Freezing of gait (FoG) is a complex symptom of Parkinson's disease (PD) that cause falls and disability in PD patients, heavily affect patients' autonomy and quality of life. Gait disturbances and FoG are difficult to manage as they usually do not complete respond to both dopaminergic treatment and subthalamic nucleus deep brain stimulation (STN-DBS). One therapeutic strategy suggested in literature for improving gait disturbances is to increase the dose of dopaminergic drugs according to the hypothesis of pseudo-ON-freezing. The pseudo-ON-FoG in patients treated with STN-DBS can easily occur as the result of a suboptimal stimulation or the consequence of a post-operative reduction of the dopaminergic therapy. Therefore, it is reasonable hypothesize both the increase of stimulation and levodopa as good therapeutic strategies to improve pseudo-ON-FoG. At present there are no evidence for suppose that one option is better than the other, even though two recent studies on gait analysis reported a positive additive effect of levodopa therapy on gait parameters in patients treated with STN-DBS. In this study, the investigators aim to objectively evaluating the improvement of FoG in PD patients treated with STN-DBS at different treatment conditions consisting of increased intensity of stimulation or higher dosage of levodopa.