the pattern of d(max) for both smooth and discontinuous surface shapes. (C) 2000 Elsevier Science Lt d. All rights reserved.