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.