Abstract
An understanding of visually evoked smooth eye movements is required to predict the visibility and legibility of
moving displays, such as might be encountered in vehicles like aircraft 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 incapable 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 anything that can be seen, but incapable of
controlling vertical vergence.
