Scientists have found "profound abnormalities" in scans of brain activity in a group of retired American football players, adding to evidence indicating that repeated blows to the head can trigger longer-term aggression and dementia.
Although the former National Football League (NFL) players in the study were not diagnosed with any neurological conditions, brain-imaging tests showed unusual activity that correlated with the number of times they had left the field with a head injury during their football careers.
Adam Hampshire at Imperial College London, who led the study, said the ex-NFL players showed "some of the most pronounced abnormalities in brain activity" he had ever seen.
"And I have processed a lot of patient data sets in the past," he said in a statement about the research.
A growing body of scientific research shows that repeated knocks to the head can lead to a condition known as chronic traumatic encephalopathy, which can lead to loss of decision making control, aggression and dementia.
Previous research has also found that former American football players have higher rates of neurodegenerative diseases such as Alzheimer's and Parkinson's disease.
These latest findings, published in the Nature journal Scientific Reports, suggest players also face a risk of subtle neurological deficits that would not necessarily show up on normal clinical tests.
John Hardy at University College London's Institute of Neurology, who was not involved in this study, said the findings showed what he and others have long suspected - that common dysfunctional behavior among former sportsmen such as American footballers, ice hockey enforcers and boxers is related to sub clinical brain injuries sustained during their careers.
The NFL agreed in August to pay $765 million to settle a lawsuit brought by thousands of former players, many suffering from dementia and health problems. They accused the league of hiding the dangers of brain injury while profiting from the sport's violence.
Hampshire's study involved 13 former NFL professionals who felt they were suffering brain problems affecting everyday life.
While their brain activity was measured using functional magnetic resonance imaging (fMRI), the former players and 60 healthy volunteers were given a test that involved rearranging colored balls in a series of tubes in as few steps as possible.
The NFL group performed worse on the test than the healthy volunteers, but the difference was modest, according to Hampshire and colleagues at the University of Western Ontario, Canada, who collaborated on the study.
More strikingly, however, the scans showed unusual patterns of brain activity in the frontal lobe, the region responsible for so-called executive functions such as decision-making that might affect someone's ability to plan and organize daily life.
The difference between the two groups was so marked that a computer program learned to distinguish between ex-football players and healthy volunteers at close to 90 percent accuracy, based just on their frontal lobe activation patterns.
Hampshire said the critical finding was that levels of brain abnormality seen in the ex-players correlated strongly with the number of head impacts they had suffered that were severe enough to warrant them being taken out of play.
"This means that it is highly likely that damage caused by blows to the head accumulate towards an executive impairment in later life," he said.