Scans have revealed "profound abnormalities" in the brain activity of retired American football players, a study into the long-term risks of the combative sport said on Thursday.
Unusual activity in the frontal lobe, observed in former National Football League (NFL) players as they carried out a cognitive test, matched records for heavy blows they had received to the head while on the field.
"The NFL alumni showed some of the most pronounced abnormalities in brain activity that I have ever seen," said lead author Adam Hampshire, a neuroscientist at Imperial College London.
"(The) level of brain abnormality correlates strongly with the measure of head impacts of great enough severity to warrant being taken out of play. It is highly likely that damage caused by blows to the head accumulate towards an executive impairment in later life."
NFL games have come under growing scrutiny for what critics say is a dangerous rate of concussions after hard blows to the head.
Some have drawn links between the onfield physical traumas and later neurological problems such as Alzheimer's or Parkinson's, which in turn have been blamed for depression and suicide.
The new study does not find evidence of disease, but highlights brain areas that may have been affected by repeated, severe impacts.
And it says standard tests do not pick up this subtle damage.
The experiment entailed asking 13 former NFL players and a comparison group of 60 volunteers to rearrange coloured balls in a series of tubes in as few steps as possible.
The test, called a spatial planning task, is a routine assessment of brain function.
For the first time, researchers scanned the participants' brain activity with magnetic resonance imaging (MRI) as they performed the test.
None of the 13 players had been diagnosed with a neurological condition, although all felt they were suffering neurological problems that affected their everyday lives.
The NFL group performed worse at the task than the volunteers, but only marginally so, said the scientists.
The MRI scans, though, showed huge differences in activity in the frontal lobe of the brain -- the part that is responsible for "executive," or higher-order functions.
The scans revealed "hyperactivation" in parts of the frontal lobe among the NFL players, which led the researchers conclude that the damaged brain was having to work harder, bringing extra areas online in order to process the task.
"The differences seen in this study reflect deficits in the executive function that might affect the person's ability to plan and organise their everyday lives," Imperial College said in a statement.
The findings show the usefulness of MRI in revealing neurological problems that are missed by standard tests, said the scientists.
"Brain imaging tests could be useful to retired players who are negotiating compensation," they said.
"Players could also be scanned each season to detect problems early."
The study, published in the journal Scientific Reports, quoted US Centers for Disease Control and Prevention (CDC) data that between 1.6 million and 3.8 million sports concussions occur annually in the United States alone.
It also cited an NFL-commissioned report showing that retired players between the ages of 30 and 49 are 20 times more likely to be diagnosed with Alzheimer's disease, dementia or another memory-related impairment than the general population.
In August, the NFL reached a settlement worth $765 million (560 million euros) in a lawsuit filed by more than 4,500 former players over concussion injuries, without admitting any liability or deficiency.