Saccade Programming in Strabismic Suppression

Individuals with strabismus frequently show a suppression phenomenon in which part of the visual input in one eye is apparently ignored when both eyes are seeing, although the eye may have normal vision when used monocularly. This is often described as an adaptive response to avoid diplopia. We have examined two patients with microstrabismus (angle of squint less than 5 deg) who show strong suppression but with only mild amblyopia. We studied saccade generation in the two eyes using a red — green anaglyph display which allowed us to present stimuli independently to each eye.

When single targets were presented in the suppressing eye, saccadic responses usually occurred. However the latencies of these saccades were increased with respect to those elicited from the normal eye (by about 70 ms for one subject and 270 ms for the other). The amplitudes of the saccades were less consistent than those of the normal eye, and saccades were sometimes made in the opposite direction to the target. We also investigated the remote distractor effect. This effect is found consistently in normal subjects and consists of an increase in the latency of a target-elicited saccade when a distractor is simultaneously presented elsewhere in the visual field. When distractors were presented in the suppressing eye, they had no effect on the latency of saccades to a simultaneous target in the other eye. We conclude that visual stimulation in a suppressing eye has no rapid access to the saccadic system.