Alzheimers Disease Neuroimaging Initiative

The cerebral cortex is like a crumpled sheet of paper: it has thickness, curves, and folds of varying depth. With age, this sheet changes unevenly — thinning in some places, losing the sharpness of its folds in others. Previously, doctors assessed brain age as a whole, as if looking at the sheet from a distance.

An old road atlas lies on the table. On its pages, lines connect cities, and the thickness of each line tells you how busy the traffic once was. A neural network looks at such a map and names the age of the car that drove along it. That is roughly how a model trained to predict a person's age from the map of their brain connections works.