Cerebellar activation during optokinetic stimulation and saccades

Objective: To investigate the activation pattern of cerebellar structures d uring small-field optokinetic stimulation (OKN) by functional MRI (fMRI) an d compare it with that obtained during voluntary saccades and fixation supp ression of OKN. Methods: Functional images were acquired from oblique trans verse slices of eight healthy, right-handed volunteers using st radio frequ ency-spoiled, single-slice, fast low-angle shot (FLASH) pulse sequence with high-spatial resolution. Results: Horizontal OKN and saccades were associa ted with bilateral activity in the cerebellar hemispheres (superior semilun ar lobule, simple lobule, quadrangular lobule, inferior semilunar lobule), the middle cerebellar peduncle, the dentate nucleus, and medially in the cu lmen and uvula of cerebellar nuclei. The pattern and extent of activation w ere independent of the stimulus direction for OKN and saccades. During fixa tion suppression, the extent of activation was significantly diminished (he mispheres) or even absent (uvula, culmen). Conclusion: The differential eff ects of fixation suppression on this complex pattern of cerebellar activati on in part allow us to separate visual and attentional from ocular motor pr ocessing. Our data agree with behavioral and physiologic animal data about ocular motor processes and motor learning in the vestibulospinal and optoki netic reflex. This suggests that hemispheric cerebellar activity may be mai nly associated with changes in attention, whereas vermal activity seems to be associated with ocular motor control, and activity of the dentate nuclei and the cerebellar peduncles seems to be associated with both.