Vergence responses to forward motion in monkeys: visual modulation at ultra-short latencies

In two monkeys, we measured the initial vergence eye movements elicited by sudden forward motion on a linear sled. Animals faced a tangent screen and experienced the translation while in darkness, fixating a small, centered spot, or viewing a large-field pattern. Forward movements elicited convergent responses that were enhanced in the presence of the visual stimuli after a latent period. The enhancement was greater with the large-field pattern than with the small spot. The latencies of these visual effects were ultra-short and less than those reported for the vergence eye movements elicited by pure visual stimuli when applied in isolation. It is possible that these ultra-short latencies resulted from the fact that there were multiple visual cues available to sense the change in viewing distance, including binocular disparity, radial optic flow, size changes, and blur. Another possibility is that the very earliest visual effects during forward motion resulted from direct modulation of the otolith-mediated, translational vestibulo-ocular reflex.