Our brains are set up to do two things at once, but not three, a French team reports in the journal Science.
The researchers reached that conclusion after studying an area of the brain involved in goals and rewards. Their experiment tested people's abilities to accomplish up to three mental tasks at the same time. The tasks involved matching letters in different ways, and for incentive, participants were paid up to a euro for doing a task perfectly.
When volunteers were doing just one task, there was activity in goal-oriented areas of both frontal lobes, says Etienne Koechlin, a professor at the Ecole Normale Superieure in Paris. That suggested that the two sides of the brain were working together to get the job done, he says.
But when people took on a second task, the lobes divided their responsibilities. "Each frontal lobe was pursuing its own goal," Koechlin says.
The lobe on the left side of the brain focused on the first task, while the lobe on the right focused on the second. When the researchers offered a greater reward for a task being supervised by one side of the brain, the amount of activity on that side increased accordingly.
Brain Maxes Out At Multitasking
But the brain has only two frontal lobes, suggesting there might be a limit to the number of goals and rewards it can handle. So the team decided to do another experiment.
They offered people rewards to do three things at once.
And when people started a third task, one of the original goals disappeared from their brains, Koechlin says. Also people slowed down and made many more mistakes. That suggests that our frontal lobes "can't maintain more than two tasks," Koechlin says.
The evidence that the brain assigns one task to each side of the brain is "very surprising," says Rene Marois, a neuroscientist at Vanderbilt University.
He says the findings, if they hold up, have implications for people trying to do more than two things at a time.
For example, Marois says, someone who is writing a report might be able to take on a second task, like checking e-mail, without losing their train of thought. But if that e-mail asked for a decision about something, that would amount to a third task, and the brain would be overwhelmed, he says.
High Stakes Could Improve Performance
David Meyer, who studies multitasking at the University of Michigan, says he doesn't think the study shows it's impossible to keep three tasks in mind.
"In the real world, there are life and death matters which hinge on exactly what happens with multitasking, which certainly wasn't the case with this study," Meyer says.
The frontal lobes of the brain might respond differently if the reward was survival, instead of the equivalent of a couple of dollars, he says. Meyer is 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 the extra brain activity didn't make people much faster or more accurate at multitasking, Meyer says.
"The effects of these motivational manipulations on the behavior were extremely small for the most part," he says. "At most only a few percentage points."
That could mean studies need to use more powerful rewards, Meyer says. Or, he says, it could mean that no reward or internal goal can make us very good at doing even two tasks at once.