There is growing evidence that repeated concussions to rugby league players have a long-term effect, says neuroscientist Dr Alan Pearce.

Key points: The study found former players had worse memories and reaction times

The study found former players had worse memories and reaction times A 2015 rule change said players suspected of having a concussion had to undergo formal head injury assessment

A 2015 rule change said players suspected of having a concussion had to undergo formal head injury assessment Over the weekend two players were allowed to play without proper checks after getting knocked down

Dr Pearce, from La Trobe University, recently completed a study comparing 25 former NRL players now aged in their fifties to 25 men of a similar age.

The men carried out cognitive tests to measure their memory and attention spans, as well as motor tests for things like reaction time and fine dexterity.

"What I've found is that the responses of retired rugby league players was significantly different to the healthy controls with no history of head injury," Dr Pearce said.

"That also correlated to the tests of cognition and motor reaction times, that were significantly worse."

Dr Pearce hopes the results could be used to start a rehabilitation program, using neuroplasticity to try and help players recover damaged brain functions.

Basically they would help participants' brains to become "rewired", and improve their memory and motor functions.

"We have seen that in other areas, such as stroke and Parkinson's, where you do exercise therapy, we do get some improvement," Dr Pearce said.

Dr Pearce will be presenting his new study next week in the US at the 12th World Congress of Brain Injury.

The study is timely, given a lawsuit lodged last month by ex-NRL player James McManus suing his former club Newcastle over its handling of his concussions.

Brendan Elliott, Josh Dugan play on despite knock

Over the weekend Newcastle Knights fullback Brendan Elliot was allowed to keep playing after a serious head knock in the first half against Souths.

The first hit to Elliot came about 25 minutes into the game and despite the fact that he lay on the ground for several minutes, he was not given a head injury assessment and was allowed to play on.

Twenty minutes into the second half he went down again and was taken off the field.

Dr Pearce said a formal assessment should have taken place.

"I was pretty surprised that they would allow him to stay on after the first impact," he said.

"Because he was obviously concussed, because when he fell you could actually see that he wasn't trying to guard himself.

"He already obviously had effects of concussion before he hit the ground."

The NRL is also examining a second incident from the weekend that involved Dragons player Josh Dugan, who went down after being elbowed in the jaw by a teammate.

After the game, Dragons coach Paul McGregor defended the decision to keep Dugan playing, saying "it was his jaw, not a head injury."

Dr Pearce said that explanation defied logic.

"When I did my anatomy the jaw was part of the head," he said.

"We know that with any sort of repetitive head injury you don't have to have a direct hit to the head to have effects of concussion.

"Even indirect hits can very violently move the brain, can affect the neurons or the cells of the brain."

Medical staff 'doing the best job they can'

NRL rules around concussion were changed in 2015, and state that players must undergo a formal head injury assessment if they are suspected of having a concussion.

But that did not appear to happen with either Dugan or Elliot.

However, Dr Pearce said that for the most part the NRL was doing a good job and there were "plenty of examples where the policy has been put in place."

"Obviously at the microscope of the community and the media looking at what happens every time, and if one gets missed it is highlighted very clearly," he said.

"The medical staff are doing the best job they can, and sometimes what happens is that they're distracted looking at other players, trying to multi-task.

"And so I think that it's possible that one or two will always slip through the net."

The ABC contacted the NRL for comment but no one was available for an interview.

In a statement it said "The NRL reviews all concussions and head injury assessments (HIA's)."

"The NRL says it's looking at a number of incidents from the weekend including the cases of Josh Dugan and Brendan Elliott."