Disconjugate adaptation of the vertical oculomotor system

Conjugate post-saccadic eye drift can be induced in normal humans if a visual pattern is made to drift after every saccade. This study examines the ability of normal humans to create disconjugate vertical post-saccadic drift. Identical fuseable patterns were presented dichoptically, one to each eye. At the end of each vertical saccade one pattern drifted up and the other down, by 5% of the saccade amplitude. Five subjects were trained for 2-3 hr. Eye movements were recorded with eye coils. Normal vertical saccades along the midline were remarkably conjugate and post-saccadic drift was minimal. Training produced only small disconjugate post-saccadic drift (0.14 deg) but substantial saccade amplitude disconjugacy (0.70 deg). For several subjects, the induced disconjugacies persisted even for saccades in the dark indicating that adaptive changes occurred in the binocular coordination of vertical saccades. Apparently vertical disparate post-saccadic retinal slip is not sufficient to stimulate significantly the saccade pulse-step matching mechanism which is believed to control post-saccadic eye drift. The changes we observed aimed to reduce position disparity and not retinal slip in each eye.