People drive poorly while they're talking on a cell phone - about as badly as when they've just downed two screwdrivers. Some states have responded by passing laws against holding a phone to your ear while driving, on the assumption that the reason it's hard to talk on a phone and drive at the same time is because your hands are busy. But it turns out it doesn't really matter what your hands are doing. Whether you talk directly into a phone that you hold to your ear or whether you use a hands-free device, you drive more poorly when you're having a telephone conversation. You have more trouble following cars ahead of you at a safe distance, you veer more from the middle of the lane, you see less of the road and the things on it that you might hit, and you cause more accidents. In sum, talking on the phone makes it harder for you to see the world around you and to control your car. The question is why. What is it about talking on the phone that interferes with seeing the world and moving your body?
In the Language and Cognition Lab, Ben Bergen and his students are trying to answer this question, by putting people in a driving simulator and having them listen and speak over a hands-free device. What they've found is that what you're talking about affects how your driving is impaired. For instance, talking about bodily actions (like opening bottles of soda) hinders use of the steering wheel. But talking about visual things (like the shape of the roof of the Taj Mahal) interferes with vision, like noticing that the car ahead is braking.
These very specific sorts of interference give us insight into how the brain processes language. Language about actions interferes with actual action because the two tasks use the same neural resources. That is, understanding language about action engages the parts of the brain that actually control real action, just as language about vision engages visual parts of the brain. The result is that real action and perception are harder when people are trying to drive at the same time as use language, because the parts of the brain that control action and perception are busy doing something else already.
Find out more at: http://www.cogsci.ucsd.edu/~bkbergen/lcl/