The purpose of this project is to elucidate the mechanisms involved in recovery from vestibular injury (vestibular compensation) in primates. Experiments will focus on the physiological processes underlying recovery of the vestibulo-ocular reflex (VOR) following unilateral labyrinthectomy or plugging of the semicircular canals. Preliminary studies have investigated the role played by volition in compensatory ocular responses during head movement by analyzing unrestrained eye and head saccadic movements (head-free gaze shifts) before and after labyrinthine lesion. These studies demonstrated that a monkey is able to utilize efference copies of neural signals that encode head movements and/or cervical or visual feedback to facilitate behavioral compensation during natural, unrestrained head movements. Single-unit recordings of neurons in the vestibular nuclei are underway to analyze changes in their oculomotor and passive angular acceleration sensitivity before unilateral labyrinthine lesion and after compensation. The hypothesis to be tested in the current study is that efference copies of neural signals encoding volitional head movements are used during those head movements to modify the response of central vestibular neurons, facilitating the stabilization of gaze in compensated animals. This hypothesis will be tested by recording responses of identified central vestibular neurons during both passive whole body rotation and horizontally head unrestrained gaze tasks. The effect of head stoppage will be interpreted in light of the input from the head accelerometer, which is recorded along with all of the other recorded and control signals. A desired benefit of this research will be the possible demonstration of efference copy at the neuronal level. This research is an outgrowth of the candidate's interest in vestibular compensation, which developed from his previous research experience and from his work as an otolaryngologist treating patients complaining of dizziness. This work is an extension and supplementation of the P.I.'s ongoing career development award project (K-08-DC00182).