After a season of college football, portions of players' brains can show signs of damage, even if the players did not experience a concussion, a new study found.

Most past research about head trauma during sports has focused on formal concussions. Whether subconcussive hits — from small dings to hard slams — similarly affect the brain has been unclear.

So, for the new study, published in Science Advances, Bradford Mahon, scientific director of the Program for Translational Brain Mapping at the University of Rochester, and other collaborators scanned the brains of the players on the University of Rochester's football team.

They took the first scans a week before the start of the preseason. During all practices and games, the players wore helmets that tracked the number and intensity of every hit to the head. At the end of the season, the researchers re-scanned players' brains.

Two of the athletes had suffered concussions; their data were removed. The other 38 players' helmets recording a total of 19,128 impacts.

When the researchers compared the scans and the helmet data, they saw that most of the players' midbrains were subtly different. The white matter that connects neurons was slightly less healthy. "There was a kind of fraying" of the tissue, said Adnan Hirad, who is completing an M.D./Ph.D.

The players whose heads had absorbed the most hits, especially if those hits involved slightly off-center impacts and head rotations, showed the greatest disruption inside their midbrain's white matter. For all of them, Mahon said, these brain injuries "were clinically silent," causing no symptoms.

New York Times