Floccular lesions abolish adaptive control of post-saccadic ocular drift in primates

After several days of exposure to optically-imposed post-saccadic retinal slip, the saccades of normal monkeys acquire an exponential ocular drift. This drift is in the direction of the imposed image motion, and persists in the dark. It has been argued that these changes result from the operation of a visually mediated adaptive mechanism that normally functions to minimize post-saccadic ocular drift. Adaptation to persistent post-saccadic retinal slip was assessed in two rhesus monkeys before and after bilateral ablations of the flocculi and portions of the paraflocculi ("flocculectomy"). After flocculectomy, both monkeys showed some post-saccadic ocular drift. Flocculectomized animals also failed to adapt to optically-imposed post-saccadic slip. We infer from this that the flocculi and/or paraflocculi are necessary for the successful suppression of postsaccadic ocular drift.