Responses of Non - Eye Movement Central Vestibular Neurons to Sinusoidal 1 Horizontal Translation in Compensated Macaques after Unilateral

25 After vestibular labyrinth injury, behavioral deficits partially recover through the process 26 of vestibular compensation. The current study was performed to improve our 27 understanding of the physiology of the macaque vestibular system in the compensated 28 state (>7 weeks) after unilateral labyrinthectomy (UL). Three groups of vestibular 29 nucleus neurons were included: pre-UL control neurons, neurons ipsilateral to the 30 lesion, and neurons contralateral to the lesion. The firing responses of neurons 31 sensitive to linear acceleration in the horizontal plane were recorded during sinusoidal 32 horizontal translation directed along 6 different orientations (30° apart) at 0.5 Hz and 33 0.2g peak acceleration (196 cm/s). This data defined the vector of best response for 34 each neuron in the horizontal plane, along which sensitivity, symmetry, detection 35 threshold, and variability of firing were determined. Additionally, the responses of the 36 same cells to translation over a series of frequencies (0.25 – 5.0 Hz) either in the 37 interaural or naso-occipital orientation were obtained to define the frequency response 38 characteristics in each group. We found a decrease in sensitivity, increase in threshold, 39 and alteration in orientation of best responses in the vestibular nuclei after UL. 40 Additionally, the phase relationship of the best neural response to translational 41 stimulation changed with UL. The symmetry of individual neuron responses in the 42 excitatory and inhibitory directions was unchanged by UL. Bilateral central utricular 43 neurons still demonstrated two-dimension tuning after UL, consistent with spatio44 temporal convergence from a single vestibular end-organ. These neuronal data 45 correlate with known behavioral deficits after unilateral vestibular compromise. 46