Much of our time is spent performing routine movements that redirect our gaze and postural orientation. Many posture and gaze movements induce a pivot, bend, or displacement of the body axis. The vestibular nuclei of the brainstem play a key role in sensing these axial movements and in adjusting the axial musculature. The goal of the proposed studies is to construct the overall logical structure of the vestibular nuclei as a sensorimotor integration unit, as part of a long-term project to provide a functional interpretation of motor control using mathematical descriptors. Because the vestibular nuclei serve the axial movements in particular, we believe that the axial movements plus the vestibular nuclei can be viewed as a unified, multilevel system within which the vestibular nuclei recombine sensory information to support particular movement phases. This system interacts with other neural centers, such as the cerebellum, as it does with the environment. In order to demonstrate the nature of this multilevel system, we will develop a mathematical framework to characterize sensorimotor integration of axial movements by the vestibular nuclei. The present proposal focuses on movements that repeat in a nearly cyclic manner. The first Specific Aim is to mathematically characterize a range of near-cyclic axial movements involving the head and trunk. For example, the gaze shifts in watching such games as tennis and ping-pong will be characterized within a range of movements differing in frequency and amplitude. Besides gaze and scanning movements, near-cyclic movements include maintaining trunk and head posture and orientation during locomotion. The second Specific Aim is to extend the mathematical characterization from a behavioral level to include the near-cyclic processes in the vestibular nuclei that mediate sensorimotor integration of each range of axial movements. Because the processes in the vestibular nuclei may have different ranges, these two Specific Aims will be repeated until the mathematical framework characterizes unified, multilevel functions of the system as a whole. The proposed studies will clarify axial movement control in healthy individuals and patients, along with the sensorimotor integration provided by the vestibular nuclei. Our studies focus on movements and mechanisms of immediate clinical relevance. For example, vestibular patients respond strongly to specific movements. Simple tasks such as grocery shopping can become extremely difficult in these cases because of the disequilibrium resulting from a combination of an intricate gaze activity -- scanning grocery shelves -- with balance and locomotion. On the other hand, head movements have been found clinically to aid a significant group of vestibular patients with standing balance. The development of a logical structure linking such repetitive movements to similarly near-cyclic processes in the vestibular nuclei should advance our understanding of these and other poorly understood impairments of sensorimotor integration.
