A Responsive Closed-Loop Approach to Treat Freezing of Gait in Parkinson's Disease

Inclusion Criteria: 1. A clinical diagnosis of idiopathic PD, without a previous DBS operation 2. Must be deemed appropriate for a GPi DBS operation by the interdisciplinary screening team (GPi must be the target chosen as most appropriate for treatment of their PD to qualify for this study) 3. Age 30-75 years (must meet UK Brain Bank criteria for diagnosis of idiopathic Parkinson's disease). 4. Experiencing significant gait and postural instability despite optimal pharmacologic management (Hoehn and Yahr Stage II or greater in the on state). 5. Best medication "on" does not reveal meaningful improvement in posture scores (Pull Test). Patients must be challenged in person with a suprathreshold dose of levodopa (1.5 times optimized regular dose of levodopa) and must have poor or no improvement in postural stability. 6\. Patients must possess a clinical history of gait freezing \> 2 episodes per month, to be included, participants must also score \> 1 on item #3 of the Freezing of Gait (FOG) Questionnaire and exhibit five or more FoG episodes during the provocation protocol in on or off state. 7\. L-dopa responsive with clearly defined "on" periods. 8. Willingness and ability to cooperate during conscious operative procedure, as well as during post-surgical evaluations, adjustments of medications and stimulator settings. Exclusion Criteria: 1. Clinically significant medical disease that would increase the risk of developing pre- or postoperative complications. Clinically significant medical disease includes uncontrolled systemic hypertension with values above 170/100mmHg; cardiac or pulmonary disease; uncorrected coagulation abnormalities or need for therapeutic anticoagulation which cannot be interrupted; any condition that would render the patient unable to safely cooperate with the study tests as judged by the screening physician. 2. Evidence of secondary or atypical parkinsonism. 3. Other neurological and musculoskeletal impairments that would negatively influence postural stability 4. Past MRI scan with significant evidence of brain atrophy or other abnormalities. 5. Dementia as evidenced by impairment in two neuropsychological domains and a Mattis Dementia Score \<130. 6\. A major untreated psychiatric disorder as revealed on psychiatric exam at screening, and a Beck Depression Inventory Score \>14. 7\. Subjects with a history of seizures. 8. Subjects who may require repeat MRI scans. 9. Subjects with a history of a cranial neurosurgical procedure. 10. Subjects with metal in the head or another implanted stimulator (e.g. vagus nerve stimulator, spinal cord stimulator, pacemaker, cochlear implant, etc). 11\. Subjects who require treatment with Electroconvulsive therapy (ECT) or repetitive Transcranial Magnetic Stimulation (rTMS). 12\. Pregnant or nursing women or women who wish to become pregnant will be excluded.