Patients : 28 subjects with Parkinson's disease duration of more than 5 years, without dementia (Mattis Dementia Rating Scale ≥ 130, MMSE ≥ 27 and DSM IV), without major depression (MADRS < 18) who have severe gait disorders including freezing of gait (defined by an answer 2 or 3 at the 3rd question of the autoquestionnaire of Giladi: Do your gait disorders impede your daily living activities and your independence: answer: yes, moderately or severely. But the patient requires no physical assistance to walk) despite an optimal dopaminergic treatment and optimal and stable subthalamic stimulation parameters. No additional therapy will be permitted during the study.