This study will evaluate the effectiveness of deep brain stimulation (DBS) for treating primary dystonia. Patients with dystonia have muscle spasms that cause uncontrolled twisting and repetitive movement or abnormal postures. Medical therapies are available, but not all patients get adequate relief from the abnormal movements or the pain associated with them. DBS is a surgical procedure that interrupts neuronal circuits in the globus pallidus interna (Gpi) and subthalamic nucleus (STN) - areas of the basal ganglia of the brain that do not work correctly in patients with dystonia. This results in decreased movement and therefore may lessen patients' symptoms and pain. The study will also examine the physiology of dystonia and determine whether the treatment effects of DBS in the Gpi differ significantly from DBS of the STN.