The goal of these experiments is to determine the mechanisms that permit humans to stabilize their heads during body movements and to make smooth, accurate head movements. Sophisticated systems analysis techniques will be used to characterize two classes of mechanisms and to analyze their interactions. The class of mechanisms comprising neurally generated motor commands to the neck muscles will be studied by observing electromyographic activity of muscles that produce horizontal rotations of the head and the head torques or movements related to that activity. The second class of mechanisms comprising mechanically generated torques will be studied at the level of torque and movement measurement before and after head mechanical properties have been altered by adding mass to the head. One important question to be answered is whether short latency vestibulocollic and cervicocollic reflexes make an important contribution to head stabilization and whether their action varies with behavioral context. A second question concerns how these reflexes interact with mechanical forces and forces generated by longer latency voluntary motor commands. The ultimate answer to these and other questions will come from the preparation of a biomechanical model that describes head stabilization and head movements in terms of neural pathways and physical parameters that control head motion. The three lines of experimentation proposed in this grant are designed to converge upon the generation and thorough testing of such a model. It is expected that such a model when completed will be extremely useful both in understanding normal head movements and in diagnosing and treating the many disorders that impair head stability and motility.