Quarterback Ben Roethlisberger, who was briefly hospitalized Sunday after sustaining a concussion, may be feeling fine when the Steelers play their first playoff game, but will he be thinking clearly?

Mr. Roethlisberger did not spend the night as originally planned and was up walking the halls at UPMC Presbyterian shortly after he was brought there by ambulance, according to team sources.

He was injured when his head bounced off the grass at Heinz Field when he was tackled by Cleveland linebackers Willie McGinest and D'Qwell Jackson late in the first half of the Steelers' 31-0 victory Sunday.

Mr. Roethlisberger is expected to play when the Steelers open the playoffs Jan. 11 at Heinz Field, although it remains in question whether he will practice this week.

A concussion is caused by a blow to the head or upper body that causes the brain to shake inside the skull. Symptoms may include disorientation, confusion, dizziness, amnesia and uncoordinated hand-eye movements.

Following a concussion, there is a change in brain function that can last anywhere from 24 hours to 10 days.

A frequent consequence of concussions like the one Mr. Roethlisberger suffered in the Cleveland game is sluggishness in thinking, which can persist after physical symptoms such as headaches have dissipated.

This effect was little known before UPMC's Sports Medicine Concussion Program developed a tool for measuring the degree of impairment, and how long it lasts.

ImPACT (Immediate Post-concussion Assessment and Cognitive Testing) is a 20- to 25-minute video game played on a computer that tests cognitive skills such as memory and reaction time. If the athlete has taken a baseline test, a post-concussion ImPACT test can determine the extent of impairment or recovery.

Mr. Roethlisberger will have taken such a baseline test, because the National Football League requires it, and because ImPACT was developed with the Steelers in mind. In the 1990s, two neuropsychologists then affiliated with the Henry Ford Health System in Michigan, Drs. Mark Lovell and Michael "Micky" Collins, worked with Dr. Joseph Maroon, team neurosurgeon for the Steelers, to develop ImPACT. Dr. Lovell has been the director and Dr. Collins the assistant director of UPMC's pioneering Sports Medicine Concussion Program since 2000.

Between 10 percent and 20 percent of all athletes -- high school, college and pro --who play contact sports suffer a concussion each season, UPMC says. Most are mild and many are undiagnosed.

Female athletes are more likely to suffer concussions than are male athletes who play the same sport. According to a 2007 study published in the Journal of Athletic Training, female soccer players in high school were 40 percent more likely than boys to suffer a concussion. Female basketball players had more than twice as many concussions as male basketball players did.

Part of the reason why, said Dr. Maroon, is because boys have bigger neck muscles. These balance the head during impact and reduce the chances of the brain being jolted during a collision.

Another explanation, said Kevin Guskiewicz, director of the Sports Medicine Research Laboratory at the University of North Carolina, may be related to why female athletes have more torn knee ligaments. They're more likely to land after a jump with their knees locked, he said. That may affect balance, which could increase the likelihood of falling and the head hitting the ground.

How long it takes to recover from a concussion depends on the individual. For some it can be only days. For others, months. But the effects on cognitive ability can be profound.

Matt Hetrick, then 17, suffered a concussion in Clarion-Limestone's first football game of the high school season in 2006. He fell from the 88th percentile in memory on his baseline test to the first (lowest) percentile after his concussion. His reaction time fell from the 22nd percentile to the first percentile.

As a high school freshman in Lancaster, Ohio, Christin Anson got kicked in the head in a soccer game. Afterward, the former honor student found she could read at only a third-grade level. It took a year of neurological therapy before she fully recovered.

The effects of concussions are cumulative and can take a long time to show up. In surveys in 2005 and 2007, the Center for the Study of Retired Athletes at the University of North Carolina found NFL players who had sustained three or more concussions during their playing days were three times more likely to have memory problems; five times more likely to develop Alzheimer's disease, and three times more likely to develop clinical depression.

The NFL has criticized that survey, but also has begun a plan to compensate former players who suffer from dementia and trauma-related impairment.