The visual back-up to the VOR: ocular tracking systems with ultra-short latencies and the role of the MST area

Abstract

In everyday life, self-movement of an observer causes the image of the visual scene on the retina to slip. Visual acuity is severely impaired when the retinal image of interest moves excessively. Naturally, the vestibular-ocular reflex (VOR) compensates for the observer's own movements through a short neural pathway from the vestibular organs to the eyes. The amplitude of the VOR is almost equal to the observer's head movement, but not perfect, and the residual retinal image motions are compensated for by visually driven ocular tracking systems.Recent behavioral studies on primates have revealed that there are three visual tracking systems that function to stabilize the gaze of the moving observer as visual back-up. One of these systems, the ‘ocular following response’, helps to stabilize gaze when the observer looks off to one side. The other two systems generate ‘vergence eye movements’ to help maintain binocular alignment on objects that lie ahead when the observer looks in the direction in which he or she is heading. One responds to the change in binocular parallax (disparity) and the other to the radial patterns of the optic flow. All three operate in a machine-like fashion to generate eye movements with ultra-short latencies.We conducted electrophysiological and chemical-lesion studies to determine whether the medial superior temporal (MST) area of the cerebral cortex, which is known to participate in visual motion processing, plays roles in eliciting these tracking eye movements or not. Despite their ultra-short latencies, electrophysiological studies in monkeys revealed a close relationship between ocular and neuronal responses in the MST, and lesions of the MST in both hemispheres significantly reduced the initial part of the tracking responses. Overall the results strongly support the hypothesis that the MST area is a primary site for producing the three visual tracking eye movements at ultra-short latencies.