Research says that "multi-tasking" skills are limited by the physical division of the brain into two hemispheres.
In a new study, boffins found that when individuals carry out two tasks simultaneously their brains divide each job up so that one is performed largely by the left side of the brain and the other is carried out mainly on the right.
The study's finding may explain why humans tend to prefer a simple choice between two options rather than three or more, reports The Independent.
To reach the conclusion, Sylvain Charron and Etienne Koechlin of France's National Institute of Health and Medical Research in Paris asked 32 volunteers to carry out two different mental puzzles while their brains were being scanned by an MRI machine.
"Each subject was performing two tasks concurrently. One task was to pair upper case letters and the other task was to pair lower case letters together. It was a very simple task and the subjects had to switch back and forth between them," Dr Koechlin said.
"We motivated them with a reward if they made no errors between trials. It was a monetary reward actually, so when the subject made an error on one of the tasks, their reward was less. We rewarded brain activity and at the same time we monitored the subjects' errors, reaction time and so on. So we could measure performance and we found that a larger reward was associated with a better performance," he said.
The study, published in the journal Science, focused on the medial frontal cortex. It is this part of the brain that is thought to drive the pursuit of rewards associated with carrying out a task.
"We found that brain activity increased with rewards and expectations in the medial frontal cortex. We found in the left hemisphere that the activity increased as the reward value of one task increased, but not the other task, whereas in the right hemisphere the brain activation was related to the reward value of the other task," Professor Koechlin said.
"The two hemispheres co-operated when there was only one task. But in two tasks, one hemisphere covers the reward of one task and the other hemisphere covers the reward of the other."
"The human prefrontal function seems to be built to control two tasks simultaneously. It means in everyday behaviour we can readily switch between two tasks but not between three. With three tasks the division is limited to only two hemispheres, so there is a problem," he said.