Limits of Adaptation in the Vergence System

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Limits of Adaptation in the Vergence System

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Vergence eye movements, used to align the eyes and avoid diplopia, are controlled by several interacting components. One, the tonic component can be thought of as a “baseline” position of the eyes, and this baseline can be adapted through recent history of eye positions. We tested the limits of this adaptation, with the dual goals of measuring the maximum increase in the amount of divergence that can be obtained through adaptation and understanding what factors determine this maximum. We used a dual stereoscope/eye-tracker to measure fusional limits (maximal divergence before experiencing diplopia) and phorias (eye movements indicative of incomplete adaptation); both were also measured behaviorally. Experiments 1-3 adapted participants stepwise in 5 minute blocks of increasing vergence demand (implemented by shifting one image location toward the periphery), and found that adaptation did shift fusional limits. But ultimately adaptation failed to keep up with vergence demand, and phorias increased in size across blocks. This indicated that adaptation was becoming less efficient as vergence demand increased; however the slope of phoria reduction did not decrease with increasing vergence demand, suggesting that the rate of adaptation did not change. We hypothesized that the 5 minute blocks were simply not enough time to adapt completely, and tested this in Experiment 4 where block duration was allowed to vary. As vergence demand increased, block duration also increased, suggesting that the rate of adaptation was in fact slowing as vergence demand increased. However, even with the variable block length, adaptation became less complete across blocks, as indicated by larger and larger phorias. Future work can determine the causes of these remaining limits on adaptation.



University of Minnesota Ph.D. dissertation. May 2022. Major: Psychology. Advisor: Stephen Engel. 1 computer file (PDF); vii, 131 pages.

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Fast, Elizabeth. (2022). Limits of Adaptation in the Vergence System. Retrieved from the University Digital Conservancy,

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