Asymmetrical adaptation of human saccades to anisometropic spectacles.

We report the extent to which effective asymmetrical saccadic adaptation was achieved by a myopic subject, who was exposed to "long-term" adaptation as he wore anisometropic corrective spectacles for about 40 years and also the extent of "short-term" adaptation in this subject and two other subjects, who initially made conjugate saccades, when they wore newly fitted anisometropic spectacles for about 8 hr. Two-dimensional binocular eye positions were measured with an accurate and precise revolving magnetic field-sensor coil technique. We found that long-term adaptation of vertical saccades was virtually perfect (almost 100% of the asymmetry introduced by the spectacles was corrected). Long-term adaptation of horizontal saccades was less complete and increased with target separation from about 40% for saccadic amplitudes of 5 degrees to about 75% for amplitudes of 60 degrees. Short-term adaptation of vertical saccades was virtually complete (100%) in one newly fitted subject and only partially complete (40%) in the other two subjects. The persistence of the adaptive asymmetry of saccades during monocular viewing showed that adaptation derived from plasticity in the programming of saccades and not from modification of vergence responses. Without the anisometropic spectacles, 30 min of self-paced, one per second changes in binocular fixation between two targets, which required a version change of 45 degrees in combination with a vergence change of 11 degrees, did not induce any asymmetrical adaptation. This result shows that a specific repeated association of version and vergence eye movements was not sufficient to induce asymmetrical adaptation, leading us to suggest that the transient fixation disparities at saccade-offset might be the necessary stimulus for the asymmetrical saccadic adaptation we observed.