You're sitting at a dull meeting and your attention drifts. You're waiting in a check-out line, thinking of nothing in particular. You're lying in bed, having just turned off the television. At such times, your conscious mind is on "idle," but your brain is not.
In such situations, the brain's "default system" takes over, a pattern of spontaneous activity that is fast becoming one of the hottest areas in neuroscience research, one that may cast light on mental illness.
Yesterday, researchers from the Massachusetts Institute of Technology, Harvard, and elsewhere reported that the brain's default functions look strikingly hyperactive in people with schizophrenia and their relatives. And the more overactive the default system is, they found, the worse the symptoms tend to be.
The findings are so strong that they raise hopes that brain scans of the default network could be used to diagnose schizophrenia before its symptoms appear, or predict how patients will respond to various treatments, researchers John Gabrieli and Susan Whitfield-Gabrieli said.
"The really exciting thing is that, probably in a year or two, for many, many diseases we'll have a huge amount of evidence about what the situation is in that disease for the default system," said Gabrieli, a professor at MIT's McGovern Institute for Brain Research.
It is already clear that various diseases affect the default system differently, he said. In autism, for example, it tends to be underactive, and in Alzheimer's, part of it seems to go dark.
For years, most neuroscientists paid little attention to what the brain was doing when on idle. But about a decade ago, researchers began to turn their attention to it.
They discovered "a very unique signature" of the brain when the mind wanders, said Marcus Raichle of Washington University in St. Louis, a neuroscientist who pioneered work on the default network. The system they found is "probably key to very core functions of how the brain operates," he said.
Their findings, Raichle said, fit in with the established idea that "the brain is always doing its thing - there is no such thing as a resting state" - which helps explain why it uses 20 percent of the body's energy. They also opened a new vista for research into brain diseases.
The Harvard-MIT paper, published yesterday in the Proceedings of the National Academy of Sciences, found that the default network in schizophrenics was hyperactive when they were at rest, and, importantly, did not ratchet down when they were asked to perform a memory test. Normally, when a subject must perform a task, the activity of the default network diminishes.
The default network findings suggest, Whitfield-Gabrieli said, that schizophrenia involves an inability for a patient's brain "to allocate resources away from their internal thoughts and feelings and toward the external world."
Gabrieli speculated that a hyperactive default network might also help fuel paranoia, by leading patients to interpret external events as being about them: thinking the television is speaking directly to them, for example.
Randy Buckner, a Harvard neuroscientist who studies the default system, said in an e-mail: "This important study suggests how a breakdown in communication between brain systems might cause the cognitive disturbances we see in schizophrenia."
"It is particularly intriguing that the default network is disrupted in schizophrenia, a disorder where the normal boundary between one's own internal thoughts and the outside world is blurred," he wrote.
It is clear that the default network becomes more active when people are thinking about themselves, whether remembering events in their lives or planning an activity.
What remains to be determined, Buckner wrote in a review paper last year, is whether the network's activity represents these self-oriented thoughts or whether its function is, rather, to act as a kind of sentinel monitoring the outside world when a person's thoughts turn inward.
Researchers say a great deal of work on the default network remains to be done, but fortunately, many studies are available for data-mining: Any experiment that looked at the brain's reaction to a task can also be used to examine what the brain was doing when there was no task at hand.
"It's like flipping the contrast," Whitfield-Gabrieli said.
Carey Goldberg can be reached at email@example.com