When a soccer player heads the ball, the brain inside the skull jiggles slightly, like jelly in a sealed jar. A single impact leaves no trace, but hundreds repeat over the years. Doctors struggle to tell when these jolts start to cause harm: standard brain scans look normal, and the person may feel perfectly healthy.
Scientists have found another way to peek into such a brain. They showed people pictures on a screen while simultaneously recording the brain's electrical activity using electroencephalography — sensors on the scalp that pick up signals from nerve cells. In a healthy person, the brain responds to a picture quickly and in a coordinated manner, like an orchestra coming in at the conductor's baton. In people who have sustained many mild impacts, the response lags and falls apart, as if the musicians have lost the rhythm.
The first 100 milliseconds after the picture appears are especially critical. At that moment, the brain hasn't yet consciously registered what it sees, but it is already preparing to process the information. In people with repeated impacts, this preparation is weaker. Based on these early signals, a computer program learned to distinguish healthy individuals from those who had sustained many impacts, and even from those who had experienced a single severe injury.
This is like inspecting a house's foundation: cracks inside aren't visible from the outside, but if you tap and listen to the echo, you can tell where the damage is. Such a method could help doctors detect danger earlier and stop an athlete before irreversible changes occur.
The brain responds to a visual image faster than you can blink: the first signals appear within 50–100 milliseconds.