Convergent properties of vestibular-related brain stem neurons in the gerbil

Three classes of vestibular-related neurons were found in and near the prep ositus and medial vestibular nuclei of alert or decerebrate gerbils, those responding to: horizontal translational motion, horizontal head rotation, o r both. Their distribution ratios were 1:2:2, respectively. Many cells resp onsive to translational motion exhibited spatiotemporal characteristics wit h both response gain and phase varying as a function of the stimulus vector angle. Rotationally sensitive neurons were distributed as Type I, II, or I II responses (sensitive to ipsilateral, contralateral, or both directions, respectively) in the ratios of 4:6:1. Four tested factors shaped the respon se dynamics of the sampled neurons: canal-otolith convergence, oculomotor-r elated activity, rotational Type (I or II), and the phase of the maximum re sponse. Type I nonconvergent cells displayed increasing gains with increasi ng rotational stimulus frequency (0.1-2.0 Hz, 60 degrees/s), whereas Type I I neurons with convergent inputs had response gains that markedly decreased with increasing translational stimulus frequency (0.25-2.0 Hz, +/- 0.1 g). Type I convergent and Type II nonconvergent neurons exhibited essentially flat gains across the stimulus frequency range. Oculomotor-related activity was noted in 30% of the cells across all functional types, appearing as bu rst/pause discharge patterns related to the fast phase of nystagmus during head rotation. Oculomotor-related activity was correlated with enhanced dyn amic range compared with the same category that had no oculomotor-related r esponse. Finally, responses that were in-phase with head velocity during ro tation exhibited greater gains with stimulus frequency increments than neur ons with out-of-phase responses. In contrast, for translational motion, neu rons out of phase with head acceleration exhibited low-pass characteristics , whereas in-phase neurons did not. Data from decerebrate preparations reve aled that although similar response types could be detected, the sampled ce lls generally had lower background discharge rates, on average one-third lo wer response gains, and convergent properties that differed from those foun d in the alert animals. On the basis of the dynamic response of identified cell types, we propose a pair of models in which inhibitory input from vest ibular-related neurons converges on oculomotor neurons with excitatory inpu ts from the vestibular nuclei. Simple signal convergence and combinations o f different types of vestibular labyrinth information can enrich the dynami c characteristics of the rotational and translational vestibuloocular respo nses.