The purpose of this study is to use neuroimaging to understand how networks in the brain change over time. Although the single most significant risk factor for developing Alzheimer's disease (AD) is age, the neurobiological processes underlying the transition from normal aging to AD are not well understood. Our group of researchers has developed ways to use MRI to detect small changes in certain parts of the brain. We will use neuroimaging to understand how the connections in the brain change over time in healthy aging. The goal is to discover which brain changes are present in healthy aging.

Participants will have two study visits (about 2 years apart) where they will undergo tests to assess mental function, fill out questionnaires, and undergo a blood draw, brain MRI and PET scan and provide a saliva sample. At the second visit, participants will not repeat PET scan. Participants will continue to be followed longitudinally every two years as long as the study is funded.

Participants are required to have a Co-Participant accompany them for the first portion of each visit. This individual must be a reliable informant that has contact with the participant at least once per week.

This is a Phase II research study to determine dosing of non-invasive brain stimulation for patients with Mild Cognitive Impairment (MCI) and depression. The brain stimulation treatment used in this study is called repetitive transcranial magnetic stimulation (rTMS). rTMS is a Food and Drug Administration (FDA)-approved treatment for depression. The goal of the study is to see if brain stimulation can be used for MCI patients to improve memory, thinking, and mood, and what dose of stimulation works best.

The study uses a form of rTMS called intermittent theta burst rTMS (accelerated iTBS). This treatment has not been FDA-approved for MCI patients. This double-blind study requires 11 study visits over the course of six months.