Brain-imaging studies have proliferated so mindlessly (no pun intended) that neuroscientists should have to wear a badge pleading, “stop me before I scan again.” I mean, does it really add to the sum total of human knowledge to learn that the brain’s emotion regions become active when people listen to candidates for president? Or that the reward circuitry in the brains of drug addicts become active when they see drug paraphernalia?

Sometimes, though, a brain-imaging study does tell us something we didn’t know. I’d wager that most people do not know how much of their brain power cuts out when they listen to a conversation that demands even a modicum of cognitive power. If polite requests have not made your significant other, kids or other passengers shut up when you're behind the wheel, maybe this will.

Researchers at Carnegie Mellon University had 29 volunteers use a trackball or mouse to drive along a (virtual) winding road at 43 miles per hour in a simulator while having their brain scanned. As they report in a study scheduled for publication in the journal Brain Research, listening and concentrating on spoken questions reduced by 37 percent the amount of activity in a brain circuit that you tap for driving. Result: drivers weaved out of their lane in the simulator, just like drunks.

In one condition the volunteers drove along undisturbed, while in the other they heard sentences and had to decide if they were true or false by pressing a button with their left hand. (Among the statements: "botany is a biological science and it deals with the life, structure, and growth of plants,” and “a phobia refers to a person’s extreme attraction to some object, situation, or person.”)

When the drivers were thinking about the sentences, activity fell by 37 percent in their brain’s parietal lobe, which integrates sights, sounds and other sensory information to form a sense of where you are in space and allows you to navigate. Activity also fell in the occipital lobe, which processes visual information. (The drivers got 92 percent of the true/false questions right, suggesting they were listening hard and focusing on them.) The scientists conclude, “the addition of a sentence listening task decreases the brain activation associated with performing a driving task, despite the fact that the two tasks draw on largely non-overlapping cortical areas.”

The consequences of that drop in brain activity showed in the simulator. Driving with one lobe tied behind their backs, as it were, the distracted volunteers hit a simulated guardrail and failed to keep to the middle of the lane significantly more than when they were driving without someone yakking at them.

“Engaging in a demanding conversation could jeopardize judgment and reaction time if an atypical or unusual driving situation arose,” says CMU neuroscientist Marcel Just, director of the Center for Cognitive Brain Imaging and leader of the study. He has a hunch that cell phones may be especially distracting—more so than listening to the radio or to a conversation with a passenger—because you can more easily tune out the radio when you have to concentrate on the road, and a passenger will usually shut up when she sees that you have to focus on the traffic. But cell phones have a certain rude insistence.

Our cell-phone laws are bedeviled by one little problem: epidemiological studies show that the rate of accidents among people who use hands-free phones is equal to that of people using hand-held phones. That makes laws requiring the former hard to justify, if well-intentioned. I’ll let Just and his colleagues say it: Findings such as those in this study "suggest that the deterioration in driving performance resulting from cellular phone usage results from competition for mental resources at a central cognitive level rather than at a motor output level, and that legislative measures which simply restrict drivers to the use of hand-free phones fail in their intent to limit an important distraction to driving."