DESCRIPTION: (verbatim from the application) The long-term goal of this project is to understand how sensorimotor integration of proprioceptive, visual, and vestibular information contributes to postural stability in humans. A complete understanding of postural control requires knowledge of how balance is maintained when the sensory integration process is challenged by conditions that limit and/or alter the available sensory cues, and that alter body dynamics. The principal hypothesis of this proposal is that this integration process is actively regulated. To demonstrate and characterize this active regulation, the focus of the research is on postural responses associated with transitions which suddenly limit or restore orientation cues or suddenly change body dynamics. The proposed experiments are motivated by recently acquired data that demonstrate characteristic transient oscillatory body sway following environmental transitions which suddenly restore accurate orientation cues. Modeling studies suggest that the observed oscillatory behavior is caused by an over generation of corrective leg muscle torque. The rapid restoration of normal, non-oscillatory body sway suggests that a sensorimotor regulatory mechanism exists that is able to quickly reduce corrective torque to an appropriate level. Additionally, preliminary results indicate that this regulatory mechanism is present in subjects with bilaterally absent vestibular function, but that this mechanism is compromised, possibly making vestibular loss subjects more susceptible than normals to instability following environmental transitions which alter the available sensory orientation cues. There are two specific aims of the proposed work. The first aim is to characterize the properties of neural mechanisms that actively regulate the human postural control system to achieve optimal dynamic control of balance in changing environments. The second is to characterize nonlinear properties of the vestibular contribution to postural control, and to understand how this nonlinear behavior contributes to the sensorimotor integration process. The proposed research relies on a model-based approach to explain and predict postural behavior determined by sensorimotor regulatory mechanisms involved in the sensory integration process. Abnormalities involving the active regulation of sensorimotor integration and gain control may be an unrecognized source of instability and falls. A better understanding of these active regulatory processes may lead to new rehabilitation methods and improved clinical balance function tests.