Most past research about head trauma during sports has focused on formal concussions. Whether subconcussive hits, which can range in intensity from small dings to hard slams, similarly affect and possibly harm the brain has remained unclear.

So, for the new study, which was published this month in Science Advances, Bradford Mahon, an associate professor at Carnegie Mellon University and the scientific director of the Program for Translational Brain Mapping at the University of Rochester, decided to document what happens inside the skulls of football players whose heads repeatedly collide with the ground and each other but who have not had concussions.

To start, he and one of his students, Adnan Hirad, who is completing an M.D./Ph.D., and other collaborators, gained permission to scan the brains of the players on the University of Rochester’s football team, a Division III program. They scheduled the scans for the week before the start of the preseason and focused in particular on each player’s midbrain.

The midbrain, which is part of the brainstem, is positionally vulnerable to pummeling blows to the head. Because it controls motor functions of the eyes and ears, those blows tend to cause ringing ears and problems focusing.

It also can be a “canary in the coal mine” for brain injury, Dr. Mahon says. If the midbrain shows tissue damage, it is likely that other portions of the brain also are being rattled and possibly harmed by impacts, he says.