During tasks requiring focused attention, regions specialized to the tasks at hand became active in the subjects whose brains were being scanned. But as those men and women mentally relaxed between tasks inside the scanners, Raichle saw that the specialized regions went quiet -- and a large and different cluster of brain structures consistently lighted up.
Raichle was particularly interested in a portion of the brain called the medial parietal cortex as a sort of central hub of this activity. He knew the area tended to become active when a person recalled his past.
And his work uncovered another key node in this curious circuit: the medial prefrontal cortex, a uniquely human structure that comes alive when we try to imagine what others are thinking.
Each region, Raichle realized, had a feature in common -- it was focused on the self, and on the personal history and relationships by which we define ourselves as individuals.
As studies continued, scientists noticed some interesting facts.
They saw that the brain parts constituting the default mode network are uniquely vulnerable to the tangles, plaques and metabolic disturbances of Alzheimer's disease -- an illness that starts by stealing one's memory and eventually robs its victims of their sense of self.
This, Raichle and colleagues would argue, suggests how important the default mode network is in making us who we are.
They saw that when operating, this network guzzles fuel at least as voraciously as do the networks that are at work when we engage in hard mental labor. That, along with other evidence, suggests to Raichle that when the default mode network is engaged, there's more than a mental vacation taking place.
So what is it doing?
Raichle suspects that during these moments of errant thought, the brain is forming a set of mental rules about our world, particularly our social world, that help us navigate human interactions and quickly make sense of and react to information -- about a stranger's intentions, a child's next move, a choice before us -- without having to run a complex and conscious calculation of all our values, expectations and beliefs.
Raichle says such mental shortcuts are necessary because the brain cannot possibly take in all the detail available to our senses at any given moment. The default mode network, he proposes, keeps a template handy that lets us assume a lot about ourselves and the people and environment we interact with.
Raichle points to another odd distinction of the default mode network -- one that suggests it plays a central role in our functioning. Its central hub has two separate sources of blood supply, making it far less vulnerable than most other regions of the brain to damage from a stroke.
"That's an insurance policy: This area is critically important," he says.
Neuroscientists suspect that the default mode network may speak volumes about our mental health, based on studies in the last three years that suggest it is working slightly differently in people with depression, autism and other disorders. (See related story.)
That fact underscores a point: Just as sleep appears to play an important role in learning, memory consolidation and maintaining the body's metabolic function, some scientists wonder whether unstructured mental time -- time to zone out and daydream -- might also play a key role in our mental well-being. If so, that's a cautionary tale for a society that prizes productivity and takes a dim view of mind-wandering.
Such social pressure, Schooler says, overlooks the lessons from studies on the resting brain -- that zoning out and daydreaming, indulged in at appropriate times, might serve a larger purpose in keeping us healthy and happy.
"People have this fear of being inadequately engaged, and as a consequence they overlook how engaging their own minds can be," Schooler says. "Each one of us can be pretty good company to ourselves if we allow our minds to go there."