So far, the long-term effects of repeated head injuries in professional football have been hard to pin down. There are a whole host of economic and social reasons for this, but the simple truth is that the scientific research itself just isn’t clear-cut. While some studies have found decreased cognitive function and an increased risk of Alzheimer’s disease in NFL players, others haven’t found strong evidence linking careers in professional football to long-lasting cognitive decline.

The problem, researchers argue, may be “ecological validity.” In other words, the skills and competencies that scientists test may not accurately represent those required in real life. The psychological tests used in laboratory settings are highly controlled and standardized, while real-world situations are much messier. Players with brain injuries might retain enough cognitive control mechanisms to pass lab-based tests yet fail miserably at the complex scenarios they face in everyday life.

A new study in Scientific Reports clears up some of this confusion, using a combination of cognitive tests and neuroimaging to get a better picture of what happens to football players’ brains as a result of repetitive traumatic brain injuries (TBIs).

The study included 13 retired NFL players with no history of psychological problems (two other ex-players also volunteered for the study, but had to be nixed since they couldn’t fit in the fMRI scanner). During their careers, each of these players had been pulled from a game at least once due to a head injury. From a control group of 60 college graduates, 20 were chosen as an age-matched control for these experiments.

First, each participant in the study performed a spatial planning task in which they were shown two different arrangements of colored balls in tubes; their goal was to figure out how many moves it would take to turn the first arrangement into the second one. Essentially, this task tests a person’s working memory and their ability to plan ahead.

As in many previous lab-based tests, the NFL group didn’t perform significantly worse than the control group did. Only in the very hardest problems—those that required four moves—was there a small decrease in the football players’ performance compared to the other participants. But overall, only two out of the 13 ex-NFL players ranked at or below the bottom tenth percentile compared to controls. Clearly, a career in football didn’t necessarily result in failure—or even underperformance—on this task.

However, when the researchers examined fMRI data showing what happened in the participants’ brains as they worked out the solution, they saw stark differences between the two groups. Compared to the control group, the ex-NFL players had decreased functional connectivity between various brain regions. In other words, different parts of the football players’ brains didn’t interact as much as they did in the other participants’ brains. This effect was particularly strong in the frontal lobe, a region involved in planning, organization, and attention. In a measure of frontal lobe connectivity, all but one of the retired players were in the bottom tenth percentile compared to the control group; that’s a much larger effect than what the researchers saw when they simply looked at performance on the task.

So why was there such a discrepancy? How could the players perform decently on the test despite the reduced connectivity?

It turns out that there was another major difference in how the two groups’ brains functioned. Two parts of the football players’ brains were surprisingly active: the dorsolateral prefrontal cortex and the frontopolar cortex, two areas in the frontal lobe. And the harder the problems got, the more active these regions became. In terms of overall activation in the dorsolateral prefrontal cortex, more than half of the players were in the top tenth percentile compared to the control group.

High activity in these areas suggests that the retired football players were devoting extra brain resources to solving the problems, possibly to overcome the effect of decreased connectivity. The control participants, on the other hand, didn’t need to compensate in this way.

Perhaps the most disturbing evidence in this study was the link between the frequency of head injuries and the extent of the player’s brain changes. As a player’s number of career TBIs went up, brain connectivity fell precipitously, and frontal lobe activity increased sharply.

Up to a certain point, people with these sorts of injuries may be able to overcome brain damage and function relatively normally by allowing regions of the frontal lobe to go into overdrive. But as tasks get increasingly complex—or the brain suffers enough damage—this compensation might not be enough to mask cognitive problems.

There are definite drawbacks to the study; the sample size is small, and we’re talking correlation, not causation. Additionally, the researchers didn’t compare activity in TBI-riddled brains with those of NFL players without repeated head injuries. But, as the study says, “NFL alumni are a particularly difficult population to recruit for research studies.”

Although this study doesn’t close the book on the relationship between TBIs, brain damage, and cognition, it suggests a very plausible explanation for the inconsistencies and conflicts in research on football players’ cognition. In the long run, this type of analysis could potentially serve as an early warning sign for players that are on the road to serious cognitive problems. These applications likely extend well beyond the NFL, as well. Soccer players, military veterans, and Parkinson’s patients could also benefit from research that helps us understand how brain abnormalities affect the way we think, act, and perform.

Scientific Reports, 2013. DOI: 10.1038/srep02972 (About DOIs).