10-8. Integrated Control of Ocular Reflexes by the Cerebellum

Whereas the VOR in rabbits is adaptively controlled by the flocculus, three specific types of ocular reflexes (VOR, OKR, and OFR) in monkeys are under the influence of a cerebellar area composed of the flocculus and ventral paraflocculus. How these reflexes are differentially represented in this area and integrated to perform purposeful eye movements is still a matter of conjecture. One difficulty is that the flocculus and ventral paraflocculus exhibit a particularly large species variation in folial morphology (Voogd, 2004). A practical solution for arguments of folial structures is to ignore them and lump these areas as one floccular complex, as adopted in physiological mapping in monkeys (Lisberger, 2009). For a radical solution, nevertheless, precise mapping is required for topographical demarcation of the cerebellar cortex in association with precise knowledge of the relevant neuronal circuits and functions. For example, the area defined as the proper flocculus receives major mossy fiber inputs from (1) the primary vestibular nerve, (2) vestibular nuclei, (3) the NRTD, and (4) the central part of the mesencephalic reticular formation. In contrast, the area defined as the ventral paraflocculus receives major mossy fiber inputs from pontine nuclei and NRTP (Gerrits and Voogd, 1989; Glickstein et al., 1994; Nagao et al., 1997a). It is apparent that the former area mediates the VOR/OKR, and the latter the OFR. On the other hand, Purkinje cells in the flocculus project to the medial and ventrolateral parts of the medial vestibular nucleus, superior vestibular nucleus, and the y group, which contain VOR relay neurons. Purkinje cells in the ventral paraflocculus also project to the medial and ventral parts of the medial vestibular nucleus, superior vestibular nucleus, and y group (Balaban et al., 1981; Nagao et al., 1997b). There is also a projection to the caudoventral part of the posterior interpositus and dentate nuclei. Hence, it is possible that the OFR is also mediated by VOR relay neurons. These findings are the basis for the assumption that the VOR, OKR, and OFR are integrated into a multi-input system sharing the controller and the controlled object (Figure 9A).