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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

The human brain isn't very good at multitasking. Now researchers in France say they've found a new reason why we struggle to carry out several tasks at once. They say a critical area of the brain can handle only two tasks at a time.

NPR's Jon Hamilton reports.

JON HAMILTON: Let's say you're at your computer working on an urgent report for your boss.

Professor RENE MAROIS (Vanderbilt University): But you also have your email open, and one email comes in from a good friend who may be inviting you to go to a restaurant...

HAMILTON: You could just ignore the email, says Rene Marois of Vanderbilt University. After all, your primary goal is to finish the report. Or your brain could do a form of multitasking. It could create a sort of mental bookmark of your current thoughts about the report while you pause long enough to answer the email.

Prof. MAROIS: Whether you're going to do the other task while maintaining your primary goal depends on the rewards associated with checking that email.

HAMILTON: And matters involving goals and rewards generally involve areas in the brain's frontal lobes. So, a French team decided to look at these areas while people tried to multitask. Their tasks involved shapes and letters, not reports and emails. And their motivation was money. Sometimes tiny amounts, sometimes a full euro for doing a task perfectly.

Etienne Koechlin of the Ecole Normale Superieure in Paris says when people were doing just one task, their left and right frontal lobes seemed to work together to achieve the goal. But when people took on a second task, he says, the lobes divided their responsibilities.

Dr. ETIENNE KOECHLIN (Ecole Normale Superieure): Each frontal lobe was pursuing its own goal.

HAMILTON: And responding to its own reward, says Marois, who is familiar with the work, but was not involved in the research.

Prof. MAROIS: So the left hemisphere represented the reward for the first task and the right hemisphere represented the information to the second task.

HAMILTON: The greater the reward, the greater the activity. But the brain has only two frontal lobes, suggesting perhaps that there might be a limit to the number of goals and rewards it can handle. So the team decided to do another experiment. Marois says they offered people rewards to do three things at once.

Prof. MAROIS: So you had to hold information about two tasks while executing a third one. And essentially people miserably failed at this task.

HAMILTON: When they started a third task, one of the original goals disappeared from their brains. The French team says that suggests the brain simply can't stay focused on more than two goals at a time.

But David Meyer, who studies multitasking at the University of Michigan, isn't so sure.

Professor DAVID MEYER (University of Michigan): I don't think there's any evidence to prove that under all conditions people can't keep three goals in mind. It's very important to distinguish between do not under some circumstances and cannot under any circumstances.

HAMILTON: Meyer says the brain might respond differently if the motivation was, say, survival instead of pocket money. He's also puzzled by what he sees as a disconnect between what was happening in people's brains and what they actually did. For example, offering people more money increased their brain activity quite a bit. But Meyer says the extra brain activity didn't make people much faster or more accurate at their tasks.

Prof. MEYER: The effects of these motivational manipulations on the behavior were extremely small, for the most part.

HAMILTON: Which means that we may not be much good at doing only two things at once, even when our brains really, really want to. The new study appears in the journal Science.

Jon Hamilton, NPR News.

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