PHILADELPHIA - A lead scientist studying the brains of deceased athletes with brain trauma has criticized the proposed NFL concussion settlement, because it would not compensate retirees who exhibit mood swings, aggression, depression or other aberrant behaviour.

Boston University researcher Robert Stern said that many of the 76 deceased NFL players found to have the brain decay known as CTE would not have qualified for awards had they lived. Some never developed the dementia, Alzheimer's and other neurological problems covered in the minimum $765 million settlement.

CTE, or chronic traumatic encephalopathy, can cause either cognitive or behavioural disorders, or both, according to Stern's research, which includes interviews with the families of 33 deceased athletes. The behavioural and mood problems can include violence, social isolation, drug overdoses and suicide, Stern wrote in a court affidavit this week.

"That's what started the whole discussion (of NFL concussions) and it's not compensated," Stern told The Associated Press in an interview Tuesday. "Repetitive hits to the head do not lead to Alzheimer's disease. They lead to CTE, if anything."

A federal judge in Philadelphia plans to weigh final objections to the settlement next month. However, the nearly 20,000 retired players eligible must decide whether to opt out by next week.

"I don't want to play an adversarial role," Stern said. "I want a settlement to go through, I just don't want a settlement to go through without people understanding what it truly means."

In his affidavit, Stern, of the Boston University Center for the Study of Traumatic Encephalopathy, noted that individuals with impaired mood and behaviour "can still experience devastating changes in their lives," even without significant cognitive impairment.

"(They include) ... the inability to maintain employment, homelessness, social isolation, domestic abuse, divorce, substance abuse, excessive gambling, poor financial decision-making, and death from accidental drug overdose or suicide," he wrote.

The NFL would pay $765 million under the plan to fund claims over the next 65 years, and more if needed. The individual awards would reach $1 million to $5 million for the most severe neurological problems, such as Lou Gehrig's disease or Parkinson's disease. The average award for moderate dementia or Alzheimer's disease is likely to be $190,000.

The settlement also provides up to $4 million for CTE-linked deaths through July 7, when Senior U.S. District Judge Anita Brody granted preliminary approval. However, they will not be covered in the future, in part out of concerns about financially-motivated suicides.

CTE can only be diagnosed after death at the moment, although Stern expects that to change within five to 10 years.

The proposed class-action settlement, which may cost the NFL an additional $112 million in plaintiffs' lawyer fees, followed more than a year of negotiations between the NFL and the lead players' lawyers.

Lead players' lawyers Christopher Seeger and Sol Weiss on Wednesday again called the deal "an extraordinary settlement for retired NFL players and their families — from those who suffer with neurocognitive illnesses today, to those who are currently healthy but fear they may develop symptoms decades into the future."

Only nine ex-players have formally opted out of the settlement, signalling they will pursue individual lawsuits against the NFL. Lawyers representing hundreds of others have raised various concerns about the deal — especially given the NFL's nearly $10 billion in annual revenues. Some have asked Brody to extend Tuesday's opt-out deadline.

Stern attached his concerns to an objection filed this week by seven former players represented by lawyer Stephen Molo.

"CTE is the industrial disease of football," Molo said, "and CTE goes effectively uncompensated in this entire sham of a deal."