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Elizabeth Fast, Linus Holm, Linda McLoon, Stephen Engel; Fusion breaks at extreme eye positions due to lack of adaptation in the vergence system. Journal of Vision 2017;17(10):1155. doi: 10.1167/17.10.1155.
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What limits our ability to diverge or converge our eyes? Current models suggest that vergence depends upon a slow, adaptable tonic component that determines the "baseline" position of our eyes, and a fast fusional component activated primarily by retinal disparity, which drives additional vergence. This framework predicts that as images require larger vergence movements to stay binocularly fused, putting the eyes in relatively extreme positions, fusion should break only if adaptation cannot maintain high enough baselines. We tested this prediction by adapting the vergence systems of 8 subjects to sequential increases of 2° image disparity, which produced divergent eye movements. Beginning at 3°, subjects adapted for 5 min blocks, before disparity increased further, to a maximum of 15°. Subjects viewed an image of an outdoor scene, while relative eye position was measured with an eye tracker and a behavioral measure. To estimate adaptation of the baseline component we measured phoria, the misalignment of the two eyes with no disparity present. Phoria should be small when adaptation is complete, and was measured every 30 s by replacing the image in one eye with a gray field for 10 s. Results showed evidence of vergence adaptation; phorias decreased on average within a block by 1.27° degrees. However, both eyetracking and behavioral estimates of adaptation became smaller across successive blocks (p < 0.01). One interpretation of these results is that adaptation became less complete as eye alignment became more extreme. Without complete adaptation of the tonic baseline, disparity became too large for the fast fusional component to overcome, and fusion broke. These results predict that longer viewing times could produce more complete adaptation, and allow vergence to reach previously unattainable positions, potentially beneficial for the treatment of strabismus.
Meeting abstract presented at VSS 2017
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