Disconjugate adaptation to long-standing, large-amplitude, spectacle-corrected anisometropia

A 12-yr-old anisometropic patient had worn corrective eyeglasses (right eye, -0.50 +1.50 x 125; LE, -9.75 +2.50 x 60) for 7 yr, and then changed to contact lenses. Eye movements were recorded before and after the change to contact lenses using binocular search coils. In habitual spectacle viewing, the patient showed disconjugate adaptation. During monocular viewing, for example, ocular alignment changed by as much as 4 degrees during a 20 degrees saccade. Also, during monocular viewing, with either eye, placing the spectacle lens in front of the eye caused an increase in the disconjugate adaptive response compared with viewing without lenses. This finding emphasizes the context specificity of adaptive responses. After switching to contact lenses, the patient still wore his spectacles for 20-40 min each day. Although there was little residual disconjugate adaptation for vertical saccades, he showed considerable adaptation for horizontal saccades, especially for gaze changes that required divergence. The persistence of a partial state of disconjugate adaptation allowed the patient to use immediate, disparity-induced, horizontal vergence to aid ocular alignment in either the contact-lens-viewing or the spectacle-viewing condition. A more complete reversion to conjugacy occurred after nine days of exclusive use of his contact lenses. Then, in a short-term experiment, two minutes of binocular viewing through the eyeglasses induced a considerable reversion toward the previous state of disconjugate adaptation (up to 1.25 degrees of vergence change during monocular viewing). Finally, the waveform of the adapted (to spectacles) intrasaccadic vergence change with monocular viewing was similar to the waveform of the unadapted intrasaccadic vergence change during binocular refixations between targets that required a combined saccade and vergence. This finding suggests a common mechanism for adaptation to spectacle-corrected anisometropia and for normal binocular vergence-saccade interactions.