Reflexive and voluntary control of smooth eye movements

An understanding of visually evoked smooth eye movements is required to predict the visibility and legibility of moving displays, such as might been countered in vehicles like air craft and automobiles. We have studied the response of the oculomotor system to various classes of visual stimuli, and analyzed the results separately for horizontal and vertical version (in which the two eyes move together), and horizontal and vertical vergence (where they move in opposite directions). Of the four types of motion, only vertical vergence cannot be performed under voluntary control, and certain stimuli (all having relatively long latencies) are in capable of evoking it. In another experiment, we instructed observers to track one of two targets, and measured weak but reliable responses to the unattended target, in which the long-latency component of the response is abolished. Our results are consistent with a system containing two distinct processes, a fast reflexive process which responds to a restricted class of stimuli, and a slower voluntary process capable of following any thing that can be seen, but incapable of controlling vertical vergence.