Bilateral processing of vestibular responses revealed by injecting lidocaine into the eighth cranial nerve in vitro.

Extracellular unit responses were recorded from the vestibular nucleus (VN) and medial longitudinal fasciculus during horizontal head rotation of an in vitro turtle brainstem in which the temporal bones remained attached. Units were characterized as type I or type II based on the responses to ipsiversive or contraversive rotation, respectively. Lidocaine injections (0.5-2 microl of 0.5%) into the root of the eighth cranial nerve within the cranium caused rapid effects on unit responses to head rotation. Responses of type I units were reduced by ipsilateral injection but enhanced following contralateral injection. On the other hand, type II units had their responses increased by ipsilateral injections yet decreased by contralateral injections. In approximately half of the type II cells, decrease of the contraversive response was accompanied by the appearance of latent ipsiversive activity. Our findings not only confirm that each eighth nerve has afferents that drive ipsiversive excitation of both vestibular nuclei but also suggest that both nerves compete to dominate a central neuron's vestibular response. These results may be inconsistent with the push-pull vestibular model in which each nerve drives the central neuron with a complementary response that enhances the vestibular output. An alternate model is described in which vestibular neurons receive bilateral excitation, and that excitatory input is antagonized by crossed inhibition during contraversive motion.