NEURAL CONTROL OF GOAL DIRECTED HEAD MOVEMENT

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Brain mechanisms that program head movements will be studied in the barn owl as a model system for understanding principles of motor system organization and sensorimotor integration. The owl's optic nerve tectum (superior colliculus) issues commands that redirect the head to stimulus sources; the site activity in the tectum represents the direction and magnitude of the desired movement. The motor code is "high-order" in that it specifies a change in orientation, but not the forces that must be generated in particular muscles to accomplish the movement. This place code for movement is subsequently transformed into an intermediate code by 4 independent saccade generators which represent orthoganal (up, down, left and right) vector components if the desired movement. This transformation of motor command signals will be studied using microstimulation, neurophysiological, anatomical, and behavioral techniques; barn owls are chosen because they make extremely accurate, rapid head movements. Specifically, we will determine which nuclei in the brainstem are responsible for transforming the place code into vector component code, and the physiological and anatomical mechanisms by which the transformation is accomplished. In addition, we will address such fundamental issues as how the motor command signals are calibrated by visual experience and whether the commands for head movement are issued in a head-centered or a body-centered frame of reference. The results of these studies will elucidate the computational strategies that underlie motor control in a complex, multi-articulated system. In addition, these studies will teach us about how the brain represents desired movements, how and what components of movement are processed in parallel, and how information is translated from one code to another and from one reference system to another. Knowledge of how the brain programs and executes movements will be important to the development of prosthetics, robotics and computational theory, and to our understanding and appreciation of principles of brain function. Such information will also provide a foundation for accurate interpretation of clinical signs relating to head movement control (such as spasmodic torticollis, supranuclear palsy, etc.), for diagnosis of movement disorders of central origin, and for designing optimal therapies and corrective procedures for certain classes of motor dysfunction.