A forest ages in different ways. In one spot the trees thin out, as if worn down, while in another they stubbornly patch up broken branches. The brain is much the same: in some people it withers, in others it fights back.
Scientists took brain scans of 952 healthy people. They divided the cortex into 148 pieces and measured how each piece changes shape with age. Then they looked at which genes are active in those pieces. It turned out there are two groups of people. In the first group, the changes resemble decline: the genes are busy with metabolism and immunity. In the second group, the genes help cells cope with stress and repair DNA.
Until now it was unclear why the brain ages so differently from person to person. Now we can see there are two paths. One leads to decline, the other to adaptation. For now this is just knowledge, but in the future it will help doctors tell ordinary aging apart from the onset of disease.
The mean age difference between subtypes was 4.5 years, but it was not statistically significant. At the same time, age explained about 93% of the accuracy in classifying subtypes, which highlights the complex relationship between chronological and biological aging.
The idea of different paths of brain aging echoes Greg Egan's novel Permutation City, where people can choose different trajectories of existence. Here, nature seems to offer two scenarios: one leads to decline, the other to adaptation.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.