If you’ve ever heard somebody scream “I can’t do two things at once”, you’re wrong… but only just.

French scientists now believe the brain has the capacity to work on two tasks at once, but that it can’t cope with more than two. That’s because the limited multitasking which people can carry out is based on a physical characteristic of the brain, which then becomes a limitation beyond two tasks.

Staff at the Pierre et Marie Curie university in Paris carried out a study (published in Science) where subjects had to look at a set of uppercase letters and keep track of whether they were in the correct order to spell out set words, with a cash reward if they succeeded. The task was then made harder with two sets of letters, which proved manageable.

However, when the task was increased to three sets — effectively creating three separate tasks — subjects not only couldn’t cope but, rather than make a bad job of all three tasks, simply acted as if one task didn’t exist.

MRI scans of brain activity during the tasks appeared to give an explanation. When the subjects increased from one to two tasks, the two frontal lobes of the medial prefrontal cortex (part of the brain which responds to rewards) showed separate activity, suggesting the tasks had been divided across the brain. When a third task was introduced there was literally nowhere for it to go.

The researchers believe that as well as showing why multitasking is limited for humans, the results may also explain why most people struggle to cope with decisions that have three or more options. It appears that the brain cannot simultaneously compare the consequences (specifically the rewards) of more than two options at once.

While the brain can cope with two tasks, it usually comes with a notable performance cost. A recent study at the University of Utah found that just one in forty drivers have the ability to “supertask”: that is, they can carry out two tasks simultaneously, both at a high degree of competence.

The Utah study looked at the skills of driving and conducting a phone conversation. Its conclusions suggested that while there are some people who can drive safely while using a phone (whether hands free or handheld), they are so rare that they shouldn’t affect decisions of distracted driving laws.

[Picture credit: Etienne Koechlin, INSERM-ENS, Paris, France, 2010.]