Listening on a cell phone, even with a headset and free hands, can make a driver as dangerous as a drunken one, a new study suggests.

Researchers have previously explored this territory, but Carnegie Mellon University scientists tried a new tack: they looked at the brain.

They used brain imaging to show that listening to a cell phone significantly reduces the brain activity that occurs during undistracted driving. This drop in brain function increases driving mistakes — such as weaving out of the lane or hitting a berm on the shoulder of the road.

Five states have enacted laws banning handheld cell phones while driving, but no state completely outlaws all types of cell phone use.

New Jersey toughened its ban effective March 1. It is now a primary offense, meaning drivers can be fined $100 for talking on a handheld cell phone even if they have committed no other violation.

Scores of studies have shown that performing a mental task such as carrying on a conversation impairs driving performance, but the new research is the first to look at what is going on in the brain.

"So how big a deal is this?" said lead author Marcel Just, a neuroscientist and director of Carnegie Mellon's Center for Cognitive Brain Imaging. "If you're on a cell phone, it's just an added risk. I certainly don't want to be crossing the road while you're driving and talking on a cell phone."