The primary objective of the current prospective monocentric pilot study is to determine the optimal stimulation parameters (site of stimulation, frequency, amplitude) of PPN-DBS in patients with severe PD. Secondary objectives are 1) to confirm the effect of PPN-DBS on disturbance of posture, balance and gait; 2) to evaluate long term tolerance of this technique; 3) to determine the components of gait that are most improved by PPN-DBS; 4) to evaluate unilateral versus bilateral stimulation; 5) to evaluate the effects of PPN-DBS on other PD symptoms; 6) to determine the impact of PPN-DBS on quality of life. Five patients fulfilling the following criteria, will be included: age between 18 and 70 years, idiopathic PD for more than 5 years, predominant axial motor signs (disturbance of gait and balance, falls, freezing of gait) responsible for disability in daily living and resistant to optimal treatment.