Net heads: huge numbers of brain cells may navigate small worlds

Science News,  Feb 17, 2007  by Bruce Bower

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Bassett and her colleagues now plan to identify the structures within the brain that hook up to the synchronized networks on the brain's surface. The researchers will use functional magnetic resonance imaging (fMRI), a technique in which scanners measure neural blood-flow changes that reflect surges or declines in cell activity.

For now, remarks Sporns, the new findings indicate that related brain networks operate at different electrical frequencies, each of which acts as a unique channel for transmitting information. The brain needs no central-control mechanism to direct mental life; interactions within and among networks do the trick.

The possibility that the brain steeps itself in flexible, chaotic activity is "an attractive idea;' notes neuroscientist Karl Friston of University College London. He says that Bassett's team now needs to formulate a theory of how low--and high-frequency synchronized networks collectively respond to mental challenges.

PERKING UP The notion that the brain thrives on chaos, in a mathematical sense, comes as no shock to neuroscientist Walter J. Freeman of the University of California, Berkeley. For the past 20 years, he has argued that the brain churns out a cascade of chaotic electrical activity that serves as a "get ready" state. From there, he theorizes, vast expanses of brain tissue shift into electrical-activity patterns that organize thought and perception.

Freeman welcomes the network approach of Bassett and her colleagues. What's critical, he says, is Bassett's observation that brain waves look the same at different frequencies. In his view, this feature allows for split-second transformations from one synchronized network to another.

At the two highest network frequencies measured during the finger-tapping exercise, brain activity synchronized over an area that's at least 22 centimeters long over the brain's folded surface. Freeman has measured a comparably sized area of high-frequency synchronization in rabbits and other laboratory animals as they perform tasks.

"This distance is astonishing, considering that it covers most of each hemisphere containing several billion neurons," Freeman says. "Explaining the large-scale reorganization of human brain activity is the central [neuroscienee] issue of our day."

With mathematicians Robert Kozma of the University of Memphis (Tenn.) and Bela Bolloba's of the University of Cambridge in England, Freeman has developed a model of how clusters of neurons generate chaotic activity in brains at rest. In that model, when an individual searches for a memory or performs other mental work, synchronized brain states of increasing frequency emerge in rapid-fire fashion. As in Bassett's study, each state produces, on its own scale, the same pattern of electrical activity.

Each brain state goes through three steps, Freeman suggests. Synchronization first emerges among individual neurons. It then spreads to interconnected populations of neurons. Finally, large neural structures with specific duties begin to reverberate in unison on each side of the brain.