The Hidden Brain

What scientists can learn from ‘nothing.’

It took Sherlock Holmes to deduce the significance of the dog that didn’t bark.* So maybe it’s understandable that neuroscientists have traditionally ignored the brain activity that just hums away quietly in the background when the brain isn’t doing much of anything. Assuming this “default” or “resting” activity was meaningless random noise, they went so far as to subtract it out—and thus ignore it—on brain images such as PET scans and fMRIs.

Oops. Neuroscience is having its dark-energy moment, feeling as chagrined as astronomers who belatedly realized that the cosmos is awash in more invisible matter and mysterious (“dark”) energy than make up the atoms in all the stars, planets, nebulae, and galaxies. For it turns out that when someone is just lying still and the mind is blank, neurons are chattering away like Twitter addicts. The very idea of default activity was so contrary to the herd wisdom that when Marcus Raichle of Washington University in St. Louis, one of its discoverers, submitted a paper about it, a journal rejected it. That the brain might be so active in regions “doing nothing,” he says, had “escaped the neuroimaging establishment.” Now the establishment is catching up, with more and more labs investigating the brain’s default activity and a June meeting in Barcelona on brain mapping devoted to it.

The brain is in default mode when we stare into space, sleep, succumb to anesthesia, make our mind a blank while sitting motionless—in short, when the brain’s only task seems to be keeping us alive and breathing. This default activity, to everyone’s surprise, is no mere murmur in the background of a loud symphony. It is the symphony, consuming 20 times as much energy as the conscious life of the mind, including thinking, feeling, and using our senses—the mental acts captured by the brain imaging that so entrances the public. “The brain at rest is not at rest,” says neuroscientist Alvaro Pascual-Leone of Harvard. “Even more important, this resting activity is not random, but is well organized and constitutes the bulk of the brain’s activity.”

That’s a lot of energy to expend on “nothing,” which makes neuroscientists pretty sure the default mode serves important functions. One seems to be preparing the brain for future contingencies. “The resting activity seems to create images about what to expect from the outside world” so the brain can react more nimbly when actual sensory information arrives, says Pascual-Leone. Resting or default activity in the motor cortex, for instance, allows us to duck in time when an errant ball speeds toward our head. In the visual cortex, nine times as many synapses carry the background chatter that goes on when there is nothing to see, such as in total darkness or when the eyes are closed, as handle signals arriving from the eyes. That suggests the default activity is constantly creating mental images that can help us make sense of real ones. Raichle compares the default activity to a conductor’s baton, keeping distinct brain circuits (instruments) always checking in with each other and, in particular, with stored memories—of other times when thrown balls have hit someone’s head, perhaps. Thanks to the constant default activity, even when we lose synapses (which are constantly being formed and broken), we do not lose memories. As long as the background music keeps playing, if the oboe drops out its replacement can easily pick up the melody.

The scenario-creating, hypothesis-forming function of the default network has led Randy Buckner of Harvard to speculate that it gives us a “prospective brain.” The default mode is “active when you are not taking in and processing information from the outside world but are just thinking to yourself, remembering, imagining the future, and taking other people’s perspectives,” he says. The default network “may be a way to play out possible scenarios to help us plan and navigate social interactions.”

On that score, it’s striking that the brains of people with autism and schizophrenia show aberrant default activity and messed-up connections in regions that seem to be its ground zero. This abnormal default activity may be the basis for the trouble schizophrenics have distinguishing reality from fantasy, and the difficulty autistics have with social interaction. More than anything, though, the default activity offers a cautionary tale about the hubris of scientists who dismiss anything the brain does as unimportant.

*The canine guarding the estate where a murder occurred must have known the killer, he realized.