Journey to the center of the brain
New look suggests brain's anatomy determines its activity
Hub systems may be frustrating for airline travelers, but a central hub keeps the brain connected and humming, a first-ever wiring diagram of the human brain reveals.
Nearly all of the information transmitted from one brain region to another passes through a core located in the center and back of the brain along the crack that separates the two hemispheres, an international group of researchers reports in the June 30 PLoS Biology.
Earlier research pinpointed an area of the brain called the default network — a group of brain regions that are active when a person is thinking about nothing in particular. The new map of the brain’s anatomy showed that, in fact, the default network also resides in this physical hub, the core of the brain.
“Our map is a very crude one,” says Olaf Sporns, a computational neuroscientist at IndianaUniversity in Bloomington. But the wiring diagram is a first step toward understanding how the brain is structured and how it communicates. Such diagrams could help therapists design strategies to improve recovery of stroke victims or people with other brain injuries.
The new study “takes the idea of the intrinsic organization of the brain to a new level,” says Marcus Raichle, a neurologist at WashingtonUniversity in St. Louis. The research reveals that “there are hubs in the brain and some hubs are more important than others. This one rises to the top of the pile.”
Differences among individuals in brain wiring may also affect how people think, and may influence how vulnerable people are to certain diseases and disorders. For instance, the core structure identified in the new study is the part of the brain most susceptible to Alzheimer’s disease.
Raichle first described the default network in 2001. He had noticed that when subjects in functional MRI studies were asked to do a specific task, such as remembering a string of words, certain parts of their brains became less active while others increased activity. Most researchers are interested only in “what goes up,” Raichle says, but he was interested in the areas that decreased activity.
He started a file on the “Medial Mystery Parietal Area”: the medial parietal area, or center and back of the brain. The mystery is why this part of the brain had lower activity when a person was actively thinking.
Deepening the mystery, Raichle and colleagues found that the area is part of the network of brain areas most active when the brain is at rest.
Of course, brains never really rest. When not actively engaged, they reflect on personal history, mind-wander, daydream or think about nothing in particular.
“It’s not resting at all. It’s going full-tilt,” Raichle says. In fact, although the brain makes up about 2 percent of an average adult’s body weight, it consumes 20 percent of calories burned each day. And the default network uses about 30 percent more energy than average for other brain regions.
“It is running very hot,” Sporns agrees.
Sporns and his colleagues Patric Hagmann at the University of Lausanne in Switzerland and Van Wedeen at HarvardUniversity used a technique called diffusion spectrum imaging to map the connections between different parts of the brain. The technique traces the path of water moving along axons, long fibers that extend from a neuron’s main body and carry electrical signals.
The researchers were gratified to find that their structural map matched the networks revealed by fMRI, Sporns says.
Computer models of the connections suggest that the brain’s activity may be a product of its anatomy.
“The fact that we engage in certain mental operations at certain times may emerge naturally from the way the brain is wired,” Sporns says. He and his colleagues plan to test whether mental illness and degenerative brain disease can be traced to short circuits in the wiring diagram.
