Abstract
Abstract: :
Purpose:Stereopsis with Gabor stimuli has been reported to deteriorate when the bandwidth becomes less than 0.5 octaves and more than 4 grating cycles become visible (Hess & Wilcox, 1994). In their experiment, bandwidth was decreased by increasing the size of the Gabor stimulus, and while the reference–test separation was maintained at a constant factor of the size of the Gabor stimulus, the angular reference–test separation concurrently increased. The purpose of our study was to separate the effects of increasing angular reference–test separation from decreasing bandwidth on the observed decrease in stereopsis. Methods:Six observers with clinically normal stereopsis performed stereoscopic depth discriminations with Gabor stimuli at contrasts 4 times their contrast threshold. The bandwidth of the Gabor stimuli was decreased (1–0.2 octaves) by increasing the standard deviation of the Gaussian envelope (0.1–1.6 arcmin) while keeping the spatial frequency of the carrier grating constant (1.31, 2.62 or 5.24 cy/deg). For each of 2 normal observers at each spatial frequency we compared the effect on stereopsis of the angular increase in separation (0.8–12.8 deg) inherent in maintaining a constant multiple of the size of the Gaussian envelope (8 x stdev) with keeping a constant angular separation (2 arcmin) between Gabor reference and test stimuli. Results:For the sample of all 6 observers, the increase in disparity threshold was significantly greater (p=0.001) with increasing angular separation as opposed to constant angular reference–test separation. Disparity threshold increased with decreasing bandwidth and increasing angular separation by an average factor of 4.5 +/–0.8 times, similar to previously reported data (Hess & Wilcox 1994). With a constant angular separation, however, disparity threshold increased with decreasing bandwidth by an average factor of 2.2 +/– 0.3 times. Conclusions:The reported effect of bandwidth on stereopsis was doubled by the concurrent increase in angular separation between the reference and test Gabor stimuli. We found that decreasing bandwidth and increasing angular reference–test separation of Gabor stimuli each reduce stereopsis by the same degree.
Keywords: binocular vision/stereopsis • depth • perception