Role of attention and eye preference in the binocular coordination of saccades in strabismus

Abstract

This study examines squint change and the binocular coordination of horizontal and vertical saccades in an adult subject with incomitant strabismus (20 or 40 prism diopters) strongly depending on the eye used for fixating. The patient had no binocular vision and was diagnosed ‘horror fusionis’ since her childhood. We found the angle of horizontal squint to be smaller when the patient was fixating with the preferred eye than with the nonpreferred eye. The squint was smaller when both eyes viewed and one eye fixated attentionally than when the nonfixating eye was closed. This suggests the importance of binocular visual stimulation. We found no significant changes in the amplitude of the disconjugacy of the saccades. However, when the preferred, left eye fixated attentionally (under binocular viewing) or monocularly, the pattern of the disconjugacy changed: the majority of the saccades showed divergent disconjugacy. This pattern is qualitatively similar to that seen in normal subjects. In this patient, divergent disconjugacy helped to decrease the convergent squint at the end of the saccades. Interestingly, vertical squint was small and did not depend on viewing conditions. The binocular coordination of vertical saccades was almost normal, at least in the binocular viewing condition. We conclude that the visual input from both eyes allows a rudimentary binocular cooperation that helps to keep the squint small and renders disconjugacy of horizontal saccades strategically divergent to reduce temporally the squint.