One thing that makes the beautiful game beautiful is the way the ball moves so effortlessly from foot to head to chest and back. But those skilled and deliberate headers may be ever so slightly but very definitely decreasing the player's cognitive skills.

Even repeated light blows to the head can harm cognition, according to a study by neuroscientist Anne Sereno and her team at the University of Texas Health Science Center at Houston. They don't know if the impact is permanent, or if there might be cumulative effects over time, and concede more research is needed. But the findings add to a growing body of knowledge about the long-term risks of head trauma in sports and evidence suggesting repeated sub-concussive blows may be detrimental over time.

"Even these sort of relatively non-violent, or thought of as so, traumas are causing cognitive changes," said Sereno. "I don't want to over interpret it, but it definitely shows that these blows are affecting cognitive performance."

Recent focus on concussions in sports left Sereno wondering what effect sub-concussive head trauma, as might be experienced during a header, could have on cognition. Her previous research into higher cognitive functions like attention, short-term memory and impulse control used sophisticated eye-tracking equipment, which helps identify exactly what parts of the brain are involved but cost $50,000 to $100,000 and must be used in a lab. Sereno developed an iPad app to test players on the field.

The test Sereno administered featured two parts. The first (A) is a pro-point task where a white arrow indicates the direction of a correct response (toward the target square). The second (B) is an anti-point task where the white arrow indicates the direction of a correct response (away from the target square).

The scientists enlisted a high school women's soccer coach and 12 players between the age of 15 and 18. The study was observational so as not to interfere with normal play. The number of headers wasn't controlled, which Sereno said helped the study because it more accurately represented what typically occurs in practices and games. The control group included 12 students of the same age who, if they were athletes, played only non-contact sports.

The iPad program consists of a simple two-part test administered immediately following practices. In it, a spot appears at the center of the tablet. During the first part of the test, the subjects were directed to touch the spot, which would cause a box to appear on the screen. They were to move their finger from the spot to the box as quickly as possible. In the second part of the test, the target appeared at random, and the subject was to touch its opposite as quickly as possible.

"We found there was no difference in pointing to a target immediately after a practice," said Sereno, "but that there were subtle changes in reaction time when they had to use these cognitive functions that lie in the frontal lobe, which is most sensitive to traumatic brain injury."

The team also recorded how many times each player headed the ball, how many hours they spent playing each week and how many years they've played. Although it was a relatively simple test and not nearly so elaborate as standard neuro-psych tests, there are fewer variables involved, making it easier for researchers to spot subtle differences in cognition.

The researchers found that a relatively small number of headers – two to 20, on average, with a median of six – slowed reaction time by 30 to 50 milliseconds. That's too small to see with the naked eye, but in this instance is a significant change.

"The results show that soccer playing in which participants headed the ball did indeed disrupt voluntary performance in female high school soccer players tested immediately following practice," the researchers note in their conclusions. "These findings demonstrate significant and specific cognitive changes in female high school soccer players who head the soccer ball during practice."

Sereno must still explore what happens over time. She didn't conduct any tests to determine whether reaction time reverts to normal, and if so, how long that might take. Although the long-term effect remains to be seen, the data did reveal a direct correlation between increased response time and the number of headers performed, time spent playing each week and how many years the subject had been playing. That suggests a cumulative effect over time, though Sereno concedes further study is needed to confirm that.

"That said," said Sereno, "I'm a neuroscientist, I would not use my head to hit something, so I am biased. But we have to better understand blows to the head, and what role repeated blows have, and what we can intervene with to potentially do something to slow down these post-traumatic brain injuries."