NFL team helmets are displayed at the NFL Headquarters in New York December 3, 2015.
Reuters/Brendan McDermid

Two decades after the NFL brushed off concussion concerns as being of interest only to journalists, the issue is at the forefront of any discussion about player safety and unlikely to go away anytime soon.

From the 2015 film "Concussion" about a doctor who fought the NFL's campaign to conceal his research on the brain damage suffered by players, to a lawsuit over brain injuries that could cost America's most popular sports league $1 billion, it is an issue that has the league playing defense.

And while some experts say there has been a sea change in the league's attitude over the past few years, concussions still occur with alarming regularity.

The NFL said there were 111 concussions reported in the 2014 season, down from 148 during the 2013 campaign, but that pattern did not continue in the 2015 season, according to NFL Players Association (NFLPA) medical director Dr Thom Mayer.

"It's safe to assume concussions are up on 2013 and 2014," Mayer, who could not reveal the actual number, told Reuters while adding that the higher number did not necessarily mean there were more concussions but rather better expertise at diagnosing the condition.

Under the NFL's concussion protocol started in 2013, each team is assigned an independent neurotrauma specialist who is not on the team's payroll. There is also a "spotter" who can stop a game if they see a player showing concussion-like symptoms.

But critics argue that the specialist can only advise a team to remove a player with suspected concussion from the game while the final decision still lies with the club doctor, potentially a conflict of interest.

There were several incidents this season when the system failed, most notably last November when St. Louis quarterback Case Keenum remained in a game even after struggling to get up and appearing woozy after his head slammed to the turf.

Another notable incident occurred when Pittsburgh Steelers quarterback Ben Roethlisberger took himself out of a game after a hard fall, even though nobody seemed to notice he was hurt.

"We had a conversation directly with the club in both those cases ... and discussed them with the league," said Mayer.

Despite these obvious issues, Mayer does not mind that the final decision on whether to remove a player resides with the team.

"It's designed to be a collaborative decision. Differences of opinion are unusual and rare."

The difficultly in diagnosing concussions has long been a bane of neurologists, but change could be on the way with an eye tracking machine that Manhattan-based neurodiagnostics company Oculogica says objectively measures brain injury.

The device tracks eye movement simply by having the subject watch a video for a few seconds. Normally, eyes move together but when a brain is injured, they dont.

"It's not possible to game the system or the medical provider," Dr Robert Glatter, a former sideline physician for the New York Jets, told Reuters. "It's ideal for evaluation on the sidelines as well as in the locker room."

Oculogica co-founder Dr Uzma Samadani, who with Glatter co-authored "The Football Decision" about the dilemma parents face when deciding whether to enroll their children in contact sports, is confident the NFL will begin using the device soon.

"Once we get FDA clearance -- I would hope within a year -- it will be obvious something will have to be used, because eye tracking is objective, and it is impossible to fake that your eye movements are coordinated when they are not," Samadani, a neurotrauma consultant for the NFL who is not authorized to speak on behalf of the league, told Reuters.

"It could potentially eliminate any human-error component to concussion diagnosis."

Mayer is interested in the Oculogica device but says more testing is needed before the NFLPA recommends its adoption by the league.

As for rule changes designed to reduce helmet-to-helmet hits, Mayer said preliminary data shows they been successful, but warned of the law of unintended consequences as other areas of the body, especially the knees, could become target areas.

"Avoiding helmet-to-helmet contact is maybe causing more (knee) injuries," he said.

(Reporting by Andrew Both in Cary, North Carolina; Editing by Frank Pingue)