Well, as a brief mention said in the article, any exercise benefit to the brain used to be thought of mainly as an increased blood flow and oxygen uptake.

But more recent research is focused on the hippocampus and neurogenesis, actually growing new neurons by exercising. Unfortunately, there's no test that can actually prove the new cells are responsible for new learning.

This happens to be the focus of my current research paper. :-)

Cool. So might the new neurons be more for motor control than say, mathematics?

Well, the hippocampus is related to long term memory and not motor control, as I understand it. So it's doubtful.

I'm having a hard time finding studies that adequately control for fitness level though. Most tend to focus on acute exercise. They run people for a while then immediately sit them down for a cognition test.

I'm more interested in the influence of long-term fitness level. That's not as often studied.