The use of cell phones, including dialing and texting, has long been a safety concern for drivers. But the Carnegie Mellon study, for the first time, used brain imaging to document that listening alone reduces by 37 percent the amount of brain activity associated with driving. This can cause drivers to weave out of their lane, based on the performance of subjects using a driving simulator.

The findings show that making cell phones hands-free or voice-activated is not sufficient in eliminating distractions to drivers. "Drivers need to keep not only their hands on the wheel; they also have to keep their brains on the road," said neuroscientist Marcel Just, director of the Center for Cognitive Brain Imaging.

Other distractions, such as eating, listening to the radio or talking with a passenger, also can divert a driver. Though it is not known how these activities compare to cell phone use, Just said there are reasons to believe cell phones may be especially distracting.

The 29 study volunteers used a driving simulator while inside an MRI brain scanner. They steered a car along a virtual winding road at a fixed, challenging speed, either while they were undisturbed, or while they were deciding whether a sentence they heard was true or false. Just’s team used state-of-the-art functional magnetic resonance imaging (fMRI) methods to measure activity in 20,000 brain locations, each about the size of a peppercorn. Measurements were made every second.

The driving-while-listening condition produced a 37 percent decrease in activity of the brain’s parietal lobe, which is associated with driving. This portion of the brain integrates sensory information and is critical for spatial sense and navigation. Activity was also reduced in the occipital lobe, which processes visual information.

The other impact of driving-while-listening was a significant deterioration in the quality of driving. Subjects who were listening committed more lane maintenance errors, such as hitting a simulated guardrail, and deviating from the middle of the lane. Both kinds of influences decrease the brain’s capacity to drive well, and that decrease can be costly when the margin for error is small.

Because driving and listening draw on two different brain networks, scientists had previously suspected that the networks could work independently on each task. But Just said this study demonstrates that there is only so much that the brain can do at one time, no matter how different the two tasks are.

The study emerges from the new field of neuroergonomics, which combines brain science with human-computer interaction studies that measure how well a technology matches human capabilities. Neuroergonomics is beginning to be applied to the operation of vehicles like aircraft, ships and cars in which drivers now have navigation systems, iPods and even DVD players at their disposal.