The vestibulocollic (VCR) and cervicocollic (CCR) reflexes help to stabilize the head during passive (imposed) and active (voluntary) rotations of the head and body. However, the specific contributions of the these two reflexes and the mechanics of the head-neck motor plant to head posture and voluntary movement is unclear. Behavioral and single- unit studies will be done in alert, trained squirrel monkeys to define these contributions. Head and gaze position will be monitored with the magnetic search-coil technique, head torques will be measured, and the EMG activity of several neck muscles involved in controlling head movements will be recorded. We will concentrate on the mechanisms responsible for stabilization and movement in the horizontal (yaw) plane. There are three specific aims: 1) Behavioral studies will be done in squirrel monkeys trained to fixate and to track visual targets with their heads. The experiments will define the contributions of the VCR, the CCR and plant mechanics to head stabilization and voluntary head movements in the yaw plane. 2) The firing behavior of secondary vestibulospinal neurons will be studied in alert, trained animals. By comparing discharge during various head-fixed and head-free conditions, the vestibular, proprioceptive and motor-command signals carried by these neurons can be estimated. This, in turn, should elucidate the role of the vestibulospinal pathways in head stabilization and voluntary movement. 3) The relative contributions of regularly and irregularly discharging vestibular-nerve afferents to head stabilization and voluntary head movements will be studied by comparing behavioral performance in the presence and absence of labyrinthine anodal currents that result in a selective functional ablation of irregular afferents. The same ablation method will be used in the single-unit studies to estimate the contributions of the two kinds of afferents to the discharge of individual vestibulospinal neurons. A major complaint arising from vestibular disorders is the instability of the head during standing posture, during locomotion, and during other voluntary movements. The proposed experiments should clarify the neurological basis for the symptoms.