The Nucleus Prepositus Predominantly Outputs Eye Movement-related Information during Passive 1 and Active Self-motion 2 3

17 Maintaining a constant representation of our heading as we move through the world requires the 18 accurate estimate of spatial orientation. As one turns (or is turned) toward a new heading, signals 19 from the semi-circular canals are relayed through the vestibular system to higher-order centers 20 that encode head direction. To date, there is no direct electrophysiological evidence confirming 21 the first relay point of head motion signals from the vestibular nuclei, but previous anatomical 22 and lesion studies have identified the nucleus prepositus as a likely candidate. While burst-tonic 23 neurons encode only eye movement signals during head-fixed eye motion and passive vestibular 24 stimulation, these neurons have not been studied during self-generated movements. Here, we 25 specifically address whether burst-tonic neurons encode head motion during active behaviors. 26 Single-unit responses were recorded from the nucleus prepositus of rhesus monkey and 27 compared for head-restrained and active conditions with comparable eye velocities. We found 28 that neurons consistently encoded eye position and velocity across conditions, but did not exhibit 29 significant sensitivity to head position or velocity. Additionally, response sensitivities varied as a 30 function of eye velocity, similar to abducens motoneurons and consistent with their role in gaze 31 control and stabilization. Thus, our results demonstrate that the primate nucleus prepositus 32 chiefly encodes eye movement even during active head movement behaviors; a finding 33 inconsistent with the proposal that this nucleus makes a direct contribution to head direction cell 34 tuning. Given its ascending projections, however, we speculate that this eye movement 35 information is integrated with other inputs in establishing higher-order spatial representations. 36