While the dangers of bone-jarring hits are obvious, even seemingly minor blows in sports like football, basketball and hockey can have an impact on an athlete’s brain.

In a new study, Indiana University researchers found differences in the brains of athletes who play contact sports and athletes who participate in non-contact sports, suggesting that a history of minor but repeated blows to the head can result in compensatory changes to the brain as it relates to eye movement function.

“The verdict is still out on the seriousness of subconcussions, but we’ve got to learn more since we’re seeing a real difference between people who participate in sports with higher risk for these impacts,” Nicholas Port, an associate professor in the IU School of Optometry, said in a statement. “It’s imperative to learn whether these impacts have an actual effect on cognitive function—as well as how much exposure is too much.”

In the study looked at 21 college-level football players and 19 cross-country runners were given a visual smooth pursuit task and a brain scan using fMRI technology.

The brains of 11 non-college-level athlete from similar socioeconomic backgrounds to the football players were also scanned  to ensure their scan results were not rooted in factors unrelated to their sport. 

The football players–most of whom were redshirted upperclassmen starters on the Indiana football team– did not have an extensive history of concussions.

During the study, eye-tracking data was analyzed using custom written software. Blinks were removed from the records and a spline interpolation was performed to recreate the eye movement missing during the blink.

 The researchers saw a difference in the regions of the brain responsible for visual processing between the two sets of athletes; the regions were much more active in football players than any of the other participants in the study.

“We focused on these brain regions because physicians and trainers regularly encounter large deficits in players’ ability to smoothly track a moving point with their eyes after suffering an acute concussion,” Port said. “Everyone from musicians to taxi drivers has differences in brain activity related to their specific skills.

“The differences in this study may reflect a lifetime exposure of subconcussive blows to the head, or they could simply be the result of playing a visually demanding sport where you’re constantly using your hands and tracking the ball,” he added.

According to the study, estimates for sub-concussive impact exposure in football players at the high school and collegiate level range from 244 to 1444 per season.

Port said the ideal way to find the root cause of the brain differences would be to do a similar analysis with only football players.

According to the study, if repetitive sub-concussive impacts have a deleterious effect on neural processing, college football players should show performance decrements on the smooth pursuit task and differences in brain activation when compared to both control groups.

Port also said that in the future wearable accelerometers could measure the physical impact during play to enhance the ability to confidently sort players of the same sport into groups based on exposure to subconcussions.

The study was published in NeuroImage: Clinical.