Abstract
Early visual deprivation impairs many aspects of vision, including the perception of global form and motion (Lewis et al., 2002; Hadad et al., submitted). We examined the effect of abnormal early visual input on later sensitivity to shapes formed by real and illusory contours. Patients treated for bilateral congenital cataracts (M age = 21.6 years, range = 12.8–30.1 years; n = 9) and comparably aged visually normal controls (n = 14/age group) discriminated fat from skinny shapes formed by real and illusory contours at high support (0.5) and low support (0.2) ratios (the ratio of the physically specified contour to the total edge length). The angle of rotation of the shapes' corners increased or decreased over trials, producing fatter or skinnier shapes. We defined threshold as the smallest angle of rotation for which the shape was discriminated accurately as fat or skinny. Testing was monocular, with the patients' results divided into those for the better and worse eye, as defined by acuity and alignment history. Z-scores, based on age-appropriate norms, indicated a large deficit in the perception of both real and illusory contours in the worse eye (all ps < 0.05) but not in the better eye (all ps > 0.10) of deprived patients. To account for the deficit in processing real contours, we calculated interpolation cost (interpolated contour minus real contour divided by real contour). For each eye of patients and controls, we found a significant effect of support ratio, with higher support associated with lower cost (all ps < 0.001). However, interpolation cost did not differ significantly between patients and controls for either eye (all ps > 0.10). Therefore, although patients with early visual deprivation show overall deficits in sensitivity to shape, there is no additional loss in sensitivity to shapes formed by illusory contours.
Canadian Institutes of Health Research (grant# MOP 36430).