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

Science News,  Feb 17, 2007  by Bruce Bower

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This approach to modeling brain function, which Freeman's group has dubbed neuropercolation, incorporates a small-world network into other neural features. For instance, the model includes some nodes that depress the activity of surrounding nodes and others that excite their neighbors, much as the brain contains cells that specialize in inhibiting or arousing each other.

Neuropercolation builds on the mathematics of percolation theory, which has long been used to model the sudden, exponential spread of forest fires and viruses. Recent research by Freeman's group appears in the March 2006 Clinical Neurophysiology. Network approaches to the brain will dampen neuroseientists' current passion for cordoning off patches of tissue presumed to specialize in various mental functions, Freeman asserts. He says that researchers err when they regard brightly colored neural spots in fMRI images as areas with unique responsibilities for a mental function being studied. These "great red spots" represent hubs of activity in larger, constantly shifting neural networks, he argues.

Both his group and Bassett's team have found hubs of particularly intense activity within networks of synchronized brain cells. These hubs arise where neural connections are especially numerous.

However, fMRI investigators such as Friston still see value in the search for brain regions with specialized duties. Activity hubs probably integrate information shuttled in from other brain locations, Friston proposes.

DARK ENERGY Studies of brain activity, mostly with fMRI, have left neuroscientists with a puzzling discovery: The additional energy required for the brain to perform other mental tasks is extremely small compared with the energy that the brain expends as an individual does nothing at all. New models of brain networks offer clues to why the resting brain generates so much energy.

In the Nov. 24, 2006 Science, neuroscientist Marcus E. Raichle of Washington University School of Medicine in St. Louis refers to the brain's intrinsic activity as "dark energy" because its functions remain mysterious. Studies indicate that fewer than 10 percent of neural connections ferry information from the external world, Raichle points out. This small proportion suggests that intrinsic activity carties out vital duties, he holds.

Raichle rinses three possible explanations for the brain's dark energy. It may in part stem from a person's random thoughts and daydreams. Intrinsic activity might also emerge from neural efforts to balance the opposing signals of cells simultaneously trying to jack up and cool down brain activity. Or it could occur during an internal process of generating predictions about upcoming environmental demands and how to respond to them.

The fractal, small-world networks observed by Bassett's team in resting volunteers, Sporns says, probably create the intrinsic activity that intrigues Raichle.

Freeman seconds that notion, emphasizing that chaotic network activity at rest reflects each individual's past experiences and expectations of upcoming events. "You see what you expect or are trained to see, not what is there" he says.