The brain at rest

Melissa Marino, Ph.D.
Published: February, 2006

The human brain is an energy glutton. Comprising only about 2 percent of body weight, it consumes nearly 20 percent of the body’s oxygen intake. Why does the brain need so much energy, even when it is at rest?

Marcus Raichle, M.D., a member of the Washington University team that developed PET in the 1970s, believes he may have an answer.

During experimental tasks, some areas of the brain become more active while other areas become less active as measured by changes in metabolism and blood flow. When subjects are studied at rest with their eyes closed, however, activity in these “task-negative” areas goes up, Raichle and his colleagues reported in 2001.

These areas, he says, are nodes of a “default” brain network that functions intrinsically and in a correlated manner during the resting or baseline state. Its activity decreases when an attention-demanding task raises the oxygen requirement in other areas.

Raichle sees purpose and evolutionary significance in this phenomenon. The default system serves as a “sentinel,” constantly monitoring the horizons of the external environment (as well as the world within). It also is “forecasting” and preparing for future events based on prior experience.

“The brain is basically in the prediction business,” he explains. “We use both our genetically endowed experience plus what we learn in our own experience, and use that to predict what’s going to happen next. And we spend most of our brain’s (energy) budget on doing that.”

Is this activity some aspect of the thing we call “consciousness?”

Perhaps, says Raichle.

If so, could fMRI determine whether a person in a persistent vegetative state really has some flicker of consciousness left?

Time, and further research, may tell.

“Whatever (the default system) is doing is a reflection of what the brain is mainly doing,” Raichle says. “We have to back away from the notion that the brain is mainly reacting to the world in which we live, and take the perspective that it is… creating, in the context of ourselves, a model of the world in which we live and expect to live.”

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