The study involved 25 boys between ages eight and 13 years who played a single season of football. The players agreed to wear special helmets that tracked impacts to the head and had MRIs done at the beginning and end of the season to note any differences resulting from their season of play.

Whitlow found that the more impacts a player had to the head, the more changes in a part of the brain called white matter, which is made up of insulated neurons that form the basis of communication between different parts of the brain. Such changes are concerning since the white matter of the brain is still developing and evolving during this age, and changes to its normal trajectory might have lasting effects on many aspects of brain function, from cognition to personality to behavior.

For now, it’s not clear what these changes may mean, or whether they have any impact on thinking or development. “There’s a lot we don’t know about the changes,” says Whitlock. “We don’t know if they persist. We don’t know if a couple weeks after the season ends, they go away.”