The subthalamic nucleus (STN) and globus pallidus interna (GPi) are two main targets in the deep brain stimulation (DBS) treatment for Parkinson's disease (PD). Large randomized controlled trials, in which patients of comparable clinical and demographics were randomized to either GPi DBS or STN DBS, have demonstrated equal effects of both targets in motor symptom improvement, the superiority of STN DBS in the medication reduction and the potential advantage of GPi DBS for cognitive, psychiatric and axial aspects. Unfortunately, in such a highly heterogeneous disease of PD, many of these randomized controlled trials didn't include sufficient subjects and thereby yielded inconsistent results. Moreover, most studies are investigating the difference between GPi and STN in different patients.

In order to address the problem, an intra-patient comparison will be made by investigating the acute turning-on effects of unilateral STN stimulation versus unilateral GPi stimulation on axial symptoms, cognition and also cardinal symptoms within each individual patient who received the treatment of combined unilateral STN and contralateral GPi DBS. Axial symptoms including gait, balance and posture, motor symptoms and cognition are comprehensively assessed under two treatment conditions.

Inclusion Criteria:

• Patients with idiopathic Parkinson's disease
• Aged between 55 and 75 years, both male and female
• Patients who have undergone combined unilateral STN and contralateral GPi DBS for more than 6 months
• A Hoehn-Yahr (H-Y) stage of less than 4 in the off-medication state
• A left-extremity to right-extremity ratio of less than 0.15 on the Unified Parkinson's Disease Rating Scale part Ⅲ (UPDRS-Ⅲ) in the both off-medication and on-medication states

Exclusion Criteria:

• History of serious psychosis
• History of intractable epilepsy (i.e., seizures)
• Diagnosed by the investigators that patients with severe cardiac, liver and kidney diseases, or other serious health conditions
• Dementia (A Mini-Mental State Examination (MMSE) score of < 24), inability to comprehend the experimental protocol or voluntarily provide informed consent
• Lack of cooperation
• Additional reasons for exclusion at the discretion of the clinical investigator
• Poorly controlled depression or anxiety
• Past history of suicidal attempt

Throughout the study, all participants, independent raters/evaluators and statisticians will be blind to the treatment conditions and parameter configurations. Independent raters who are blind to the conditions will be responsible for the conduct of all the assessments. Participants are blind to the treatment conditions. Movement disorders clinician responsible for programming are blind to the study protocol. They are simply told to adjust parameter to optimize therapeutic effects.