They also experienced an improvement in executive function. Several studies have shown that aerobic exercise causes a robust increase in brain-derived neurotrophic factor (BDNF), which increases the growth of new neurons and likely explains the increase in brain volume these people experienced.

From the article:

Using a new MRI technique, researchers found that adults with mild cognitive impairment (MCI) who exercised four times a week over a six-month period experienced an increase in brain volume in specific, or local, areas of the brain

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The analysis revealed that for both the aerobic and stretching groups, brain volume increased in most gray matter regions, including the temporal lobe, which supports short-term memory.