The role of the cerebellum in smooth pursuit eye movements and optokinetic nystagmus in primates.

Experimental evidence demonstrates that the primate cerebellum plays an essential role in the generation of smooth pursuit eye movements (SPEM). The same neural elements also take part in the generation of the 'direct' component of OKN, which is responsible for an initial jump in slow phase eye velocity during constant velocity stimulation and the optokinetic response above 0,05 Hz. In addition to the extensively studied floccular region, recent experiments also indicate a role of the mid-vermis (lobule VI and VII) in SPEM control. The 'indirect' or 'velocity storage' component of OKN produces more gradual changes in slow phase eye velocity, and basically depends only on brainstem mechanisms. It is, however, under the inhibitory control of the nodulus and ventral uvula. The loss of this control after lesions could explain disorders like periodic alternating nystagmus.