Inclusion Criteria:  All participants must be aged >55 years with English as the 1⁰ language, with stable permitted medications for <4 weeks, geriatric depression scale score <6, visual and auditory acuity adequate to perform tests, and history of education excluding developmental cognitive abnormalities.  Exclusion Criteria:  Exclusion criteria are history of significant neurologic disease or mental illness, unstable medical conditions that could interfere with completion of study, substance abuse within 2 years, contraindications to MRI, abnormal findings on MRI, history of chronic kidney disease, and investigational agents within 1 month of participation.