Deep brain stimulation (DBS) improves debilitating symptoms of movement disorders when conventional medical therapies and novel surgical therapies fail. Despite the remarkable efficacy of DBS, its therapeutic mechanism remains unclear. There is controversy regarding whether the therapeutic effects of DBS are associated with inhibition or excitation of target neurons, the introduction of new activity into the network, or a combination of these mechanisms. Additionally, it is unclear why stimulus frequency plays an important role in the clinical response to therapy. The fundamental hypothesis of this proposal is that unilateral subthalamic nucleus (STN) DBS in PD alters neuronal activity in the bilateral basal ganglia-thalamic-cortical motor system in a manner that is dependent on stimulation frequency.