Outcome measures: To determine the distribution of muscimol in the STN and to provide an anatomic correlate for clinical effects of neuronal suppression, real-time 3D-volumetric MR-imaging will be used during infusions. To assess safety, tolerability and clinical effects of muscimol infusion, standard PD rating scales (motor subsection of the Unified PD Rating Scale, timed-up-and-go gait assessment and peg board bradykinesia testing) will be performed following infusion and correlated to real-time infusion MR-imaging studies. To compare the effects of muscimol infusion to STN DBS, the assessments obtained during infusion will be compared to the similar assessment 6 months after DBS placement.