"I ended up going to the hospital with a Grade II concussion," he said. "I was in la-la land."

The Giants didn't track the number of shots Matheny took to the head before he went on the disabled list, nor can they be sure which were most significant. The club is in the process of examining video of every pitch he caught over the last three years to determine how many foul tips a catcher typically takes over the course of a season.

The last straw for Matheny came when, during a three-game series with Florida in late May, the four-time Gold Glove winner was hit in the head several times by foul balls.

After taking two foul tips to the head in the series opener, he sat out the middle game against the Marlins. He saw a doctor, but passed the neurological testing and was back in the lineup the next day. In that game, a ball hit him square on the chin so hard that it traveled back toward the pitcher. That's when trainer Stan Conte pulled him from the game.

Matheny, who turns 36 next month, remembers those, as well as a few at Colorado in April. An umpire contacted him to tell the catcher he recalled several foul tips Matheny took during a series at Oakland.

After going on the DL, Matheny became frustrated because he was incapable of focusing on his most basic daily duties, such as sitting down with pitchers to study opposing hitters.

Eventually, he left the team so he could recover away from the sport's pressures. He now spends most of his time back home in Chesterfield, Mo.

"Some days," he said, "doing absolutely nothing at home, I feel good."

But when Matheny exerts himself through physical activity, even minor household chores, the chemicals in his brain can become stirred up again.

Despite their frequency, it can be difficult to determine the severity of concussions. CT scans and MRIs can reveal bleeding but fail to show how a brain is functioning.

"We've learned more about concussions in the past five years than the previous 50," said Collins, of the University of Pittsburgh concussion center. "It wasn't long ago that it was smelling salts and 'How many fingers am I holding up?"'

Collins' concussion center developed a computerized test called "ImPACT" that puts the brain to work and derives data about points of trauma — measuring attention, memory, processing speed and reaction time.

"How can you manage something if you can't measure it?" Collins said. "It's one of those things where this type of evaluation is important."