Aerobics grows your brain, but does it make you smarter?

Here’s what looks to be a straightforward claim:

Increased hippocampal volume translates to improved memory function(.)

But a simple line in the Discussion section may not convey the trickiness of the analysis in the Results section.

This paper, by Erickson and company, is looking for ways to prevent or reverse cognitive decline as people age. The hippocampus is part of the brain critical to the formation of memory, something we’ve known from many unfortunate people like Henry Molaison (known in the scientific literature as HM) or Clive Wearing who have suffered damage to their hippocampi. There’s good evidence that the size of the hippocampus can be affected by experience in humans.

The experiment had two groups. One group did aerobic exercise, the other did stretching exercises. They tested people after six months of exercise, and again after a year.

People who did the aerobic exercise had an anterior hippocampus that was a couple of percentage points larger than when they started. This is pretty cool, because hippocampus size decreases with age. The authors estimate that this is the equivalent of “rolling back the clock” by one or two years. The pattern they saw with the people doing stretching exercises was more typical: their hippocampi, on average, shrank.

Hippocampus is involved in forming new memories. Aerobic exercise makes your hippocampus bigger, as shown in this paper. So the statement I quoted at the top seems to follow, not just logically, but inevitably.

But here’s my problem. In the Results, the authors write:

Both groups showed improvements in memory(.)

Wait. Both groups got better at the memory task? That’s not what I would predict if hippocampus volume relates to memory function. After all, the hippocampi of the stretching control group decreased in size. You might think, “Well, okay, maybe both groups improved, but the aerobic exercise group must have improved more than the stretching control group, right?” Wrong.

(T)he aerobic exercise group did not improve performance above that achieved by the stretching control group(.)

Wait. What? How can you claim that  bigger hippocampus volume translates into better memory, when people whose hippocampi are shrinking perform just as well at the memory task as those whose hippocampi are growing? Erickson and colleagues make this claim based on an analysis of the people in the aerobic control group only, and show that there’s a correlation between the amount of increase in the hippocampus and the improvement on the memory task.

Am I missing something blindingly obvious? I don't see how you can claim bigger hippocampus means better memory when you only analyze the test group and not the control.

This paper is very cool in what it shows about flexibility of brain size, but I am not sure what the take-home message is about whether aerobics can keep your memory sharp.

Reference

Erickson K, Voss M, Prakash R, Basak C, Szabo A, Chaddock L, Kim J, Heo S, Alves H, White S, Wojcicki T, Mailey E, Vieira V, Martin S, Pence B, Woods J, McAuley E, & Kramer A. 2011. Exercise training increases size of hippocampus and improves memory Proceedings of the National Academy of Sciences, 108 (7), 3017-3022 DOI: 10.1073/pnas.1015950108

Photo by rikomatic on Flickr; used under a Creative Commons license.