Multi-tasking: Can we do it?

Sure. But only up to a point. Our brains can handle two activities, but not three. Which might explain why we have a hard time making decisions when we’re faced with more than two choices.

In the brain, the medial prefrontal cortex (MFC) keeps track of what we’re doing. When we’re working on two tasks, it can divide its attentions, with one half of the region focusing on one task, and the other half on the second task. But, according to researcher Etienne Koechlin of the Universite Pierre et Marie Curie in Paris, we’re actually “divide tasking”, rather than multi-tasking. And things get pretty muddled if we try to add a third task.

Koechlin’s research, “Divided Representation of Concurrent Goals in the Human Frontal Lobes,” was published in the journal Science.

In an interview for Live Science, Koechlin said:

What the results really show is that we can readily divide tasking. We can cook, and at the same time talk on the phone, and switch back and forth between these two activities. However, we cannot multitask with more than two tasks.

Koechlin’s study used fMRI brain scans to monitor 32 subjects as they watch upper case letters on a screen. The subjects had to determine if the letters were presented in the correct order to spell a certain word, such as B-R-A-I-N. They received a monetary reward if they made no errors. As the rewards increased in value, the researchers saw more activity in the MFC.

The subjects were then presented with lower case letters as well, and had to determine if both the upper case and lower case letters spelled a word, B-R-A-I-N and b-r-a-i-n. This required the subjects to switch back and forth between tasks.

During this dual task, the MFC divided up the labor. One hemisphere of the brain encoded the reward associated with the upper case letter task, and so showed activity during that task, while the other region encoded the reward associated with the lower case task.

Essentially, the brain behaved “as if each frontal lobe was pursuing its own goal,” Koechlin said.

When researchers introduced a third letter-matching task, they saw the subject’s accuracy drop considerably. In essence, there was no where for the third task to go.

As for decision-making, Koechlin thinks his results may explain why it’s difficult for us to decide between more than two options:

Previous work has indicated that people like binary choices, or decisions between two things. They have difficulty when decisions involve more than two choices, Koechlin said. When faced with three or more choices, subjects don’t appear to evaluate them rationally; they simply start discarding choices until they get back to a binary choice.
This is perhaps because your brain can’t keep track of the rewards involved with more than two choices, Koechlin said.

If you’re interested in reading more about multi-tasking, check out this episode from NPR’s Talk of the Nation, featuring NPR science correspondent Jon Hamilton and University of Michigan professor Daniel Weissman.

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