Disparity range for binocular summation.

Binocular summation of contrast and stereopsis have been linked because they both disappear under certain pathological conditions. The dependence of stereopsis on spatial frequency prompted us to examine how binocular summation varies with both spatial frequency and binocular disparity. We therefore measured binocular summation at different disparities using spatially localized stimuli which were also restricted in their Fourier composition. Contrast thresholds were measured using three interleaved forced-choice staircases for left and right eye monocular stimuli and a binocular stimulus composed of the two monocular stimuli presented simultaneously. At zero disparity binocular thresholds were 1.4 to 1.6 times lower than monocular. As disparity was increased the ratio between the thresholds became smaller, such that at large disparities it was near 1.2, the value expected from probability summation. The range of disparities over which probability summation was exceeded varied with the spatial frequency of the stimulus. At 6.0 cpd the range was 2-3 deg, but at 2.0 cpd or 0.75 cpd the range increased to 4-6 deg. These values closely parallel the range of disparities over which stereoscopic depth sensations occur, but they exceed the limits within which disparate images of an object can be fused into a single percept. The results support the contentions that "neural" summation occurs in the mechanism for stereopsis, that this mechanism uses spatial frequency selective channels, and that this mechanism is separate from the mechanism which mediates fusion.