A recent study led by neuroscientist Susan Tapert of the University of California, San Diego compared the brain scans of teens who drink heavily with the scans of teens who don't.
Tapert's team found damaged nerve tissue in the brains of the teens who drank. The researchers believe this damage affects attention span in boys, and girls' ability to comprehend and interpret visual information.
The more teens drank the worse it got. Check out the NPR story and read the comment section. It's got a lot of good information.