Nature of coarse-to-fine constraints on binocular fusion.

Several stereo-matching algorithms posit that processing on coarse (low spatial frequency) scales constrains processing on fine (high spatial frequency) scales by shifting the disparity range over which fine mechanisms operate. If this were the case, stereo increment thresholds for high frequencies in the presence of low frequencies would be constant, regardless of base disparity. In contrast, we find that in the presence of low spatial frequencies, increment thresholds for high spatial frequencies increase with base disparity, as do increment thresholds for high frequencies presented alone. As a further test of whether information on coarse scales enhances processing on fine scales, diplopia thresholds were measured. In the presence of low frequencies, fusion ranges for high frequencies were severely reduced. These constraints were found to exhibit a power-law dependence on low frequency contrast and to operate over relatively localized areas of the visual field.