An interesting article by Thomas et al. recently measured the effect of six weeks of exercise on neuroimaging measures. Contra to expectations, they found that cerebral blood volume did not significantly change, while white matter volumes did show a significant increase. This suggests that the increases in cognition and hippocampal volume that occur following exercise may be due more to myelin changes than general blood flow changes in the brain per se.
Since blood perfusion is especially poor in white matter regions, it may have been that their study wasn’t able to detect finely grained improvements in blood flow following exercise (which we would have expected from theory and previous studies). Instead, it’s possible that their study detected increased white matter volume as a key consequence of improved blood flow in the brain selectively in the poorly perfused white matter brain regions. Either way, this is interesting data and helps illuminate the mechanisms behind a healthy amount of aerobic exercise.
Thomas AG, Dennis A, Rawlings NB, et al. Multi-modal characterization of rapid anterior hippocampal volume increase associated with aerobic exercise. Neuroimage. 2015.