Ars Technica summarizes a new report in Science:
Humans are capable of pursuing multiple goals at once—for example, I am pursuing writing an article and eating a bowl of Froot Loops—but how those activities get divided by the brain is still somewhat of a mystery. A new study, published in Science this week, imaged human brains and watched them try to multitask as subjects performed a set of variously interrupted tasks. They saw that our brains can divide resources fairly easily for two tasks, but have a much harder time juggling three or more.
Here’s a link to the original article, but it’s subscription-only.