Understanding balance signals when walking freezes

Inclusion Criteria: Participants with Parkinson's disease (with and without Freezing of Gait) * Diagnosis of idiopathic PD. * Age 40-80 years. * Able to ambulate independently without the use of an assistive device (e.g. cane) for 50 meters. Healthy Older Adults (Control participants) * Age 40-80 years (this group will be age- and sex-matched to the PD and FOG group). * Able to ambulate independently without the use of an assistive device (cane or walker). * Normal capacity to perform complex activities of daily living independently based on informant or physician report. Healthy Young Adults * Age 21-44 years (this group will be sex-matched to the PD and FOG group) * Able to ambulate independently without the use of an assistive device (cane or walker) Exclusion Criteria: Exclusion Criteria (for all participants): * Any musculoskeletal disorder that affects the ability to stand or walk. * History of musculoskeletal disorders that significantly affect movement of lower limbs. * Uncorrected visual impairment that may affect participation or performance in the study. * History of visual and/or vestibular conditions that may affect participation or performance in the study. * Meet criteria for dementia with Lewy bodies, Multiple Systems Atrophy, Alzheimer's disease, dementia diagnosis, or other neurodegenerative disorder other than Parkinson's disease. * History of seizures, epilepsy, stroke, multiple sclerosis, or traumatic brain injury or other significant neurological disorders that may affect participation or performance in the study. * History of muscular conditions of the neck and back, including whiplash. * 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 * Reduced capacity to consent. This will be assessed using a 2-stage process. Initially, participants will be tested using the University of California, San Diego, Brief Assessment of Capacity to Consent (UBACC). If the results of the UBACC are inconclusive the individual will be further tested using the MacArthur Competency Assessment Tool for Clinical Research (McCAT-CR). Additional exclusion criteria for EVS Experiments (Experiments 1 and 2): * History of motion sickness (as EVS can exacerbate motion sickness). * Intense physical exercise or consumption of alcohol in the 24 hours prior to any experiment that will include EVS testing. * Recent history of migraine headaches. * Intracranial metallic or magnetic devices. * Pacemaker or any implanted device. * Implanted deep brain stimulator or other neurosurgeries to treat PD. * Use of medications that can cause dizziness. Additional exclusion criteria for Control Participants: * Presence of parkinsonism or cognitive impairment (including dementia or mild cognitive impairment). * Active central nervous system, systemic, psychiatric conditions or use of psychoactive medication that would adversely affect cognitive, neuropsychiatric, motor, or autonomic functioning.