An Investigation of Prism Adaptation Latency

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Purpose. The latency of adaptation of the human vergence system to a change in convergence or divergence forced upon it by a prism was investigated in this pilot study. Methods. Adaptation was stimulated by a 5–s period of binocular vision through a prism of 0 Δ, –8Δ, or 8 Δ. Immediately thereafter and for a further period of 45 s, lateral heterophoria was monitored subjectively by an automated version of Duane's screen and parallax test. Adaptation was calculated from the phoria 40 s after the end of binocular vision. Tests were performed at 0.4 m. In the first test session, there was screening during the binocular period to test for flaws in the screening method. In the second session, there was no screening during the binocular period and phoria measurement was started without changing the power of the prism. In the third session, the prism was restored to 0 Δ after the period of binocular vision. Immediately after the last 5–s test, tests were repeated with 1 s of binocular vision. The subject had excellent visual acuity, stereoacuity, and stereolatency. Results. The test functioned correctly and showed good repeatability. The greatest adaptation to 8 Δ was 59%. This was obtained with only 1 s of binocular vision. There was adaptation to – A with 5 s of binocular vision but it was obscured by adaptation to 0 Δ or 8 Δ which persisted from previous tests. Discussion. The reason why the subject's latency was not found more precisely is explained. Additional evidence is presented in support of the finding that prism adaptation can take place within 1 s (perhaps less) of binocular vision, and the persistence and dominance of adaptation to base-out prism at near is pointed out. Guidelines are proposed for the experimental measurement of prism adaptation latency.

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