The purpose of this study is to determine if changes in brain serotonin affects the
accumulation of amyloid in the brain. The investigators will use brain imaging methods to
measure the amount of serotonin and amyloid in the brain of individuals with Parkinson's
Disease (PD) and otherwise healthy older people. PD participants will undergo repeat brain
imaging to assess amyloid accumulation two years after their first brain imaging session. All
participants will undergo examinations to assess their motor function, and asked questions to
assess their mood and thinking.