The overarching goal of this proposed project is to maximize TBS effects to non-invasive spaced TBS in MCI. For the comparison, the investigators will include cognitively normal adults. There will be a screening session plus three consecutive days including baseline outcome measures on Day 1, TMS interventions and outcome measures on Day 2, and follow-up measurements on Day 3. In this project, on Day 2, participants will receive three TBS blocks inside the MRI scanner with each TBS block separated by 30-90 minutes to enhance the possibility of maximizing the TBS effect. This will be repeated in two blocks, each with a different TMS protocol. The protocols will be in randomized order.
Emerging evidence indicates that dysfunction of hippocampal synaptic plasticity, which precedes neuronal degeneration during the progression of Alzheimer's disease (AD), underlies the hallmark cognitive impairment. Although there are currently no effective disease modifying treatments for AD, recent preclinical studies in animal models of AD have suggested that repetitive transcranial magnetic stimulation (rTMS) promotes hippocampal synaptic plasticity and, ultimately, improves learning and memory abilities. Interleaved TBS-MRI is a paradigm, which allows the investigators to study human brain functionality with real-time MRI, to better understand modulations of brain activity in response to the non-invasive brain stimulation, TBS. The interleaved TBSfMRI paradigm can more accurately estimated the immediate brain activity compared to the offline TBS-MRI studies in which TBS is applied outside the MRI. With this interleaved TBS-MRI approach, the investigators will be able to measure immediate changes in targeted brain activity, such as hippocampus activation, following each TBS session. This approach has created the unprecedented potential enabling the investigators to model the dose-dependent effects of TBS more accurately on brain function.