A special corner of hell is reserved for drivers who weave from one lane to another at a crawl while blithely chatting on their cell phones. Even a simple form of multitasking — driving while listening to someone else talk — disrupts the ability to navigate a car safely, a new study finds.

An intriguing neural response underlies vehicular mishaps associated with such distractions, say neuroscientist Marcel Just of Carnegie Mellon University in Pittsburgh and his colleagues. Attending to what someone says galvanizes language-related brain areas while simultaneously reducing activity in spatial regions that coordinate driving behavior.

This finding suggests that people who combine relatively automatic tasks, such as speech comprehension and car driving, exceed a biological limit on the amount of systematic brain activity they can accommodate at one time, the researchers propose. As a result, the less-ingrained skill — in this case, driving, which is learned long after a person grasps a native language — takes a neural hit.

“What’s exciting is that now we have a biological account of how multitasking affects driving behavior,” Just says.

The new findings appear in the April 18 Brain Research.

If merely listening to someone talk dents the ability to maneuver a car, then other common driver activities may do the same, Just suggests. These behaviors include tuning or listening to a radio, eating and drinking, monitoring children or pets, and conversing with a passenger.

Cell phones stand out as particularly problematic for drivers, Just notes. Cell phone conversations require a driver’s constant attention in order not to appear rude or insulting to an unseen partner. In contrast, a talking passenger can willingly cut off conversation upon spying an approaching ambulance or some other demand on a driver’s attention.

Psychologist David Strayer of the University of Utah in Salt Lake City agrees, adding that the new results offer a conservative estimate of the neural impact of multitasking on driving. Strayer and his colleagues have documented steep declines in simulated driving skill, as well as a marked drop in driving speed, among volunteers using handheld or hands-free cell phones.

Just’s team studied 29 adults, ages 18 to 25. Each participant lay in a functional MRI scanner equipped with a screen that displayed a simulated driving exercise. These machines measure blood-flow changes in the brain, providing indirect signs of rises and falls in neural activity.

In one trial, volunteers steered a car along a virtual winding road using either a computer mouse or a computer trackball. The virtual car maintained a constant, moderate speed. Drivers encountered no intersections, hazards or other vehicles. Still, simulated driving while lying down in a noisy brain scanner proved challenging for participants.

Undisturbed driving activated areas toward the back of the brain involved in spatial perception.

In a second trial, participants steered a car down a virtual road with one hand while listening to general-knowledge sentences that they had to identify as true or false by pressing response buttons with the other hand.

Drivers responded correctly to nearly all sentences. This verbal task prompted strong activity in midbrain structures necessary for language comprehension, as well as a 37 percent decline in activity in spatial regions that had been employed during undisturbed driving.

During one-minute virtual trips, participants listening to sentences drove onto the shoulder of the pavement or into the wrong lane 13 times on average, compared with 9 times on average for undisturbed drivers.

“Listening to talk radio or to spoken directions from a navigation system while driving probably have similar effects to what we found,” Just says. “Multitasking puts high demands on the brain.”

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