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Secret life of the brain

People have been rummaging around inside the human body for millennia, so to find a new organ in the 21st century is an extraordinary achievement. Thats effectively what two researchers have done. Their discovery came from asking a simple question: what happens when the brain is restingwhen its doing nothing? Douglas Fox takes up the story.

In 1953, a physician named Louis Sokoloff laid a 20-year-old college student onto a gurney, attached electrodes to his scalp and inserted a syringe into his jugular vein.

For 60 minutes the volunteer lay there and solved arithmetic problems. All the while, Sokoloff monitored his brainwaves and checked the levels of oxygen and carbon dioxide in his blood.

Sokoloff, a researcher at the University of Pennsylvania in Philadelphia, was trying to find out how much energy the brain consumes during vigorous thought. He expected his volunteers brain to guzzle more oxygen as it crunched the problems, but what he saw surprised him: his subjects brain consumed no more oxygen while doing arithmetic than it did while he was resting with his eyes closed.

People have long envisaged the brain as being like a computer on standby, lying dormant until called upon to do a task, such as solving a sudoku, reading a newspaper or looking for a face in a crowd. Sokoloffs experiment provided the first glimpse of a different truth: that the brain enjoys a rich private life. This amazing organ, which accounts for only 2 percent of our body mass but devours 20 percent of the calories we eat and drink, fritters away much of that energy doing, as far as we can tell, absolutely nothing.

There is a huge amount of activity in the [resting] brain that has been largely unaccounted for, says Marcus Raichle, a neuroscientist at Washington University in St Louis. The brain is a very expensive organ, but nobody had asked deeply what this cost is all about.

Raichle and a handful of others are finally tackling this fundamental questionwhat exactly is the idling brain up to, anyway? Their work has led to the discovery of a major system within the brain, an organ within an organ, that hid for decades right before our eyes. Some call it the neural dynamo of daydreaming. Others assign it a more mysterious role, possibly selecting memories and knitting them seamlessly into a personal narrative. Whatever it does, it fires up whenever the brain is otherwise unoccupied and burns white hot, guzzling more oxygen, gram for gram, than your beating heart.

Its a very important thing, says Giulio Tononi, a neuroscientist at the University of Wisconsin-Madison. Its not very frequent that a new functional system is identified in the brainin fact it hasnt happened for I dont know how many years. Its like finding a new continent. The discovery was slow in coming. Sokoloffs experiment in 1953 drew little attention. It wasnt until the 1980s that it started to dawn on researchers that the brain may be doing important things while apparently stuck in neutral. In the 1980s a novel brain-scanning technique called PET (for positron emission topography) was all the rage. By injecting radioactive glucose and measuring where it accumulated, researchers were able to eavesdrop on the brains inner workings. In a typical experiment they would scan a volunteer lying down with their eyes closed and again while doing a mentally demanding task, then subtract one scan from the other to find the brain areas that lit up.

Raichle was using PET to find brain areas associated with words when he noticed something odd: some brain areas seemed to go at full tilt during rest, but quieted down as soon as the person started an exercise. Most people shrugged off these oddities as random noise. But in 1997, Raichles colleague Gordon Shulman found otherwise.

Shulman sifted through a stack of brain scans from 134 people. Regardless of the task performed, whether it involved reading or watching shapes on a screen, the same constellation of brain areas always dimmed as soon as the subject started concentrating. I was surprised by the level of consistency, says Shulman. Suddenly it looked a lot less like random noise. There was this neural network that had not previously been described.

Raichle and Shulman published a paper in 2001 suggesting that they had stumbled onto a previously unrecognized default modea sort of internal game of solitaire which the brain turns to when unoccupied and sets aside when called on to do something else. This brain activity occurred largely in a cluster of regions arching through the midline of the brain, from front to back, which Raichle and Shulman dubbed the default network.

The brain areas in the network were known and previously studied by researchers. What they hadnt known before was that they chattered non-stop to one another when the person was unoccupied, but quieted down as soon as a task requiring focused attention came along. Measurements of metabolic activity showed that some parts of this network devoured 30 percent more energy, gram for gram, than nearly any other area of the brain.

All of this poses the questionwhat exactly is the brain up to when we are not doing anything? When Raichle and Shulman outlined the default network, they saw clues to its purpose based on what was already known about the brain areas concerned.

One of the core components is the medial prefrontal cortex (see figure above), which is known to evaluate things from a highly self-centered perspective of whether theyre likely to be good, bad or indifferent. Parts of this region also light up when people are asked to study lists of adjectives and choose ones that apply to themselves but not to, say, Madonna. People who suffer damage to their medial prefrontal cortex become listless and uncommunicative. One woman who recovered from a stroke in that area recalled inhabiting an empty mind, devoid of the wandering, stream-of-consciousness thoughts that most of us take for granted.

Parts of the default network also have strong connections to the hippocampus, which records and recalls autobiographical memories such as yesterdays breakfast or your first day of kindergarten.

To Raichle and his colleague Debra Gusnard, this all pointed to one thing: daydreaming. Through the hippocampus, the default network could tap into memoriesthe raw material of daydreams. The medial prefrontal cortex could then evaluate those memories from an introspective viewpoint. Raichle and Gusnard speculated that the default network might provide the brain with an inner rehearsal for considering future actions and choices.

Randy Buckner, a former colleague of Raichles, now at Harvard, agrees. To him the evidence paints a picture of a brain system involved in the quintessential acts of daydreaming: mulling over past experiences and speculating about the future. Were very good at imagining possible worlds and thinking about them, says Buckner. This may be the brain network that helps us to do that. There is now direct evidence to support this idea. In 2007, Malia Mason, now at Columbia Business School, New York City, reported that the activity of the default network correlates with daydreaming. Using the brain-imaging technique functional magnetic resonance imaging (fMRI), Mason found that people reported daydreaming

Daydreaming may sound like a mental luxury, but its purpose is deadly serious: Buckner and his Harvard colleague Daniel Gilbert see it as the ultimate tool for incorporating lessons learned in the past into our plans for the future. So important is this exercise, it seems, that the brain engages in it whenever possible, breaking off only when it has to divert its limited supply of blood, oxygen and glucose to a more urgent task.