A new MRI technique has revealed that it is the structural integrity of the hippocampus more than its size that reflects fitness and correlates with cognitive performance.
Research has focused on hippocampal size because it is easier to measure, and in children and older adults there are strong correlations between hippocampal size and memory. But this is less true for healthy, young adults. This new, subtler, technique reveals that something else is going on — something that has probably been masked by the effects of size in older adults (whose hippocampi are shrinking) and younger children (whose brains are still growing).
The technique measures viscoelasticity. If the hippocampus is more elastic, memory is better. When it’s more viscous, memory is worse. Those with better aerobic fitness had better hippocampal elasticity.
 . Aerobic fitness, hippocampal viscoelasticity, and relational memory performance. NeuroImage [Internet]. 2017 ;153:179 - 188. Available from: http://www.sciencedirect.com/science/article/pii/S1053811917302859