This grant proposal tests the global hypothesis that the cerebellum is the site of both forward and inverse internal models of the arm. Internal models provide for representations of the input-output properties of the motor apparatus or their inverses. A forward model predicts the state of system, either the motor variables or the sensory output, as a consequence of the current state of the arm and the motor commands. An inverse dynamics model transforms the desired trajectory into the torques and forces needed to control the arm. The results of numerous psychophysical studies support the hypothesis that the central nervous system utilizes internal models to control movements. Although widely hypothesized that the cerebellum acquires and stores internal models of the arm, there are few explicit tests of this hypothesis based on single cell recordings. Our recent findings demonstrate that Purkinje cells in the intermediate and lateral zones of the anterior lobe do not signal movement dynamics or muscle activity and therefore, cannot provide the neural substrate for an inverse dynamics model of the arm. Instead, the simple spike discharge of these Purkinje cells encodes arm kinematics, potentially consistent with the output of a forward model that predicts the state of the arm. The first two Specific Aims expand our testing of the hypothesis that the cerebellum is the site of an inverse dynamics model of the arm. Specific Aim 1 examines whether the discharge of cerebellar nuclear neurons is consistent with the output of an inverse dynamics model of the arm, recording nuclear neurons while imposing viscous and elastic force fields. Specific Aim 2 addresses whether Purkinje cell discharge is consistent with the output of an inverse dynamics model, if force control or force feedback is critical to task performance. Using kinematic, isometric and haptic tracking, these experiments will test whether the movement parameters signaled by Purkinje cell simple spike discharge is conditional based on the type of control strategy or feedback used to perform the task. Specific Aims 3-5 test the hypothesis that Purkinje cells are the output of a forward internal model of the arm. Specific Aim 3 tests whether Purkinje cell simple spike discharge encodes arm kinematics as opposed to target/cursor motion. Specific Aim 4 uses a random tracking task to examine whether the discharge of Purkinje cells predicts the upcoming state of the limb and whether the prediction generalizes to reaching movements. In Specific Aim 5 both random and cued perturbations are used to manipulate both the state of the arm and the motor command to further test the forward internal model hypothesis. PUBLIC HEALTH RELEVANCE The cerebellum is essential for the production of smooth, coordinated movements. The results from these studies may lead to a better understanding of both normal function and the abnormal processing that occurs in cerebellar disease.