Pathways Mediating Impaired Postural Control in Parkinson's Disease

Inclusion Criteria: Participants with Parkinson's disease * Diagnosis of idiopathic PD or dystonia as determined by a movement disorders neurologist in accordance with the UK Society Brain Bank diagnostic criteria. * Age 45-80 years. * Able to ambulate independently without the use of an assistive device (e.g. cane) for 50 meters. Healthy Older Adults (Control participants) * Age 45-80 years (this group will be age and sex-matched to the PD group) * Able to ambulate independently without the use of an assistive device (cane or walker) Healthy Young Adults * Age 21-44 years (this group will be age and sex-matched to the PD group) * Able to ambulate independently without the use of an assistive device (cane or walker) Exclusion Criteria: * Subjects who describe a history of a frequent vasovagal syncope (fainting) in response to blood, emotional stress, or sensory triggers. * Subjects who are on anti-coagulant medications. * Any musculoskeletal disorder that affects the ability to stand. * History of seizures, epilepsy, stroke, multiple sclerosis, or traumatic brain injury. * Intracranial metallic or magnetic devices. * Pacemaker or any implanted device. * History of surgery on blood vessels, brain or heart. * Unexplained, recurring headaches or concussion within the last six months. * Moderate to severe hearing impairment. * Subjects who are pregnant. * Dementia diagnosis * Other significant neurological disorders that may affect participation or performance in the study * Implanted deep brain stimulator or other neurosurgeries to treat PD.