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.