Simply listening to someone over a mobile cell phone while driving may distract the brain enough to cause an accident, a new brain-imaging study suggests.

Previous studies have suggested that drivers who use cell phones run a greater risk of accidents, and that hands-free phones do not appreciably lower the odds.

The new findings, reported in the journal Brain Research, cast further doubt on the idea that hands-free cell phones are safer for drivers. Just the act of listening, researchers found, appears to divert much of the brain resources that would normally go toward navigating the road.

The study included 29 volunteers who used a driving simulator while inside an MRI brain scanner. Participants steered along a winding “virtual” road, once with no distractions and once while listening to various sentences and trying to decide whether they were true or false.

The researchers found that in the second scenario, the drivers’ brain activity changed - including a 37 percent drop in activity in the parietal lobe, a brain area involved in spatial sense and navigation.

Moreover, this shift in brain activity was accompanied by an increase in driving errors; drivers tended to drift more in their simulated lanes and were more likely to hit the virtual guardrail.

“Drivers need to keep not only their hands on the wheel; they also have to keep their brains on the road,” lead researcher Dr. Marcel Just, of Carnegie Mellon University in Pittsburgh, said in a statement.

According to Just, conversing on a cell phone may well be more distracting than listening to music or to someone in the passenger seat, for instance.

“Talking on a cell phone has a special social demand, such that not attending to the cell conversation can be interpreted as rude, insulting behavior,” he explained.

In contrast, a passenger in the car can recognize when the driver needs to focus on the road, and may stop talking. Listening to music, for its part, does not require the cognitive processing necessary for having a conversation and can be more readily tuned out.

Because driving and listening rely on different brain networks, some scientists had speculated that the brain could handle both tasks at the same time. But the current findings, according to Just, suggest that there is only so much the brain can accomplish simultaneously.

“Drivers’ seats in many vehicles are becoming highly instrumented cockpits,” he noted, “and during difficult driving situations, they require the undivided attention of the driver’s brain.”

SOURCE: Brain Research, online March 5, 2008.