A simple formula may explain a great deal about the overall structure of the human brain.
By Nathan Collins
Probably the most recognizable feature of the human brain is those oddly shaped folds. Technically known as cortical gyrification, they’re actually an essential feature, because they make it possible to pack more neurons into the small space inside our heads. Now, researchers have discovered an intriguing connection between those folds and aging: Basically, as we get older, our brains slowly start to unfold.
The new study, published today in Proceedings of the National Academy of Sciences, follows one recent theoretical and experimental work on mammal brains, which suggested there was a universal formula connecting a brain’s total unfolded size, its folded size, the thickness of the brain tissue (or cortex), and mechanical tension—essentially, how tightly individual neurons hold a brain’s folds together. In a 2015 paper published in Science, researchers reported a simple formula that explains nearly all of the differences in brain size and folding across mammal species, from tree shrews to primates.
That led Newcastle University computer scientist Yujiang Wang to wonder whether the same formula that seems to apply across mammal species also works for human brains. The researchers used publicly available MRI scans of more than a thousand different human brains, from which they computed folded and unfolded size and cortical thickness. As they suspected, the same rule that explained differences between mammal brains also appeared to account for the dissimilarities among human brains.
As we grow older, our brains start to go slack.
The researchers didn’t stop there. Next, they looked to see whether subtle changes might emerge as we age, which they found to indeed be the case. While the basic relationship between folded and unfolded size and cortical thickness does not change, mechanical tension diminishes—in other words, as we grow older, our brains start to go slack.
Where the formula really starts to break down, however, is in people with Alzheimer’s disease: Not only do their brains show signs of substantially less mechanical tension, but folded brain size actually scaled differently with unfolded size in Alzheimer’s patients compared with healthy controls.
The researchers are careful to point out they can’t explain all the variability in our brains, but the results could help lead to a better understanding of brain structure and disease. “By investigating and quantifying what remains invariant and what changes in each case, we shed some light on the underlying mechanisms through which the cortex changes in health and disease and argue that morphological complexity could emerge from a few simple rules,” the authors write.