Saccadic binocular coordination in alternating exotropia

Abstract

We studied the coordination of binocular eye movements in human subjects with alternating exotropia (divergent strabismus). Binocular saccades were recorded in six subjects during binocular and monocular viewing. Subjects were instructed to make saccades between two continuously lit targets (LED's) presented in an isovergence array (with the straight-ahead target 130 cm from the eyes) in a dimly lit room. For saccades up to 20 degrees amplitude, there were no large differences in the dynamics of the saccades between control and exotropic subjects. However, for larger amplitudes subjects frequently alternated the eye of fixation during saccades. That is, subjects fixated the left target with the left eye and the right target with the right eye. The alternation in eye fixation at the end of the saccade was taken into account in the programming of the saccades. The amplitudes of the alternating saccades were approximately equal to the target amplitude minus the strabismus angle. We conclude that for those saccades where alternation occurs, there is not only a change in the eye of fixation, but also a change in the target representation provided by either eye. Thus, in this group of strabismic patients, saccades may be programmed in a retina-centered coordinate system, if we assume that for making a saccade to a new target in the contralateral visual field its representation on the temporal retinal field of the currently fixating eye is suppressed and the retinotopic target information is derived from the non-fixating eye. In executing the saccade, the non-fixating eye automatically becomes the fixating eye.

Ocular Motor Behavior in Macaques With Surgical Exotropia

“…The monkeys in this report displayed a behavior typical of humans with a large angle exotropia: they alternated fixation by saccading to left-sided targets with the left eye and to right-sided targets with the right eye (van Leeuwen et al 2001). In principle, this behavior means they could acquire peripheral targets more rapidly than normal subjects because smaller saccades were generally required.…”

“…It remains unclear how they program saccades under such circumstances (van Leeuwen et al 2001). One possibility is that the fixating eye detects the new target in its peripheral retina, and the correct saccade amplitude is calculated for the other eye by adding the size of the exotropia.…”

Spatial Patterns of Fixation-Switch Behavior in Strabismic Monkeys

“…However, if amblyopia is minimal, patients with strabismus develop the ability to fixate targets with either eye and can spontaneously change the eye of fixation depending on the location of the target. [4][5][6][7] A saccadic eye movement that results in switch in the fixating eye is an alternating saccade. 8 Alternating saccade (fixation-switch) behavior has been demonstrated in both esotropes and exotropes.…”

“…8 Alternating saccade (fixation-switch) behavior has been demonstrated in both esotropes and exotropes. 4,5,9 Fixation-switch is likely driven by visual suppression of specific retinal areas of each eye, although mechanisms are yet undetermined. [4][5][6] Mechanisms of visual suppression in exotropia (divergent strabismus) and esotropia (convergent strabismus) also are controversial.…”

“…4,5,9 Fixation-switch is likely driven by visual suppression of specific retinal areas of each eye, although mechanisms are yet undetermined. [4][5][6] Mechanisms of visual suppression in exotropia (divergent strabismus) and esotropia (convergent strabismus) also are controversial. 10,11 Behavioral studies in human exotropes suggest that temporal hemi-retinae are suppressed.…”