Objective: To investigate the activation pattern of cerebellar structures during small-field optokinetic stimulation (OKN) by functional MRI (fMRI) and compare it with that obtained during voluntary saccades and fixation suppression of OKN.
Methods: Functional images were acquired from oblique transverse slices of eight healthy, right-handed volunteers using a radio frequency-spoiled, single-slice, fast low-angle shot (FLASH) pulse sequence with high-spatial resolution.
Results: Horizontal OKN and saccades were associated with bilateral activity in the cerebellar hemispheres (superior semilunar lobule, simple lobule, quadrangular lobule, inferior semilunar lobule), the middle cerebellar peduncle, the dentate nucleus, and medially in the culmen and uvula of cerebellar nuclei. The pattern and extent of activation were independent of the stimulus direction for OKN and saccades. During fixation suppression, the extent of activation was significantly diminished (hemispheres) or even absent (uvula, culmen).
Conclusion: The differential effects of fixation suppression on this complex pattern of cerebellar activation in part allow us to separate visual and attentional from ocular motor processing. Our data agree with behavioral and physiologic animal data about ocular motor processes and motor learning in the vestibulospinal and optokinetic reflex. This suggests that hemispheric cerebellar activity may be mainly 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.