Inclusion Criteria:  Diagnosis of idiopathic Parkinson's disease based on presence of at least 2 cardinal PD features (tremor, rigidity, or bradykinesia) Undergoing deep brain stimulation surgery for treatment of advanced Parkinson's disease, according to clinical evaluation, including the following criteria: advanced idiopathic PD as determined by OFF unified Parkinson's Disease Rating Scale (UPDRS) part III motor subscale > 25 L-dopa responsive symptoms with at least 30% improvement in UPDRS III scores on vs. off medication OR medication refractory disabling tremor Persistent disabling motor symptoms or drug side effects (dyskinesias, motor fluctuations, disabling "off" periods) despite optimal medical therapy preoperative MRI without evidence of cortical or subdural adhesions or vascular abnormalities Willingness and ability to cooperate during conscious operative procedure for up to 40 minutes  Exclusion Criteria:  medical contraindication to surgery, including use of anticoagulant or antiplatelet therapy within 1 week significant cognitive or psychiatric disease based on clinical neuropsychological testing