Multi-modal characterization of rapid anterior hippocampal volume increase associated with aerobic exercise

- Human anterior hippocampal volume can be increased by only six weeks of regular aerobic exercise.
- This increase in anterior hippocampal volume returns to baseline in the absence of continued aerobic exercise.
- These changes can occur in middle adulthood, not only in the presence of age related atrophy.
- Multimodal neuroimaging techniques support a change in myelination but not a change in vasculature within anterior hippocampus.

The hippocampus has been shown to demonstrate a remarkable degree of plasticity in response to a variety of tasks and experiences. For example, the size of the human hippocampus has been shown to increase in response to aerobic exercise. However, it is currently unknown what underlies these changes. Here we scanned sedentary, young to middle-aged human adults before and after a six-week exercise intervention using nine different neuroimaging measures of brain structure, vasculature, and diffusion. We then tested two different hypotheses regarding the nature of the underlying changes in the tissue. Surprisingly, we found no evidence of a vascular change as has been previously reported. Rather, the pattern of changes is better explained by an increase in myelination. Finally, we show that hippocampal volume increase is temporary, returning to baseline after an additional six weeks without aerobic exercise. This is the first demonstration of a change in hippocampal volume in early to middle adulthood suggesting that hippocampal volume is modulated by aerobic exercise throughout the lifespan rather than only in the presence of age related atrophy. It is also the first demonstration of hippocampal volume change over a period of only six weeks, suggesting that gross morphometric hippocampal plasticity occurs faster than previously thought.