Years ago a cruel joke suggested that President Ford couldn't chew gum and walk at the same time.
Silly, of course, but it turns out that there was a bit of truth in it. Not just for Mr. Ford, but for all of us.
Any time we try to perform two things at once, one performance suffers, albeit only slightly when it comes to nearly brainless tasks like walking and chewing at the same time. The going gets tough, though, when either task becomes more demanding.
Take talking on a cell phone while driving a car, for example, and this is not part of an anti-cell phone crusade. New research shows, however, that the price we pay for trying to listen intensely comes at the expense of our ability to see clearly.
When we turn the "listening knobs" up, says psychologist Steve Yantis of Johns Hopkins University, we turn the "visual knobs" down.
It doesn't make any difference if the phone is hands-free. It's the listening that makes the difference, not the nature of the instrument.
That may be a tad surprising because we humans pride ourselves on being able to carry out more than one task at once, or multitasking, as they say in the computer biz. In fact, some research suggests we are the only animals that have developed high skills at multitasking.
"People definitely are optimized for handling multiple tasks," says Yantis, who has spent years studying how we control the flow of information into our brains, and what we do with it when we've got it. "We've evolved to be effective multitaskers because it's useful to be able to do multiple things at once to be able to survive. But there are limitations."
Yantis and his team of researchers have been using one of the most effective tools in the field of cognitive science, functional magnetic resonance imaging, to look inside the brains of their subjects during various experiments. Different parts of the brain literally light up, showing such things as increased blood flow, when the subjects are given different tasks.
The evidence shows clearly that when the participants concentrated on listening, the part of the brain that controls vision became less active, and vice versa.
It's as though we have a certain amount of gray matter, and if we're going to put a lot of it in our listening basket, we're going to have to take it from somewhere else.
Yantis' primary interest is in the flow of information in the brain, and how that relates to such things as drug abuse and some mental illnesses, which is why much of his research is funded by the National Institute on Drug Abuse. But the ubiquitous instrument that has become such an intricate part of our lifestyles -- the cell phone -- lends itself perfectly to his research.
Especially since it is so often used while we are trying to do something else, like drive a car.
Yantis thinks a cell phone can be particularly intrusive, thus compromising our multitasking abilities. If you're talking with someone else in the car, he says, that person knows you're trying to drive and will probably lighten up on the conversation if red lights are flashing in the distance. But if you're on a cell phone, the other person may not even know you are driving and may ratchet up the complexity, or the emotional content, of the conversation without knowing you are in a very precarious situation.
That requires you to concentrate more on the conversation, and thus less on the driving, because you can't devote equal attention to both. That's true even if the person on the cell phone is simply trying to give you directions.
"I have to form a mental map of where I'm going and that sort of visual processing is going to be taking away from what I'm seeing out the windshield," he says.
Either reaction time or performance will be impaired, he adds.
I interviewed Yantis on the telephone, and while we were talking I noticed personal evidence of the validity of his research. When I concentrated on understanding what he was saying, my vision literally blurred. My eyes weren't focused on anything in my office, and everything was a bit fuzzy.
Try it some time.
"If you really want to concentrate close your eyes," he says, "because the visual scene will distract you a little bit."
Of course, not while driving a car.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.