Abstract
Visuomotor learning relies on a complex circuitry that adjusts motor and perceptual representations to reduce saccade errors for high-acuity vision. The cerebellum is considered a key hub to drive the motor changes. Yet, it remains to be examined how the cerebellum adjusts the motor command and whether it is also involved in perceptual changes. We fitted a state-space model to the visuomotor learning data of eight cerebellar patients and eight healthy subjects. The model, based on Masselink & Lappe (2021), explains motor and perceptual changes by plasticity at three stages of the visuomotor circuitry — first, at the stage of visual target representation, second, at the stage of an inverse model that derives the motor command, and, third, at the stage of a forward dynamics model that computes the displacement of visual space due to the saccade based on corollary discharge. These three stages can adapt to nullify postdictive motor error, i.e. the error of the motor command with respect to a postdictive update of visuospatial target position. Modeling results show that inverse model adaptation is impaired in patients, especially in response to outward target steps, i.e. when adaptation is controlled by upregulation of saccade duration. Insufficient adjustment of saccade duration can also be found when no target step occurs, i.e. when longer saccade durations usually compensate for saccade peak velocity decline, known as oculomotor fatigue. Our modelling results suggest that this impairment may have led to perceptual compensation, explaining the significant visual outward localization of the target that we found during patients’ baseline fixation. We conclude that cerebellar impairments of the inverse model account for reduced visuomotor adaptation in patients, especially with respect to regulation of saccade duration. Consequently, learning at perceptual level, e.g. upstream of the cerebellum, may partially compensate for these deficits.