PITTSBURGH, March 5 -- Using a cellphone while driving -- even a hands-free model -- sharply decreases activity in the brain regions used to control the vehicle, researchers here said.

Functional magnetic resonance imaging (fMRI) showed a 37% decrease in the brain activity associated with driving when the task is complicated by listening to speech, Marcel Just, D.O., of Carnegie Mellon University, and colleagues reported in the journal Brain Research.

Regulations that allow drivers to use hands-free or voice-activated cellphones are missing the point, Dr. Just said in a statement.

"Drivers need to keep not only their hands on the wheel," he said. "They also have to keep their brains on the road."

In an experiment, 29 volunteers underwent fMRI scanning while they guided a virtual vehicle down a winding road, either undistracted or having to listen to recorded sentences and decide if they were true or false.

In both cases, the researchers calculated how well the volunteers kept their virtual cars on an ideal path and how often they had "road maintenance errors" such as hitting a simulated guardrail.

The researchers also calculated what areas of the brain were active when volunteers were just driving and when they also had to listen.

The study found:

• On average, the root mean squared deviation from the ideal path increased from 2.48 to 2.64 when the listening task was added to driving. The change was significant at P<0.01.
• Mean road maintenance errors increased from 8.7 to 12.8, when listening was added to driving. The change was significant at P<0.05.
• There were large decreases in activity in the areas of the brain involved in the spatial processing associated with driving when listening was added. In the parietal lobe the decrease was 37%, which was significant at P<0.0001.

Dr. Just and colleagues noted that the driving task did not include having to react to potentially dangerous situations, so that the finding probably underestimates the degree of distraction.

Equally, they said, the task didn't require participants to speak, so that the listening task was probably less disruptive than a true cellphone conversation.

"The clear implication is that engaging in a demanding conversation could jeopardize judgment and reaction time if an atypical or unusual driving situation arose," Dr. Just said.

"Heavy traffic is no place for an involved personal or business discussion, let alone texting," he added.

The same considerations might apply to conversing with passengers, the researchers said, although there is evidence that passengers in a car are more likely to be aware of competing demands on the driver's attention and stop talking when it's inappropriate.

The researchers noted that the "striking" decrease in brain activity associated with driving while listening comes despite the fact that driving and listening draw on two largely non-overlapping cortical areas.

One possible interpretation is that the brain has only a limited amount of resources that can be devoted to paying attention, Dr. Just and colleagues said.

The study was supported by the Office of Naval Research. Dr. Just reported no conflicts.