DYNAMICS OF RABBIT VESTIBULAR NUCLEUS NEURONS AND THE INFLUENCE OF THE FLOCCULUS

1. We recorded single vestibular nucleus neurons shown by electrical s timulation to receive floccular inhibition [flocculus receiving neuron s (FRNs)] and/or to project toward midbrain motoneuronal pools [midbra in projecting neurons (MPNs)] in awake, head-fixed rabbits during comp ensatory eye movements. Stimuli included head rotation in the light, h ead rotation in the dark, and rotation of an optokinetic drum about th e animal. We employed sinusoidal and triangular position profiles in t he 0.05- to 0.8-Hz frequency band. We also examined transient response s to step changes in eye position. 2. We found identified vestibular n ucleus cells (i.e., FRN/nonMPNs, FRN/MPNs, and non-FRN/MPNs) in the pa rvocellular and magnocellular portions of the medial vestibular nucleu s, at the rostrocaudal level of the dorsal acoustic stria. 3. All iden tified vestibular nucleus neurons were excited during ipsilateral (rel ative to side of recording) head rotation and contralateral eye rotati on. 4. The neuronal firing rates could be related to eye position and its time derivatives, and that relationship could be approximated by a two-pole, one-zero linear transfer function. As with abducens neurons , a more detailed approximation requires inclusion of two nonlineariti es-a hysteresis and a variable sensitivity term that increases as eye movement amplitude decreases. 5. When the vestibuloocular reflex is su ppressed by a conflicting full-field visual stimulus [visual vestibula r conflict condition (VVC)], vestibular nucleus neuron modulation is l argely suppressed. The remaining modulation is motoric in nature, beca use it can be related to the residual eye movements. Cells with ''sens ory vestibular signals,'' i.e., cells whose modulation during VVC corr elates better with head rotation than eye movement, were not encounter ed. 6. We examined the dependence of firing rate parameters on stimulu s modality. All neurons exhibited increased phase lead with respect to abducens nucleus neurons during stimuli involving head rotation. This finding could indicate that vestibular-derived inputs are inhomogeneo usly distributed on premotor neurons and that the studied premotor pop ulation receives a stronger vestibular input than another premotor gro up, not recorded in the current experiments. 7. FRNs and non-FRNs were similar in their qualitative response to fast phases, the applicabili ty of the two-pole, one-zero transfer function, hysteresis, and the am plitude nonlinearity. 8. FRNs differed from non-FRNs in having a phase -advanced firing rate at all stimulus frequencies during visual and ve stibular stimuli. The phase difference suggests that one role of the r abbit flocculus is to regulate phase of the net premotor signal.