Responses of Central Vestibular Neurons to Sinusoidal Yaw Rotation

24 After vestibular labyrinth injury, behavioral measures of vestibular function partially 25 recover through the process of vestibular compensation. The current study was 26 performed to improve our understanding of the physiology of macaque vestibular 27 nucleus neurons in the compensated state (>6 weeks) after unilateral labyrinthectomy 28 (UL). The responses of neurons to sinusoidal yaw rotation at a series of frequencies 29 (0.1 – 2.0 Hz) and peak velocities (7.5 210°/s) were examined to determine how the 30 behavior of these cells differed from those in animals with intact labyrinths. The 31 sensitivity of neurons responding to ipsilateral rotation (type I) did not differ between the 32 intact and injured sides after UL, though this sensitivity was lower bilaterally after lesion 33 than pre-lesion. The sensitivity of neurons that increase firing with contralateral rotation 34 (type II) was higher ipsilateral to the UL than pre-lesion or in the nucleus contralateral to 35 the UL. UL did not increase asymmetry in the responses of individual type I or II 36 neurons to ipsilateral vs. contralateral rotation, nor does it change the power law 37 relationship between neuronal firing and level of stimulation. Increased sensitivities of 38 contralesional type I neurons to the remaining vestibular nerve input and increased 39 efficacy of inhibitory vestibular commissures projecting to the ipsilesional vestibular 40 nucleus appear responsible for recovery of dynamic function of central vestibular 41 neurons in compensated animals. The portion of type I neurons on the ipsilesional side 42 is reduced in compensated animals, which likely accounts for the asymmetries in 43 vestibular reflexes and perception that characterize vestibular function after UL. 44 45