When it comes to video games, Dr. Vincent Mathews' teenage sons keep him honest.

For the last few years, he has been studying the effects of violent video games on the brains of the people who play them. His work tends to give cheer to critics of violent games. Take last week, for example, when Indiana University's School of Medicine — where Mathews is a professor of radiology — announced the results of the doctor's two-year study. Mathews' team detected differing brain activity between players who played a violent game and a nonviolent game.

The proof that he was onto something is that his kids didn't raise a fuss. Typical teens, they have played enough "Halo" and other violent games at parties to give dad grief for picking on games. "My oldest one who is a 19-year-old always ... accurately points out the shortcomings of the studies," Mathews said Monday. "He's a bright kid. As we've moved along and refined our process and done better and better experiments, it becomes harder to argue that [violence in games] doesn't really mean anything. [My kids] are sort of coming around to, 'Maybe there is something to this.' "

"This" is an idea with which many avid gamers are ill at ease: that violent games mess with your brain. Such reports make for common headlines and can stiffen rights aimed at knocking violent games out of the reach of young gamers.

Mathews' Indiana University study started two years ago with a pool of 44 teens age 13 to 17, a mix of guys and girls of varying levels of gaming expertise. The IU group picked two games, the World War II first-person shooter "Medal of Honor: Frontline" and "Need for Speed Underground" — both are from Electronic Arts but designed to be easy for players to jump into and play. After ensuring that the kids agreed that the "MoH" game was violent and that "NFS" was not, the subjects were each assigned one of those games.

The kids and their parents were interviewed by the IU team. The subjects were given 40 minutes to familiarize themselves with their game and brought back a couple of days later for a 30-minute session with their game. For "Need for Speed" their directions were "to drive as fast as they can and go as far as they can," Mathews said, noting that players can't run over pedestrians in the game. For "Medal of Honor," "they're basically playing the game to shoot and advance and kill as many people as they can over the course of the time they're playing."

The teens were then given a series of MRI tests that detect which parts of the brain are most active. The teens were given the Stroop Tests, in which patients are asked to pay attention to one thing while ignoring another — like naming the color of a word and not what the word says (even if it's the word "green" written in blue type, for example).

Mathews' team found that the type of game each teen played didn't affect their performance on the tests. But the type of game affected where the blood was flowing. As the IU press release stated: "Adolescents who had played violent video games exhibited more brain activity in a region thought to be important for emotional arousal and less activity in a brain region associated with executive functions. Executive functions are the ability to plan, shift, control and direct one's thoughts and behavior."

What Mathews' study hasn't determined is how long-term these effects are. None of the teens were tested more than 90 minutes after their half-hour session playing the game. That's what the doctor wants to check next. He also wants to find out if the effects can be undone with other types of games or from even just taking a break from playing.

Meanwhile, Mathews' study will provide more fodder for national debate. For the last two years, U.S. politicians have tried to get laws on the books that would ban the sale of explicit, violent games to minors, and almost all of those laws have been struck down by courts. The courts have cited a lack of reliable scientific evidence demonstrating that games cause any greater physical harm than other forms of entertainment protected by the First Amendment.

What's clearly needed, at minimum, for such laws to pass are studies that show that games affect brains in ways that movies, music and books do not. That kind of study isn't in Mathews' plans, but he hopes people will perform such tests to see if interactivity really does affect people in special ways.

When the study was announced last week, some gaming bloggers jumped all over the source of Mathews' funding. In the past — and now with this current $650,000 study — backing has been provided by the Center for Successful Parenting, whose Web site advises parents to make sure their children are not "gamewashed" by violent games. But Mathews denies that such a funding source could taint his findings. "I can see how people would jump on that point," he said. "I'm happy to take money from the video game industry and do the same experiments."

Despite what headlines about Mathews' study have indicated, they haven't decided that violent games inspire actual violent behavior, that they are worse than violent movies or that they affect brains long-term. Mathews' team just figured out that a violent game does something to your head that a nonviolent one doesn't. He's making two believers in his household. Who's next?

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