California, Connecticut, New Jersey, New York, Washington, the District of Columbia and the Virgin Islands prohibit drivers from using handheld cellphones, but no jurisdiction bans hands-free phones, says Jonathan Adkins, spokesman for the Governors Highway Safety Association, which represents state and territorial highway safety offices.

Allowing hands-free phones "really gives drivers a false sense of safety," Adkins says. He adds that he has seen no evidence that bans on handheld phones have prevented accidents.

Neuroscientist Marcel Just, director of the Center for Cognitive Brain Imaging at Carnegie Mellon University in Pittsburgh, agrees. Just studied 29 volunteers who used a driving simulator while inside an MRI brain scanner. The volunteers steered a car along a virtual winding road undisturbed or while deciding whether a sentence they heard was true or false.

Listening while driving led to a "significant deterioration in driving accuracy," Just and his co-authors write in the latest issue of the journal Brain Research. The drivers hit the guardrail and veered out of the center of the lane more often while listening.

In the listening situation, MRI brain scans found a 37% decrease in parietal lobe activity. The parietal lobe is associated with spatial processing, so it is critical for navigation. Activity also decreased in the occipital lobe, which processes visual information.

"Certain activities in life are inherently multitasking, but driving and cellphone use isn't something Mother Nature thought about when she was designing our brains," Just says.

But banning cellphones outright is "too draconian," Just believes. "I could imagine banning them during rush hour, maybe during inclement weather."

Besides, say Just and Joy Hendrick, who has found that college-age drivers don't brake as quickly when talking on either a handheld or a hands-free phone, it's unlikely that busy lawmakers would support a ban.

For now, the researchers say, they would just like to raise awareness of the problem. Hendrick, a kinesiology professor at State University of New York-Cortland, says drivers need to ask themselves: "Do I need to make this call?" If the answer is yes, she says, then they should think about pulling over or at least keeping the call as brief as possible.

Enlarge Carnegie Mellon University Images show parietal lobe activity while driving undisturbed, left, and while listening to sentences, when it dropped 37%.