The nucleus prepositus predominantly outputs eye movement-related information during passive and active self-motion

Maintaining a constant representation of our heading as we move through the world requires the accurate estimate of spatial orientation. As one turns (or is turned) toward a new heading, signals from the semicircular canals are relayed through the vestibular system to higher-order centers that encode head direction. To date, there is no direct electrophysiological evidence confirming the first relay point of head-motion signals from the vestibular nuclei, but previous anatomical and lesion studies have identified the nucleus prepositus as a likely candidate. Whereas burst-tonic neurons encode only eye-movement signals during headfixed eye motion and passive vestibular stimulation, these neurons have not been studied during self-generated movements. Here, we specifically address whether burst-tonic neurons encode head motion during active behaviors. Single-unit responses were recorded from the nucleus prepositus of rhesus monkeys and compared for head-restrained and active conditions with comparable eye velocities. We found that neurons consistently encoded eye position and velocity across conditions but did not exhibit significant sensitivity to head position or velocity. Additionally, response sensitivities varied as a function of eye velocity, similar to abducens motoneurons and consistent with their role in gaze control and stabilization. Thus our results demonstrate that the primate nucleus prepositus chiefly encodes eye movement even during active head-movement behaviors, a finding inconsistent with the proposal that this nucleus makes a direct contribution to head-direction cell tuning. Given its ascending projections, however, we speculate that this eye-movement information is integrated with other inputs in establishing higher-order spatial representations.