While those reports were anecdotal and admittedly involved the heavy, old leather ball that went out of fashion many years ago, there was enough circumstantial evidence — compiled with the help of a leading neuropathologist and the then-medical officer of England’s Football Association — to inspire a later study led by Norwegian medical researchers.

Over time, the ball, the equipment and the professionalism shown by players have all advanced. But so has the speed of the game. So while the ball itself might be lighter and made of materials that do not soak up weight during play, players rarely are given a yard of space today to head the ball correctly. Instead, they are crowded and jostled so they cannot make clean or deliberate contact. That, in the modern game, would be deemed a dereliction of duty by the defenders, who are instructed to deny both the time and the space need to attempt what is called a “clean” header.

So FIFA’s research study will be complicated indeed. But it dare not be complacent with the findings.

Pereira graphically described what happened to him at the World Cup as “the lights going out” after he was concussed when his head collided with the knee of England’s Raheem Sterling.

That followed other televised incidents of head trauma, such as last November, when Tottenham Hotspur goalkeeper Hugo Lloris was knocked out during a Premier League match. Lloris and his coach, André Villas-Boas, overruled the club doctor, who advised that the keeper be taken out of the game and sent to a hospital for observation. Lloris played on.

As a consequence of that and other incidents, the English F.A. and the Premier League have ruled that from now on a third doctor, independent of the two teams, should be at games to make the final on-field assessment as to whether a player can continue.

Rugby, which unlike the N.F.L. is played without helmets and padding, has declared this summer that there will be “a zero tolerance approach to gambling with the welfare of players” regarding head injuries.”