The investigators aim to use the new PET radioligand, 18F-MK-6240, to detect tau pathology in cognitive healthy and mild cognitive impairment (MCI) elders. The investigators will then examine the interactions between differential tau burden and performance on cognitive tasks, functional magnetic resonance imaging (fMRI) neural activation patterns, and other cognitive and behavioral measures. By investigating these relationships, the investigators hope to understand the cognitive and behavioral outcomes of tau deposition found in specific brain regions in cognitively normal/mildly cognitively impaired adults. Furthermore, the study aims to examine how the presence of tau may contribute to the risk of subsequent cognitive decline, neurodegeneration, and dementia.