The boys' responses were measured in two ways, the first being skin conductance responses, basically a measure of sweat levels. "We know that when people are shown an emotionally provocative picture or video they have exaggerated sweat responses," Grafman explained.
The participants' brains were also scanned to measure blood flow in different areas, the idea being the more blood flow, the more active that part of the brain.
"They became desensitized over time to these mild and moderate aggressive scenes so they, in essence, had much less sweat over time," Grafman noted.
And brain activity in the orbital frontal cortex, the part of the brain involved in modulating aggression, decreased.
"Based on prior studies, we know that the size of those brain regions vary depending on their exposure, which means you're shaping a very plastic brain in adolescents for their future," Grafman said. "There's a likelihood that you're shaping their behavior in the real world as well."
"There's a debate going on as to whether or not violent video games are cathartic or inure teenagers to violence," said Alan Hilfer, director of psychology at Maimonides Medical Center in New York City. "It's generally the understanding with most of us that if kids are playing them excessively, they do tend to make a certain kind of a kid -- a more fragile, more vulnerable kid -- less sensitive to violence. We all have a bit of a concern as to the extent that teenagers play video games and are engaged in watching violent acts . . . This study reinforces that concern."
Craig S. Fabrikant, of the department of psychiatry at Hackensack University Medical Center in New Jersey, recommends that parents "monitor what your kids are doing and watch. Be aware."
There's more on kids and media at the American Academy of Pediatrics.
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