The brain's division of gray and white matter optimizes speed of signal traffic.

The physical structure of the brain is an efficient road network designed to minimize delays in the transmission of information, says a paper published in the December, 2005 edition of PLoS Computational Biology.
The human brain is two-toned: A 2 mm-thick layer of gray matter covers an interior composed of white matter. The gray matter acts as a sort of global travel network, while the thinner gray matter handles local traffic flow.

“People can drive long distances to go on vacation; you can drive locally to a store or something,” said coauthor Dmitri Chklovskii, of Cold Spring Harbor Laboratory. “Basically, the same happens in the brain. There are wires that connect nearby regions to communicate signals locally and there are wires that go a long way to connect remote regions.”

Chklovskii and colleague Quan Wen, of the State University of New York at Stony Brook, used engineering to try to determine why brain tissues are divided in this way.

“The idea is that evolution has somehow tinkered the brain and perfected its function and structure,” Chklovskii said. “We’re trying to understand why that physical structure is the way it is, so we can sort of boot-strap our way to understand how it works.”

Chklovskii and Wen tested several theoretical brain designs—such as pipes of white matter running through gray matter, layered planes of gray and white matter, and intermixed gray and white matter— to see which would enable signals to be transmitted the fastest. They found that a design in which gray and white matter are separate can transmit information much more efficiently than a design in which they are intermixed.

“It is better to divide in space the long-range connections so that highways are not interfering that much with local connections,” Chklovskii said.

The theoretical conclusions of Chklovskii and Wen matched real-world observations.

“We can sort of predict that distance where you get around just by using local connections and beyond what distance you have to use the highway system,” Chklovskii said. “That distance is 2 mm, which is the thickness of the gray matter. So that seems to agree.”

Previous studies have proposed that gray and white matter segregation may have developed in order to minimize brain volume.

“The brain may, and probably does, optimize over a variety of criteria which have shaped its current form, acting indirectly as evolutionary selection forces,” said Eytan Ruppin, a neuroscientist at Tel Aviv University, via e-mail.

Jaap Murre—of the University of Amsterdam, who also studied volume minimization in brain design—agrees with Ruppin. Murre cites conduction delays as another factor that may have lead to the design. Although he admits that volume minimization does tend to trump all other restrictions.

“No matter how fast a brain might be, the size of the birth channel remains a pretty hard limit on how large a brain can become.”

Originally published January 31, 2006


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