Nystagmic modulation of neuronal activity in rabbit cerebellar flocculus.

1. The responses of neuronal elements in the flocculus of the awake, restrained rabbit were recorded during horizontal vestibular nystagmus in the dark. 2. Purkinje cells showed both vestibular (Types I and II) and eye movement modulation of simple spike activity. Type I Purkinje cells most commonly were inhibited in association with the ipsilaterally directed fast phase of nystagmus and excited during contralaterally directed fast phases. Type II Purkinje cells had a similar modulation but in the opposite direction. Variations on this pattern included an increase in firing during fast phases in both directions. 3. Presumed mossy fibers and granule cells also exhibited both vestibular and nystagmic modulation in various combinations. The nystagmic modulation often began during the fast phase and continued into the early part of the slow phase. Mossy fibers showing both vestibular and nystagmic modulation probably arise from the vestibular nuclei and/or the perihypoglossal nuclei. 4. Floccular control of brain stem nuclei utilizes not only vestibular but also eye movement signals and probably all sensory and internal signals involved in the regulation of gaze.