(USA TODAY) -- Wayne Clark, an NFL backup quarterback in the 1970s, says he had one concussion in college and two in the pros. He says the most severe one was in 1972 when he was holding on a field goal try for the San Diego Chargers. The kick was blocked. He was hit.

"I really didn't, quote, wake up until the plane trip home. Somewhere over New Mexico or Arizona I finally started regaining awareness of where I was and what was going on," says Clark, 65.

A study released Tuesday said a new testing method showed five living former NFL players, including Clark, had signs of CTE -- a concussion-related brain disease linked to depression and dementia that has only been identifiable after death.

The other four unnamed players in the study have conditions ranging from mild cognitive impairment to depression and dementia. Clark said he was doing fine. One question about CTE is why some players with similar concussion histories show symptoms of CTE and others do not.

The investigators say more research is needed on their method of using a low-dose radioactive compound developed at UCLA to mark evidence of CTE -- the accumulation of tau protein in brain cells. But it might be a significant step toward detecting, treating and preventing CTE.

"We had to be able to discover it in living people. To me, that is the Holy Grail of CTE," said Julian Bailes, Chicago neurosurgeon and co-author of the report.

The study is based on research at UCLA headed by Gary Small, professor of psychiatry and biobehavioral sciences. Bailes and Small collaborated in launching the study.

"We've got to do more studies," said Small, lead author. "What we're hoping is that we'll be able to test new treatments and have better ways of diagnosing problems earlier. We're looking toward preventive treatments to protect them rather than repair the damage."

Last May, Pro Football Hall of Famer Junior Seau committed suicide. This month, the National Institutes of Health said Seau's brain showed CTE (chronic traumatic encephalopathy).

"One of the problems with CTE cases is that some of them end in suicide. The suicides are often precipitous, without warning," Bailes said. "The hope would be if you could identify them while they are in the early states that they could be treated."

Clark is one of more than 4,000 former players suing the NFL on accusations it failed to protect players from long-term effects of concussions. He said it was a privilege to participate in the study. "This, I hope, will advance everybody's understanding of what's going on," he said.

The researchers used a marking compound developed at UCLA for Alzheimer's research. A PET scan was used to spot tau protein in the brains of the five players.

"What was really striking in this study was that the areas of the brain that showed the deposits were identical to that seen in the autopsy studies of patients with CTE," Small said.

Pending further research, Bailes said, widespread screening of players could be merited.

What if an active player was diagnosed? "You could see someone is accumulating tau in their brain to the extent that you think maybe they ought to end their career," Bailes said. "It's going to conjure up a lot of questions. And certainly I don't have the answer for all those."

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