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Zahra Hussain, Andrew Astle, Ben Webb, Paul McGraw; Position matching between the eyes in strabismus. Journal of Vision 2017;17(10):1064. doi: https://doi.org/10.1167/17.10.1064.
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Misalignment of the visual axes in strabismus disrupts retinal correspondence and therefore fusion. Fusional status is usually assessed centrally where the perceptual consequences of misalignment are most prominent, but the periphery is also affected. We used a dichoptic spatial mapping procedure to measure the perceived horizontal offset of a stimulus presented to the deviating eye of a large and heterogeneous group of strabismic observers (N = 32), at thirty-two positions in the visual field. The free-localization task required observers to move a response probe presented to the deviating eye, to a position diagonally opposite a reference probe presented to the fixing eye. The reference and response probes were placed in opposite hemifields to reduce luminance cues and interocular suppression. Observers with normal binocular vision were also tested (N = 20), with the reference and response probes randomized between the eyes. Perceived offsets were quantified at each location to produce a spatial map for each observer; spatial maps across observers were examined for patterns associated with the direction and magnitude of ocular deviation. We found a reliable and striking non-uniformity in the magnitude of perceived offset across the visual field of strabismic subjects: at 1 deg, the offset was larger in the visual field in the direction of ocular deviation, than in the visual field against the direction of deviation. This effect was reversed at 7 deg, and was absent in individuals with negligible mean perceived offset. Whereas offset direction was consistent with the direction of ocular deviation, offset magnitude was uncorrelated with the objective angle of deviation indicating the presence of anomalous retinal correspondence in a number of observers. The asymmetric visual field distortion may partly be accounted for by retinal geometry and interocular suppression, but additional cortical adaptations must contribute to the overall pattern observed.
Meeting abstract presented at VSS 2017
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