Version and vergence eye movements in humans: open-loop dynamics determined by monocular rather than binocular image speed

We examined the velocity dependence of the vergence and version eye movements elicited by motion stimuli that were symmetric or asymmetric at the two eyes. Movements of both eyes were recorded with the scleral search coil technique. Vergence was computed as the difference in the positions of the two eyes (left-right) and version was computed as the average position of the two eyes ((left+right)/2). Subjects faced a large tangent screen onto which two identical random-dot patterns were back-projected. Each pattern was viewed by one eye only using crossed-polarizers and its position was controlled by X/Y mirror galvanometers. Viewing was always binocular and horizontal velocity steps (range, 5-240 deg/s) were applied to one (asymmetric stimulus) or both (symmetric stimulus) patterns approximately 50 ms after a centering saccade. With the symmetric stimulus, the motion at the two eyes could be either in the opposite direction (eliciting vergence responses) or in the same direction (eliciting version responses). The asymmetric stimuli elicited both vergence and version. In all cases, minimum response latencies were very short (<90 ms). Velocity tuning curves (based on the changes in vergence and version over the time period, 90-140 ms) were all sigmoidal and peaked when the monocular (i.e., retinal) image velocities were 30-60 deg/s. The vergence (version) responses to symmetric stimuli were linearly related to the vergence (version) responses to asymmetric stimuli when expressed in terms of the monocular rather than the binocular image velocities. We conclude that the dynamical limits for both vergence and version are imposed in the monocular visual pathways, before the inputs from the two eyes are combined.