ANALYSIS OF STEREOTHRESHOLDS FOR STIMULI BELOW 2.5-C DEG/

We analyze published data on disparity detection thresholds for a wide range of conditions. This type of detection changes behavior dramatic ally at the spatial frequency of 2.5 c/deg; above this frequency thres hold remains constant while below it threshold grows at a uniform rate . Many other types of threshold, such as upper disparity limits for de pth perception and threshold amplitudes for stereo and monocular motio n, show similar behavior. These data lead to the postulate that there are no foveal stereo channels peaking below 2.5 c/deg, so that foveal stimuli in the whole range below 2.5 c/deg are processed by a single c hannel tuned to this frequency. Consequently, disparity detection thre sholds at frequencies below this frequency are controlled by the singl e parameter of effective contrast in the 2.5 c/deg channel, whose outp ut depends jointly on the contrast and spatial frequency of the stimul i. We develop this idea to explain the relations between spatial and c ontrast tuning functions for disparity thresholds. To validate our con clusions, we describe an experiment with difference-of-Gaussian stimul i over a range of interocular widths and contrast differences. For a d ichoptic width ratio of 2:1, the dichoptic contrast ratio required to minimize disparity detection thresholds was 1:4, just as predicted by the model.