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Agostino Gibaldi, Avi M Aizenman, Dennis M Levi, Martin S Banks; Binocular Coordination and Interocular Balance in Amblyopia. Journal of Vision 2020;20(11):1623. doi: https://doi.org/10.1167/jov.20.11.1623.
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Saccades are ballistic eye movements that humans make to direct gaze to an object of interest. When landing on a new fixation point, effective binocular coordination facilitates accurate convergence of the eyes, helping to fuse a single image. Without accurate convergence, a double image may be perceived. We have shown that the visual system uses statistical regularities in the natural environment to aid eye alignment at the end of saccades (Gibaldi & Banks,2019), defining the empirical oculomotor horopter. This behavior is consistent when stereovision is functional and effective. In this study we investigated whether the same statistical regularities are exploited in stereoanomolous binocular vision in amblyopia and strabismus.
Nine subjects participated, six with normal vision and stereovision, and three with amblyopia (at least two lines difference between the eyes) and impaired stereovision (stereothreshold > 200arcmin). Subjects performed saccades to LED targets arranged vertically, while their eye movements were recorded with an Eyelink II eyetracker. We measured horizontal vergence after completion of monocular and binocular saccades to assess whether vergence is consistent with the natural-disparity distribution. We additionally measured perceptual interocular balance by asking observers to indicate the apparent location of dichoptic stimuli with interocular contrast differences.
The oculomotor horopter of healthy subjects presents a consistent pattern of divergence for upward saccades and of convergence for downward saccades. This matches environmental scene statistics, and this effect is larger with monocular viewing. In two amblyopic subjects this pattern is not present, and vergence shows greater variability as compared to healthy subjects, demonstrating a lack of binocular coordination. Interestingly, the third amblyopic observer showed only a slight imbalance, which was accompanied by better binocular coordination.
The evaluation of binocular coordination could be used for an objective assessment of binocular visual dysfunction, useful for screening young, non-collaborative patients and possibly early intervention.
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