The researchers said that they did not see a delay if the participants were given the tasks one at a time. But the researchers found that response to the second task was delayed by up to a second when the study participants were given the two tasks at about the same time.

Image Juggling activities in Times Square. When people divide their attention, they react more slowly and make more mistakes, scientists say. Credit... Photographs by Hiroko Masuike for The New York Times

In many daily tasks, of course, a lost second is unimportant. But one implication of the Vanderbilt research, Mr. Marois said, is that talking on a cellphone while driving a car is dangerous. A one-second delay in response time at 60 miles an hour could be fatal, he noted.

“We are under the impression that we have this brain that can do more than it often can,” observed Mr. Marois, who said he turns off his cellphone when driving.

The young, according to conventional wisdom, are the most adept multitaskers. Just look at teenagers and young workers in their 20s, e-mailing, instant messaging and listening to iPods at once.

Recently completed research at the Institute for the Future of the Mind at Oxford University suggests the popular perception is open to question. A group of 18- to 21-year-olds and a group of 35- to 39-year-olds were given 90 seconds to translate images into numbers, using a simple code.

The younger group did 10 percent better when not interrupted. But when both groups were interrupted by a phone call, a cellphone short-text message or an instant message, the older group matched the younger group in speed and accuracy.

“The older people think more slowly, but they have a faster fluid intelligence, so they are better able to block out interruptions and choose what to focus on,” said Martin Westwell, deputy director of the institute.