Contrary to popular belief, a concussion is not a bruise to the brain caused by hitting a hard surface. Indeed, no physical swelling or bleeding is usually seen on radiological scans. The injury generally occurs when the head either accelerates rapidly and then is stopped, or is spun rapidly.
This violent shaking causes the brain cells to become depolarized and fire all their neurotransmitters at once in an unhealthy cascade, flooding the brain with chemicals and deadening certain receptors linked to learning and memory. The results often include confusion, blurred vision, memory loss, nausea and, sometimes, unconsciousness.
Neurologists say once a person suffers a concussion, he is as much as four times more likely to sustain a second one. Moreover, after several concussions, it takes less of a blow to cause the injury and requires more time to recover.
Studies on Head Injuries
A 2000 study surveyed 1,090 former N.F.L. players and found more than 60 percent had suffered at least one concussion in their careers and 26 percent had had three or more. Those who had had concussions reported more problems with memory, concentration, speech impediments, headaches and other neurological problems than those who had not, the survey found.
A 2007 study conducted by the University of North Carolina's Center for the Study of Retired Athletes found that of the 595 retired N.F.L. players who recalled sustaining three or more concussions on the football field, 20.2 percent said they had been found to have depression. That is three times the rate of players who have not sustained concussions.
As scrutiny of brain injuries in football players has escalated in the past few years, with prominent professionals reporting cognitive problems and academic studies supporting a link more generally, the N.F.L. and its medical committee on concussions have steadfastly denied the existence of reliable data on the issue.
But in September 2009, a study commissioned by the N.F.L. reported that Alzheimer's disease or similar memory-related diseases appear to have been diagnosed in the league's former players vastly more often than in the national population — including a rate of 19 times the normal rate for men ages 30 through 49.
The study, which was conducted by the University of Michigan's Institute for Social Research, has not been peer-reviewed, but the findings fall into step with several recent independent studies regarding N.F.L. players and the effects of their occupational head injuries.
After the study results were reported, Representative John Conyers Jr., a Michigan Democrat and chairman of the House Judiciary Committee, held a hearing on the impact of head injuries sustained by N.F.L. players.
The league and Commissioner Roger Goodell had insisted that the N.F.L.'s policies were safe and that no third-party involvement was necessary, pointing to research by its committee on concussions as proof. But after an embarrassing hearing on the issue before the House Judiciary Committee in which the league was compared to the tobacco industry, the doctors leading the league's committee resigned.
In December 2009, the league announced that it would impose its most stringent rules to date on managing concussions, requiring players who exhibit any significant sign of concussion to be removed from a game or practice and be barred from returning the same day.
Several members of Congress portrayed Mr. Goodell and the league as impeding proper player care and obfuscating the long-term effects of concussions. The league and a former co-chairman of its committee on brain injuries, Dr. Ira Casson, have consistently played down studies and anecdotal evidence linking retired N.F.L....