Objective: For patients with cervical dystonia who have failed medical therapy, stereotactic deep brain stimulation (DBS) of the globus pallidus interna (Gpi) or subthalamic nucleus (STN) can provide significant symptom improvement, but it is not known which site provides maximum benefit or the physiologic mechanisms by which these improvements may occur. The study goals are to determine if DBS of Gpi or STN provides significant clinical improvement in patients with primary cervical dystonia, whether the anatomic DBS site (if either) provides optimal improvement, and to gain deeper insight into the underlying pathophysiology of this disorder.