In this research study we want to learn more about the effects of non-invasive brain stimulation on motivation, memory, and brain-network function in cognitively unimpaired older adults and individuals with preclinical Alzheimer's disease. This study will use a form of non-invasive brain stimulation called repetitive Transcranial Magnetic Stimulation (rTMS). rTMS will slightly alter activity in an area of your brain that controls cognition. Changes resulting from this stimulation will be measured with behavioral tests, as well as by taking brain images with Magnetic Resonance Imaging (MRI). Participants will come in for one baseline visit followed by 10 days of daily rTMS study visits (Monday through Friday) and an evaluation visit. Then, there will be a 2-week break. After this break, they will return for another baseline visit, an additional 10 days of rTMS, and a final evaluation visit.