<qa>  What is the main cause of aging?Normal aging reduces cerebral blood flow and cognitive function. Aging also appears to alter functional connectivity within the brain, which is associated with cognitive functioning. Observational studies suggest that regular physical activity is associated with higher cerebral blood flow and improved cognitive function. However, the mechanistic links among regular physical activity and cerebral blood flow with advancing age are unknown. In this context, it is unclear if aging or exercise training status alters the neurovascular coupling of blood flow in the brain. Thus, the overall goal of this study is to examine the age-related changes in cerebral vasodilatory capacity, an important homeostatic mechanism and marker for effective regulation of cerebral perfusion, in order to determine how it is mechanistically linked to cognition. Additionally, the investigators will explore the potential beneficial effect of physical activity on the relationships between cerebral vasodilation and cognition in humans. 