Non-concussion head impacts in sports that involve physical contact as normal play can lead to brain changes and result in lower test scores, a new study suggests.

Based on a recent study, repeated blows to the head during a season of contact sports such as football, hockey or boxing, may cause changes in the brain's white matter - tissue that contain nerve fibers -  and affect cognitive abilities even if none of the impacts resulted in a concussion.

Researchers at the Indiana University School of Medicine and the Geisel School of Medicine at Dartmouth College found significant differences in the brain white matter of varsity football and hockey players compared with a group of non-contact sport athletes following one season of competition.

"The contact sports and non-contact sports groups differed, and the number of times the contact sports participants were hit, and the magnitude of the hits they sustained, were correlated with changes in the white matter measures," Thomas W. McAllister, M.D., chair of Indiana University's Department of Psychiatry, said in a statement.

Researchers also found that head impact in contact sports may also affect an athlete's performance.

"In addition, there was a group of contact sports athletes who didn't do as well as predicted on tests of learning and memory at the end of the season, and we found that the amount of change in the white matter measures was greater in this group," McAllister said.

For the study, two groups of Dartmouth athletes were studied: 80 football and ice hockey players in the contact sports group, and 79 athletes drawn from such noncontact sports as track, crew and Nordic skiing.

Football and hockey players wore helmets equipped with accelerometers, which enabled researchers to compile the number and severity of impacts to their heads. Players who sustained a concussion during the season were not included in the analysis.

The athletes were then administered a form of MRI test known as diffusion tensor imaging, which is used to measure the brain's white matter. They were also given the California Verbal Learning Test II to measure their verbal learning and memory.

In a press release, researchers said they did not find large-scale, systematic differences in the brain scan measures at the end of the season, which they found "somewhat reassuring  and consistent with the fact that thousands of individuals have played contact sports for many years without developing progressive neurodegenerative disorders."

However, the results do suggest that some athletes may be more susceptible to repeated head impacts that do not involve concussions, although much more research would be necessary to determine how to identify those athletes.

"This study raises the question of whether we should look not only at concussions but also the number of times athletes receive blows to the head and the magnitude of those blows, whether or not they are diagnosed with a concussion," McAllister said.