(A) Over a nine-year period, greater amounts of physical activity in the form of walking are associated with greater gray matter volume in several regions including prefrontal, temporal, and hippocampal areas. (B) The effect of walking on greater gray matter volume was dependent on the amount of walking. Those individuals walking at least 72 blocks or roughly 6 to 9 miles per week showed the greatest amount of gray matter volume. Given the association between exercise and brain-derived neurotrophic factor (BDNF), it is possible that these effects are at least partially mediated by BDNF (data adapted from Erickson, Raji, and others 2010).