Cholinergic Mechanisms of Attentional-motor Integration and Gait Dysfunction in Parkinson Disease (UDALL)

Inclusion Criteria: * Age ≥21 for normal control subjects (Male/Female) and ≥45 for Parkinson's disease (PD), Progressive supranuclear palsy (PSP), or Alzheimer's disease (AD) participants (Male/Female). * For normal control subjects, no significant neurological or psychiatric symptoms and normal neuropsychological examination for age. * PD diagnosis (with or without Mild Cognitive Impairment/dementia) will follow the Movement Disorder Society-revised clinical diagnostic criteria for PD or Parkinson-PSP patients. * Modified Hoehn and Yahr stages 1-4. * AD subjects meeting the criteria listed in Guy M. McKhann et al. * All PD subjects are required to have nigrostriatal dopaminergic denervation as demonstrated by vesicular monoaminergic transporter type-2 (VMAT) \[18F\]9-fluoropropyl-(+)-dihydrotetrabenazine (DTBZ) positron emission tomography (PET) imaging. This may be based on a prior DTBZ PET scan or the DTBZ PET scan performed as part of this study. Exclusion Criteria: * Disorders which may resemble PD, such as dementia with Lewy bodies, vascular dementia, normal pressure hydrocephalus, multiple system atrophy, corticobasal ganglionic degeneration, or toxic causes of parkinsonism. The use of the Movement Disorder Society-revised clinical diagnostic criteria will mitigate the inclusion of PD subjects with atypical parkinsonism. * Subjects on neuroleptic, anticholinergic (trihexyphenidyl, benztropine), or cholinesterase inhibitor drugs. Subjects with prior exposure to disa…