Amyloid Plaque and Tangle Imaging in Alzheimer's Disease and Down Syndrome

Inclusion Criteria: * Age 21 years or older * No significant cerebrovascular disease - modified Ischemic Score of \< 4 * Adequate visual and auditory acuity to allow neuropsychological testing * Screening laboratory tests and ECG without significant abnormalities that might interfere with the study Additional Inclusion Criteria for Controls * MMSE score between 24 and 30 (unless \< 8 years of educational achievement) * The following medications are allowed if stable for \> 1 month: antidepressants (without anticholinergic effects) if not currently depressed and no history of major depression for 2 years; estrogen replacement therapy; thyroid replacement therapy as long as individual is euthyroid; antihypertensives that do not influence cognitive function Additional Inclusion Criteria for Individuals with Down syndrome * Family member or caregiver available; caregiver relationship 2 years or longer * Karyotype DX of trisomy or translocation DS Mosaic form of Down syndrome * English-speaking Exclusion Criteria: * Evidence of neurological or other physical illness that could produce cognitive deterioration; volunteers with a history of TIAs, carotid bruits, or lacunes on MRI scan will be excluded * Parkinson's Disease * History of myocardial infarction within the previous year or unstable cardiac disease * Uncontrolled hypertension (systolic BP \> 170 or diastolic BP \> 100), history of significant liver disease, clinically significant pulmonary disease, diabetes, or cancer * Major psychiatric disorders, such as bipolar disorder or schizophrenia * Medicines that could influence psychometric test results * Use of any of the following drugs: centrally active beta-blockers, narcotics, clonidine, anti-Parkinsonian medications, benzodiazepines, systemic corticosteroids, medications with significant cholinergic or anticholinergic effects, anti-convulsants, or warfarin * Current diagnosis or history of alcoholism or drug dependence * Evidence of untreated depression or untreated anxiety * Use of any investigational drugs within the previous month or longer, depending on drug half-life * Contraindication for MRI scan (e.g., metal in body, claustrophobia) * Diagnosis of possible or probable AD or any other dementia (e.g., vascular, Lewy body, frontotemporal) or MCI Additional Exclusion Criteria for Individuals with Down syndrome * Mosaic form of Down syndrome * History of clinically significant neurological disorder or disease and Psychiatric diagnosis or treatment within 3 months prior to screening