The effect of disparity on the very earliest ocular following responses and the initial neuronal activity in monkey cortical area MST.

Movement of the visual scene evokes tracking movement with the eyes (ocular following response, OFR) at short latency (Miles, F.A., Kawano, K., Optican, L.M., 1986. Short-latency ocular following responses of monkey. I. Dependence on temporospatial properties of visual input. J. Neurophysiol. 56, 1321-1354). We examined the effect of binocular disparity on the initial OFR. The dependence of the OFR on horizontal disparity steps was studied in three monkeys (Macaca fuscata), and the associated unit discharges in the medial superior temporal area (MST) were studied in two of these. Based on the changes in eye position over the period 50-83 ms (measured from stimulus onset), the initial OFR showed clear dependence on the disparity imposed during the preceding centering saccade. The disparity tuning curves were S-shaped with a peak at a small crossed disparity and a trough at uncrossed disparities. Based on the changes in discharge rate over the period 40-73 ms (measured from stimulus onset), almost all OFR-related MST neurons (80/83, 96.4%) showed significant dependence on the disparity step (Student's t-test, P < 0.05). The early neuronal responses of the majority of units (41/75, 55%) had disparity tuning curves resembling those for the OFR, which peaked at small crossed disparities. These findings suggest that the neurons in the MST contain information on binocular disparity in their short-latency discharges, and are involved in the neural basis of the OFR's dependence on horizontal disparity.