A yearlong study of boxers’ and mixed martial-arts fighters’ brain activity has found those who fight for more than six years begin to experience damage and those who fight longer than 12 years expose themselves to an even greater decline each time they return to the ring.

“What we’ve found suggests changes and damage in the brain happens years before symptoms emerge,” said Dr. Charles Bernick, author of the study. “It’s what we see in Alzheimer’s and Parkinson’s patients.”

Bernick has supervised MRIs and computerized and cognitive tests of an estimated 170 fighters at the Cleveland Clinic’s Las Vegas center in the past year.

“If we’re going to protect these athletes, we need to follow them earlier in their course,” Bernick said.

Bernick said the study found fighters begin losing brain volume — as brain cells die — after six years of fighting.

“We still need to follow these guys over time,” Bernick said. “If someone’s having damage, a commission might want to limit the number of fights he has. There is a hierarchy of protective things, interventions to help. This is still being sorted out.

“We are looking at this ‘threshold effect.’ How much punishment can a brain take? What are the markers we can see for long-term problems?”

The study received a $12-million boost in funding earlier this year from a Las Vegas dinner and auction. One of the highlights of the night was Ultimate Fighting Championship Chairman Lorenzo Fertitta outbidding Dallas Cowboys owner Jerry Jones, spending $1.1 million for a pair of autographed gloves belonging to Muhammad Ali.