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

Science News,  Feb 17, 2007  by Bruce Bower

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About 40 years ago, the late psychologist Stanley Milgram tapped into the commonsense notion that "it's a small world." Milgram asked 60 people to send a folder to a certain individual whom none of them knew. Participants were given a little information about the target person and asked to mail the folder to a friend or acquaintance who, in their view, was more likely to know the stranger than they were. Each recipient of the folder was asked to do the same, until the material reached its destination.

Only one-quarter of the chains were completed. In those cases, though, the folder passed through an average of six intermediaries. Milgram's project inspired the phrase "six degrees of separation" and led to, for example, people calculating movie actors' working relationships to actor Kevin Bacon.

The small-world phenomenon got a big boost in 1998. Steven Strogatz of Cornell University and Duncan Watts of New York University used mathematical simulations to show that all sorts of large networks can be traversed in a small number of steps. Strogatz and Watts demonstrated how this effect applies to the more than 4,300 elements of the electric-power grid in the western United States and to the collaborative relationships of more than 225,000 professional actors.

Strogatz and Watts also demonstrated the relevance of the small-world idea to the array of 282 brain cells in worms called nematodes.

Small-world networks have a distinctive structure: There's a cluster of nodes, each connected to its immediate neighbors, with a few that connect to distant nodes. This structure enhances the power and efficiency of these systems, Strogatz and Watts argued.

More and more neuroscientists agree. Motivated by Milgram and his mathematical progeny, researchers are now devising models grounded in the small-world effect to explain how the human brain works. These scientists are looking for small-world setups within the brain's massive, interconnected cell networks and for moment-to-moment electrical manipulations that, they suspect, foster thinking and learning. Their efforts are a sharp departure from popular brain-imaging efforts to pinpoint neural niches that specialize in particular mental capabilities.

"Researchers have just begun to apply a huge arsenal of approaches to understanding how brain networks are patterned, how they evolve and grow, and how they generate dynamic structures," says neuroscientist Olaf Sporns of Indiana University in Bloomington.

FRACTAL FREQUENCIES The 22 volunteers recruited by neuroscientist Danielle S. Bassett of the National Institute of Mental Health in Bethesda, Md., and her colleagues didn't draw a tough assignment. Each participant simply lay under sensors that, at 275 points across the scalp, measured the magnetic field produced by neurons' electrical discharges on the brain's surface. Half watched a computer screen and tapped their right index fingers when a designated shape such as a square appeared. The rest saw the shapes but weren't asked to do anything in response.

Their brains did plenty, though. Bassett's team analyzed the six types of brain waves that showed up in all the participants. Each wave type crackled at a specific frequency, the result of millions of cells at various locations emitting synchronized signals.

From the electrical-activity associations that the researchers noted at pairs of scalp points, they constructed a simulated brain network. This provided an outline of which brain

areas were working together at each of the six frequencies. It also revealed that at each frequency, brain networks exhibited a small-world arrangement. Clusters of closely grouped neural junctions typically incorporated a few connections to distant locations.

Intriguingly, each frequency-specific brain wave looked like all the others did, although it operated on a unique scale. Biological patterns that repeat in this way over different scales of measurement are known as fractals.

Fractal, small-world brain networks reverberate in an electrical limbo state that almost, but not quite, comes unglued, Bassett's group reports in the Dec. 19, 2006 Proceedings of the National Academy of Sciences. Especially at higher frequencies, these networks operate "on the edge of chaos," the researchers say. In that precarious condition, synchronized activity relegated to a small brain area can rapidly expand into far-flung neural regions to deal with new challenges or situations, the team proposes.

Although brain networks looked much the same whether volunteers tapped their fingers or did nothing, one notable difference emerged. During the tapping task, the networks delineated by the two highest frequencies of synchronized cell firing displayed novel long-distance connections between the frontal brain and an area toward the back of the brain. Since high-frequency, synchronized neural activity may foster perception, memory, and consciousness (SN: 11/13/04, p. 310), Bassett suspects that this neural response guided a participant's decision to tap when shown the various visual cues.