Adaptation of tonic accommodation.

We compared the resting (dark) focus of accommodation before and after adapting to accommodative stimuli placed nearer or farther from an initial baseline resting focus. Short-term monocular adaptation (less than 2 min) did not result in consistent after-effects that were correlated with the adaptation stimulus. After short-term adaptation, accommodation returned to its resting level in 2-15 s. Long-term monocular adaptation (30 min) to a 6-D near stimulus resulted in a small (0.5-D) average increase in the resting focus of accommodation beyond the normal 2-15-s short-term decay. These observations illustrate a tonic adaptation of accommodation that is small and requires longer durations of adaptation than an analogous adaptation of the fusional vergence system to prism.