Human optokinetic nystagmus is linked to the stereoscopic system

It was previously proposed that a linkage between the optokinetic system and the stereoscopic system in higher mammals serves to allow these animals to selectively stabilize those parts of the visual scene which lie in the plane of convergence as the animals move forward in a three-dimensional world (Howard and Ohmi, 1984). A new procedure is now described by which OKN gain can be measured as a function of the binocular disparity of the stimulus. With vergence locked on a vertical line, the gain of the slow phase of vertical optokinetic nystagmus (OKN) was recorded in four human subjects as the binocular disparity (stereo depth) of the moving display was changed from -3 degrees to +3 degrees. The gain of OKN was found to be inversely proportional to binocular disparity. Evidence for cells in the visual cortex, MT and MST that are sensitive both to visual motion and binocular disparity is reviewed. It is argued that the activity of cells responsive to direction of motion and zero disparity selectively augments OKN and that this enables humans to stabilize the images of parts of the scene in the plane of regard while ignoring competing motion signals arising from other distances.