Parkinson's disease (PD) is a disabling, progressive condition characterised by severe problems with movement for which medical treatment in the longer term can be unsatisfactory. Deep brain stimulation (DBS) is a treatment, which directly stimulates the nerve cells affected inside the brain to help overcome the difficulties with movement. Classically, DBS stimulates in a manner that is constant. Many patients develop severe problems with walking, so-called freezing of gait, which can be unresponsive to constant DBS. In this study the investigators will test if left-right alternating DBS helps to improve gait problems by potentially promoting left-right alternating up- and down-modulated brain activity, which was found during walking in a previous study. The investigators will test if alternating DBS improves the ability to adjust walking speed and to walk freely compared to constant DBS.

Inclusion Criteria:

• PD patients with bilateral subthalamic nucleus DBS electrodes and stimulators implanted for at least 6 months
• Able to give consent.

Exclusion Criteria:

• Lack of capacity to consent
• Cognitive impairment/lack of capacity to perform experimental task. The investigators will additionally conduct a short (10 min) quantitative assessment of cognitive function using a validated test (MMSE). Patients with a score < 20 will be excluded from the study.

Anonymised data will be made available at https://data.mrc.ox.ac.uk/ once collected and fully analysed

Anonymised data will be made available at https://data.mrc.ox.ac.uk/ once collected and fully analysed

Please see https://data.mrc.ox.ac.uk/

Participants and the UPDRS examiner are blinded to the stimulation condition.