The area of spatial integration for initial horizontal disparity vergence.

We investigated over what central area disparity in a random dot stereogram is integrated to stimulate an initial vergence response. Vergence was measured subjectively, with a forced choice dichoptic nonius vernier task following a brief (230 msec) stimulus presentation. Stimuli were random-dot stereograms showing a central circular disc of 12.5 min arc crossed retinal disparity in front of, and occluding, a same density fixation plane surround. The size of the disc was varied. All ten observers responded to the brief stimulus. Initial vergence increased with increasing disc diameter and, for nine out of ten subjects, reached a maximum with the disc ca 6 deg, suggesting this is the extent of the spatial integration region. Below 6 deg diameter, surround and target disparities were averaged together. Initial horizontal vergence responds automatically to a cyclopean target presented in the centre of gaze by pooling disparities within a limited but surprisingly large area.