Eye movements in relation to loss and regaining of fusion of disjunctively moving random-dot stereograms.

Horizontal eye movements of four subjects were recorded with a scleral induction-coil technique during dichoptic viewing of a random-dot stereogram. The stereogram contained two depth planes. The two half-images of the stereogram were slowly moved in opposite lateral directions beyond the limit of divergence; subsequently the movements of both images were reversed. Ocular vergence followed image vergence of the foveally viewed part of the stereogram during fusion. Binocular disparity increased beyond a certain angle of divergence. Before loss and after regain of fusion the same relation was found between binocular disparity and angle of divergence. The size of binocular disparity that did evoke vergence responses was limited to about 4 deg arc; this range was larger than the maximum amount of binocular disparity (1 to 2 deg arc) that could be fused. Shifts in fixation from the one depth plane to the other one were executed by rapid vergence movements. These vergence movements were slower than saccades but still exceeded the maximal velocities generally assumed for vergence.