Cortical Correlates of Gait in Parkinson's Disease: Impact of Medication and Cueing

Inclusion Criteria: * Diagnosis of idiopathic Parkinson's Disease from movement disorders neurologist with the United Kingdom Brain Bank criteria of bradykinesia with 1 or more of the following - rest tremor, rigidity, and balance problems not from visual, vestibular, cerebellar or proprioceptive conditions * Without musculoskeletal or peripheral or central nervous system disorders (other than PD) that could significantly affect their balance and gait * All subjects will be capable of following directions for the protocols and to give informed consent. * Hoehn \& Yahr Levels II-III. Exclusion Criteria: * Severe dyskinesia that may affect quality of fNIRS. * Major musculoskeletal or neurological disorders, structural brain disease, epilepsy, acute illness or health history, other than Parkinson's Disease, significantly affecting gait and turning i.e., peripheral neuropathy with proprioceptive deficits (detected as lack of toe proprioception assessed during the neurological exam at Day 1), musculoskeletal disorders, vestibular problem, head injury, stroke. * Montreal cognitive assessment (MoCA) score \< 21 or dementia that precludes consent to participate or ability to follow testing procedures * Inability to stand or walk for 2 minutes without an assistive device. * Idiopathic Parkinson's Disease exclusion criteria: Parkinson plus syndromes such as progressive supranuclear palsy, multiple system atrophy, or corticobasal syndrome or implanted electrodes for deep brain stimulation (DBS), possible vascular parkinsonism, current use of dopamine-blocking agents or cholinesterase inhibitor (as may affect Prefrontal cortical activity while walking).