Abstract
Binocular fusion requires a vergence mechanism to bring the two eyes’ images into alignment. To determine the accuracy of vergence we measured Nonius alignment under a range of viewing conditions. Specifically, we presented random-Gabor-patch (RGP) stereograms (14deg x 14 deg) for 400ms, followed 100ms later by two briefly presented (100ms) vertical Nonius lines. Participants judged the relative position of the Nonius lines to provide a psychophysical estimate of the vergence induced by the preceding RGP stereogram. The RGP stereograms consisted of vertical Gabor patches with random positions and phases, but with fixed spatial frequencies (1.5, 3.0 or 6.0 cpd). Identical arrays of patches were presented to the two eyes, except that one eye’s array could be shifted horizontally to produce binocular disparity, with either correlated or anti-correlated luminance profiles. For each spatial frequency, 38 staircases, one for each of the 19 stimulus disparities x 2 correlation conditions, were interleaved. For both correlated and anti-correlated conditions, the vergence shift first increased with stimulus disparity, reaching a maximum when disparity was 2-4 times the Gabor wavelength, and then decreased. We implemented and tested a binocular motor fusion model that assumes that vergence is driven by phase-disparity energy under a coarse-to-fine process to align the two eyes images until phase disparity is eliminated. Our modeling shows that a simple model with a single second-order phase-disparity detector with a spatial wavelength of 8-16 times the Gabor wavelength, provides a reasonable fit to both correlation conditions. Adding another second-order phase-disparity detector with a shorter spatial wavelength might significantly improve the model fits. Adding a first-order phase-disparity detector further improved the model fits significantly, providing a reasonable account of the differences between the two correlation conditions. We conclude that vergence eye movements are driven by multiple pathways at different scales, mainly by second-order phase-disparity energy.