In the mid-1990s, neuroscientist Marcus Raichle noticed something funny going on in some of his brain-scanning experiments.

Here’s what usually happens, more or less: someone lies inside of the scanner and performs a specific task, like pressing a button. In response to that particular task, some parts of the brain become more active. Voilà: you have identified the regions involved in button-pressing.

But Raichle observed that a couple of areas actually quiet down during a goal-directed task—during many different tasks, in fact. One of these spots is in the parietal lobe, in the middle of the back of the brain. Raichle thought the data was curious, but didn’t dwell on it. He threw the scans into a folder, labeled it