Saccadic binocular coordination in alternating exotropia

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° 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.