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S. P. Karim, B. T. Crane, J.-R. Tian, J. L. Demer; Asymmetry and Optically Inappropriate 3 Dimensional (3D) Characteristics of Prism-Induced Step Vertical Vergence. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4122.
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© ARVO (1962-2015); The Authors (2016-present)
We investigated 3D human motor mechanisms underlying vergence in response to step vertical disparity.
Eye movements of 8 young adults with normal binocular vision and stereo thresholds 40 arcsec or better were studied using binocular, 3D scleral search coils. During viewing of a black cross straight ahead at 400 cm, vertical vergence was induced by placing 1° base up prism over one eye for 10 sec intervals alternating with equal periods of normal vision.
All subjects reported single binocular vision 1-4 sec after prism placement over either eye. With prism over the right eye (RE), RE abducted (1.35° ± 0.24 (SEM)), infraducted (0.99° ± 0.18) and excycloducted (1.35° ± 0.62), while the left eye (LE) demonstrated no horizontal duction, small supraduction (0.31° ± 0.11), and substantial incycloduction (2.20° ± 0.59). Eye movements with prism over LE were remarkably similar, as RE abducted (1.01° ± 0.27), infraducted (0.82° ± 0.22), and excycloducted (2.48° ± 0.76), while LE abducted (0.24° ± 0.07) and infraducted (0.57° ± 0.09) slightly, with substantial incycloduction (3.77° ± 0.90). In 14 out of 16 trials obtained in 8 subjects, there was dextro-cycloversion (upper poles of both eyes toward right shoulder). There was incyclovergence of 0.87° ± 0.58 and 1.31° ± 1.09, for RE prism and LE prism, respectively.
Vertical vergence induced by step placement of base up prism generated dextro-cycloversion exceeding prism power, regardless of which eye viewed through the prism. Levo-cycloversion did not occur. This remarkable asymmetry contradicts Enright (1992), who found reversal of cycloversion with reversal of vertical disparity. Infraduction and excycloduction by RE with prism over either eye excludes sole implementation by the superior oblique muscle, an infraductor and incycloductor.Vertical vergence during prism viewing was strikingly asymmetric: always optically appropriate for RE prism viewing, but consistently increasing vertical disparity for LE prism viewing. Despite reported binocular single vision, RE always paradoxically infraducted during LE prism viewing, suggesting central perceptual compensation of disparity that may be related to proprioception or efference copy of disproportionate ocular torsion.
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