The effect of forced vergence on retinal correspondence.

The spatial characteristics of the changes in retinal correspondence produced by forced convergence were studied. The vertical extent of lateral shifts in binocular correspondence were quantified by comparing the convergence of the eyes measured with binocular search coils to the convergence of the eyes as determined using nonius lines having vertical separations (gaps) between the nonius lines of 0.5-4.8 degrees. Lateral shifts in binocular correspondence only occurred for nonius gaps < 3-4 degrees. The effects of horizontal retinal eccentricity on lateral shifts in correspondence were determined by measuring the nonius horopter of the subject under forced convergence, using 11 nonius line eccentricities between 4.5 degrees left and right. The nonius horopter was shifted toward the fusion target maximally near the fixation point. There was no shift beyond 3 degrees of eccentricity. We conclude that the nonius horopter is 'dimpled' vertically and horizontally, facilitating local fusion by shifting the line horopter and the region of single binocular vision toward the point of regard over a region of 3 degrees around the fixation point.