The study also proposes two complementary approaches to investigating the neural correlates of the beneficial effects of aerobic exercise on cognition: 1) imaging -- a combination of structural, metabolic, and cognitive activation fMRI studies to evaluate the neural substrates of the effect of aerobic exercise on cognition will be used. 2) important correlates -- the effects of apolipoprotein E (APOE) genotype, inflammatory markers and cognitive reserve on the cognitive effects of aerobic exercise will be explored.