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.

Parkinson's disease (PD) is a common problem of older people often causing significant problems with thinking and memory. Abnormal accumulation within in the brain of a protein called amyloid is felt to contribute to impaired thinking and memory in PD. Excessive accumulation of this protein is also involved in Alzheimer disease (AD) and may influence thinking and memory in otherwise healthy older people. Recent evidence suggests that changes in the amount of another brain chemical, serotonin, influences the amount of amyloid accumulating in the brains of people with PD and otherwise healthy older people. The purpose of this study is to determine if changes in brain serotonin affects the accumulation of amyloid in the brain. The investigators use brain imaging methods to measure the amount of serotonin and amyloid in the brain of individuals with PD and otherwise healthy older people. PD participants will undergo repeat brain imaging to assess amyloid accumulation 2 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.

Inclusion Criteria:

• Persons who have been diagnosed with Parkinson disease age 45 years and older are eligible to participate in this study.
• Healthy older individuals (ages 60-80) without neurologic problems are eligible to participate.

Exclusion Criteria:

• Individuals using some drugs cannot participate. These include anti-depressant medications, buproprion, or St. John's Wort, as well as dopamine antagonist medications.
• Women of childbearing age may not participate.
• Individuals unable to have MRI scans because of a pacemaker or metal fragments may not participate.
• Individuals with a history of stroke may not participate. People with any contraindication of PET imaging such as previous participation in research procedures involving ionizing radiation may not be eligible to participate in this study.