Interocular differences in contrast and spatial frequency: effects on stereopsis and fusion.

Anisometropia produces interocular differences in contrast and spatial frequency. The influence of these two parameters on Panum's fusional limit (PFL) and stereoscopic depth thresholds was investigated with sinusoidal gratings and one-dimensional band-pass-limited targets. Vertical fusion limits were unaffected by large interocular differences in contrast (40-10%) at two spatial frequencies (0.8 and 1.6 cpd). However, when tested with a low spatial frequency (0.8 cpd), stereothresholds increased 150% with an interocular difference in contrast as small as 50-25%. Stereoacuity was reduced less by differential contrast when tested with higher spatial frequencies (3.2 cpd). When tested with low spatial frequencies the stereothreshold was elevated more by reducing the contrast of one image than by equal contrast reductions of both ocular images. Stereothresholds appear to be elevated by binocular suppression evoked by interocular differences in contrast. Vertical as well as horizontal fusion limits decreased with increasing interocular size difference. Horizontal fusion limits fell off more gradually with increasing size difference than did vertical fusion limits, particularly at higher spatial frequencies (2.4 cpd). Similarly, stereothresholds increased with increasing interocular size differences. Changes in the fusion limit and stereothreshold that occur with interocular size differences are predicted from positional disparities between edge features rather than from differences in spatial frequency.