Floccular efferents in the rhesus macaque as revealed by autoradiography and horseradish peroxidase.

To fulfill its putative role in short- and long-term modification of the vestibulo-ocular reflex, the flocculus of the cerebellum must send efferents to brainstem nuclei involved in the control of eye movements. In order to reveal the sites of these interactions, we determined the projections of the flocculus by autoradiography and orthograde transport of horseradish peroxidase in five rhesus macaques. Anterogradely labeled axons collected at the base of the injected folia and coursed caudally and medially between the middle cerebellar peduncle and the flocculus. They swept medially over the caudal surface of the middle cerebellar peduncle, over the dorsal surface of the cochlear nuclei, and then caudally along the lateral surface of the inferior cerebellar peduncle to pass over its dorsal surface in the cerebellopontine angle and terminate exclusively in the ipsilateral vestibular nuclei. Three contingents of axons could be differentiated. The axons of one group flowed caudally and medially into the y-group, which clearly received the densest floccular projection. Other, notably thicker, axons of this group continued rostrally and medially to terminate chiefly in the large-cell core of the superior vestibular nucleus. A second large contingent of thin axons streamed caudal and ventral to the y-group to form a compact tract adjacent to the lateral angle of the fourth ventricle and dorsal to the medial vestibular nucleus. Fibers from this tract (the angular bundle of Löwy) supplied a sizable projection to the rostral part of the medial vestibular nucleus and modest projection to the ventrolateral vestibular nucleus. A final group of fibers extended caudally and medially from the y-group in a plexus ventral to the dentate and interposed nuclei to terminate in the basal interstitial nucleus of the cerebellum (Langer, '85), a broadly distributed cerebellar nucleus on the roof of the fourth ventricle. The flocculus can affect vestibulo-ocular behavior only through these efferents to the vestibular nuclei and the basal interstitial nucleus of the cerebellum.