Short-latency vergence eye movements elicited by looming step in monkeys.

The looming associated with forward and backward motion of the observer has been shown to elicit vergence eye movements with short-latency (approximately 80 ms) in human subjects. We studied the vergence eye movements elicited by looming in three monkeys (Macaca fuscata). The animals faced a large tangent screen onto which a random dot pattern was back-projected. The movements of both eyes were recorded with an electromagnetic induction technique. Fifty milliseconds after a centering saccade, this first pattern was replaced with a new one that showed the same image viewed from a slightly different distance. This looming step (two-frame movie) included both radial optic flow and a size change. As expected from the optical geometry, centrifugal flow coupled with enlargement (signaling forward motion) increased the vergence angle, whereas the converse combination decreased the vergence angle. In both cases, the optimal step-change in apparent viewing distance was 2%. The latency of these vergence responses was very short and similar to those induced when disparity steps are applied to such large patterns (approximately 60 ms). We suggest that these two systems act in synergy to help maintain binocular alignment during forward and backward motion of the observer.