Transcranial magnetic stimulation and motor plasticity in human lateral cerebellum: dual effect on saccadic adaptation.

The cerebellum is a key area for movement control and sensory-motor plasticity. Its medial part is considered as the exclusive cerebellar center controlling the accuracy and adaptive calibration of saccadic eye movements. However, the contribution of other zones situated in its lateral part is unknown. We addressed this question in healthy adult volunteers by using magnetic resonance imaging (MRI)-guided transcranial magnetic stimulation (TMS). The double-step target paradigm was used to adaptively lengthen or shorten saccades. TMS pulses over the right hemisphere of the cerebellum were delivered at 0, 30, or 60 ms after saccade detection in separate recording sessions. The effects on saccadic adaptation were assessed relative to a fourth session where TMS was applied to Vertex as a control site. First, TMS applied upon saccade detection before the adaptation phase reduced saccade accuracy. Second, TMS applied during the adaptation phase had a dual effect on saccadic plasticity: adaptation after-effects revealed a potentiation of the adaptive lengthening and a depression of the adaptive shortening of saccades. For the first time, we demonstrate that TMS on lateral cerebellum can influence plasticity mechanisms underlying motor performance. These findings also provide the first evidence that the human cerebellar hemispheres are involved in the control of saccade accuracy and in saccadic adaptation, with possibly different neuronal populations concerned in adaptive lengthening and shortening. Overall, these results require a reappraisal of current models of cerebellar contribution to oculomotor plasticity.