Neurophysiological and anatomical investigation-The paraflocculus-cerebellar nuclear control mechanisms of the primate smooth pursuit eye movement system

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The neural mechanisms for the short-term adaptation of the vestibulo-ocular reflex (VOR) has been investigated as an experimental model of the cerebellar motor control. My results of lesion, single unit recording and circuitry analysis studies in the rabbit and monkey experiments consistently support the hypothesis that the flocculus adaptively controls the gain of the VOR through the long-term depression (LTD) of transmission efficacy of the synapses on the Purkinje cells within the flocculus. To this flocculus hypothesis, a different role of the flocculus is proposed in the monkey VOR that the adaptation of the VOR is induced by the cooperation of the flocculus and its target neurons in the brain stem (Lisberger and Sejnowski 1993). To this proposal, however, my neuroanatomical study using retrograde and/or anterograde axonal flow methods and study of unit recording experiments reveal that the caudal half of the monkey flocculus has the characteristics for the control of the VOR,but the rostral half of it has the characteristics for the control of smooth pursuit. So I suggest that the rostral half of the monkey flocculus belongs to the ventral paraflocculus. The experimental observations reported by the Lisberger's group seem to include thoses obtained in the ventral paraflocculus. Therefore, I suggest that the flocculus and ventral paraflocculus respectively controls the vestibulo-ocular and smooth pursuit eye movements in the primate. My behavioral study using the adaptation of the primate VOR and those for the adaptation smooth pursuit eye movements also support the view that these two slow eye movements are independently controlled by the different groups of neurons in the cerebellum.