Objective measurement of binocular fixation misalignment.

The amount of deviation from central fixation during binocular fusion of a vertical border was compared with conventional fixation disparity measured at the same time, forced convergence serving as the independent variable. Fixation eccentricity was measured objectively by monitoring a scleral blood vessel with a video camera and by analyzing the movement, greatly magnified, on a video screen. Fixation disparity was measured conventionally by interocular nonius alignment of vertically dissociated line segments. The results agree well with previous comparisons of these responses, in which the fixation eccentricity was measured by using a technique based on the effect of retinal stimulus location on border enhancement. As in the previous experiment, the fixation misalignment was found to be many times larger than the corresponding disparity for most forced convergence values. The large discrepancy between actual fixation misalignment and fixation disparity has thus been documented objectively as well as subjectively.