But researchers still don't know whether athletes should be worried about big hits that cause concussions, or smaller "pre-concussive" collisions that occur as a routine part of the game.
"This area is a rush to judgment over what we know or don't know," said Thomas McAllister, chairman of the department of psychiatry at Indiana University and a leading expert in traumatic brain injuries. "Because there is a lot we don't know."
McAllister explained that scientists have known about the dangers of getting hit on the head since the 1920s when they diagnosed a form of dementia in boxers. They called it neurologica pugilistica. In recent years, the same condition has become known as CTE.
NEWS: Pro Soccer Players Have Sharper Mental Skills
Last year, McAllister and his colleagues completed a study of NCAA football players over the course of a season, as compared to a similar group of track and field athletes. They tested both groups for cognition and memory, as well as performed sophisticated diffusion tensor imaging of their brains. While they all performed equally on the tests, they found about one in five football players experienced changes to a part of their brain called the corpus collossum, which lies in the middle of the two hemispheres.