Currently, there is a lack of comprehensive knowledge about the role of vestibulospinal drive and cortical activity during self-initiated movement transitions in older adults and people with PD (both with and without FOG). This set of experiments has two primary purposes: to (1) understand the pathological neurophysiology underlying freezing of gait (FOG) during movement transitions and FOG-inducing movements and (2) identify neurological biomarkers associated with FOG and FOG-inducing movements. To achieve this, the investigators will assess vestibular activity using the noninvasive neuromodulation technique of electrical vestibular stimulation (EVS, Experiments 1 and 2) and assess cortical activity by recording via electroencephalography (EEG, Experiments 3 and 4, no stimulation included). These experiments will investigate the vestibular (EVS Experiments) and cortical (EEG experiments) contributions to movement transitions during standing, walking, turning, and changing movement rates. Upon completion of this project, the investigators expect to provide a new understanding of key neural systems (vestibular and cortical) involved in the pathogenesis of movement impairment and freezing episodes during movement transitions including gait initiation, turning, and changing movement rates, in people with PD. An increased understanding of the temporal dynamics of systems involved in FOG and FOG-inducing movements could later guide the development and delivery of novel interventions (e.g. closed-loop deep brain stimulation [DBS] or non-invasive brain stimulation) to decrease the incidence and severity of FOG episodes, reducing fall risk and morbidity.

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.