We measured the ability to fuse dichoptic images of a horizontal line alone or in the presence of a textured background with different vertical disparity. Nonius-line measurements of vertical vergence were also obtained. Diplopia thresholds and vertical vergence gains were much higher in response to an isolated vertically disparate line than to one with a zero vertical-disparity background. The effect of the background was maximum when it was coplanar with the target and decreased with increasing relative horizontal disparity. We conclude that vertical disparities are integrated over a restricted range of horizontal disparities to drive vertical vergence. 2000 Elsevier Science Ltd. All rights reserved.

The region of retinal disparity that is effective in maintaining binocular alignment of the eyes was investigated by measuring the horizontal fusional amplitudes for fixation targets consisting of a small central cross to which peripheral lines having various disparities were added. It was found that the addition of peripheral targets significantly facilitated binocular alignment, but only if the peripheral lines had less than about 0.5 ° of disparity. This result indicates that the fusional mechanism responsible for binocular alignment is only narrowly tuned for retinal disparity. In light of current evidence that indicates that much larger disparities are capable of initiating vergence eye movements, the results support suggestions that fusional vergence consists of functionally dichotomous vergence initiating (coarse) and sustaining (fine) channels. Invest Ophthalmol Vis Sci 30:1638-1642,1989 Considerable evidence now exists that fusional vergence consists of two functional components. ' A coarse, transient mechanism initiates the vergence response to a retinal disparity, while a slower, spatial feature-sensitive mechanism sustains binocular alignment within the limits of Panum's area. The