CONTRIBUTIONS OF REGULARLY AND IRREGULARLY DISCHARGING VESTIBULAR-NERVE INPUTS TO THE DISCHARGE OF CENTRAL VESTIBULAR NEURONS IN THE ALERT SQUIRREL-MONKEY

The discharge of neurons in the vestibular nuclei was recorded in aler t squirrel monkeys while they were being sinusoidally rotated at 2 Hz. Type I position vestibular-pause (PVP I) and vestibular-only (V I) ne urons, as well as a smaller number of other type I and type II eye-plu s-vestibular neurons were studied. Many of the neurons were monosynapt ically related to the ipsilateral vestibular nerve. Eye-position and v estibular components of the rotation response were separated by multip le regression. Anodal currents, simultaneously delivered to both ears, were used to eliminate the head-rotation signals of irregularly disch arging (I) vestibular-nerve afferents, presumably without affecting th e corresponding signals of regularly discharging (R) afferents. R and I inputs to individual central neurons were determined by comparing ro tation responses with and without the anodal currents. The bilateral c urrents, while reducing the background discharge of all types of neuro ns, did not affect the mean vestibular gain or phase calculated from a population of PVP I neurons or from a mixed population consisting of all type I units. From this result, it is concluded that I inputs are canceled at the level of secondary neurons. The cancellation may expla in why the ablating currents do not affect the gain and phase of the v estibule-ocular reflex. While cancellation was nearly perfect on a pop ulation basis, it was less so in individual neurons. For some neurons, the ablating currents decreased vestibular gain, while for other neur ons the vestibular gain was increased. The former neurons are interpre ted as receiving a net excitatory (I-EXC) I input, the latter neurons, a net inhibitory (I-INH) input. When compared with the corresponding R inputs, the I inputs were usually small and phase advanced. Phase ad vances were larger for I-EXC than for I-INH inputs. The sign and magni tude of the I inputs were unrelated to other discharge properties of i ndividual neurons, including discharge regularity and the phase of ves tibular responses measured in the absence of the ablating currents. Un ilateral currents were used to assess the efficacy of ipsilateral and contralateral pathways. Ipsilateral pathways were responsible for almo st all of the effects seen with bilateral currents. The results sugges t that the vestibular signals carried by central neurons, even by thos e neurons receiving a monosynaptic vestibular-nerve input, are modifie d by polysynaptic pathways.