Scientists don't know exactly when head injuries from contact sports cross the line into dangerous territory.

But a new study published in the journal Frontiers of Neurology shows that brain changes from contact sports — even games less violent than football, like basketball and soccer — start to appear earlier than most people think.

Even brains of college athletes show signs of changes.

"To us, the biggest surprise was that we observed any reliable effects of contact exposure on the brains of young athletes who are relatively early in their sport careers, and do not show any health problems," Nathan Churchill, the study's lead author and a postdoctoral fellow at St. Michael’s Neuroscience Research Program in Ontario, told Business Insider. "This seems to show that the brain is remarkably sensitive to the effects of bodily contact."

For the study, researchers conducted brain scans on 65 healthy university-level varsity athletes before their season started (to avoid any skewed effects from recent games). The group included 23 participants who played "collision" sports with intentional contact like football and hockey, 22 who played contact sports like soccer and basketball, and 20 who played non-contact sports like volleyball.

The more contact in the sport, the more brain changes researchers saw. Researchers noted changes in white matter and reduced communication between certain brain areas, especially in brain regions associated with vision and movement. Effects of older concussions could also be spotted in the brains, even though no one with a recent concussion was allowed to participate in the study.

The authors wrote that their results could help identify a sort of "neurobiological 'signature' of contact exposure."

It's not too surprising that young football and rugby players showed signs of brain changes. There's significant evidence that professional football players may develop the degenerative brain disease CTE due to the repeated hits they receive. The latest findings from one ongoing study of deceased former players' brains found that 110 out of 111 had developed the disease (though in most of those cases, they were in the study due to suspicions of CTE).

But it's noteworthy that brain changes were also apparent in athletes whose sports don't involve as many collisions.

College players who are still perfectly healthy show signs that their brains have been exposed to contact. Darren Abate/Getty Images

The recent study didn't find anything similar to the brain changes seen in former NFL players. To be clear, Churchill said that "we cannot definitively call our findings evidence of 'brain damage,'" since the typical markers of damage from brain injuries were not seen in the group studied.

The observed changes could even be a sign of a healing process. But they could also indicate that there's a point at which head contact makes long-term health problems more likely.

"It would be more accurate to say that we are seeing how the healthy brain responds to contact exposure, which includes possible changes in organization of white matter, and altered patterns of brain activity," he said.

The study implies, however, that brain changes that could later become significant start to appear at a young age. And it's an indication that some brain changes linked to contact occur even if kids aren't playing hard-hitting sports like football or lacrosse (though players of those sports showed more brain changes than players of lower contact sports).

Given the findings on professional football players and the long-term impacts of head injuries, many people have expressed concerns about letting their kids play contact sports. Even a number of NFL players have said they'd only let their kids play football if tackling and collisions could be limited.

Gaining a better picture of what a brain that has experienced collisions looks like could help researchers determine which types of long-term health outcomes result from different kinds of contact. And that could help parents of young or aspiring athletes make more informed decisions.