Vertebrate brains generally contain two kinds of tissue: gray matter and white matter. Gray matter contains local networks of neurons that are wired by dendrites and mostly nonmyelinated local axons. White matter contains long-range axons that implement global communication via often myelinated axons. What is the evolutionary advantage of segregating the brain into white and gray matter rather than intermixing them?

In this study, the authors postulate that brain functionality benefits from high synaptic connectivity and short conduction delays—the time required for a signal from one neuron soma to reach another. Using this postulate, they show quantitatively that the existence of many fast, long-range axons drives the segregation of the brain into gray and white matter. The theory not only provides a possible explanation for the structure of various brain regions such as cerebral cortex, neostriatum, and spinal cord, but also makes several testable predictions such as the scaling estimate of the cortical thickness.

The theory is based on the idea that maximum brain function requires a high level of interconnectivity among brain neurons but a low level of delays in the time it takes for signals to move through the brain ("conduction delays").

Based on no fewer than 62 mathematical equations and expressions, the theory ("Segregation of the Brain into Gray and White Matter: A Design Minimizing Conduction Delays") provides a possible explanation for the structure of various neurological regions including the cerebral cortex and spinal cord.

The research was carried out at Cold Spring Harbor Laboratory on Long Island by theoretical neuroscientist Dmitri Chklovskii and graduate student Quan Wen. "We present our theory in Results, which is organized into seven sections. In the first, we consider competing requirements between small conduction delays and high connectivity in local circuits. We show that local conduction delay limits the size of the local network with all-to-all potential connectivity to the size of the cortical column. The second section models full brain architecture as a small-world network, which combines high local connectivity with small conduction delay. We derive a simple estimate of conduction delay in global connections as a function of the number of neurons. In the third section, we consider spatially integrating local and global connections. We argue that mixing local and global connections substantially increases local conduction delay, while the global conduction delay may be unaffected. In the fourth section, by minimizing local conduction delay we derive a condition under which white/gray matter segregation reduces conduction time delays. The fifth section gives a necessary condition for the segregated design to be optimal, and an example of such design is given in the sixth section. Finally, the seventh section restates our results in terms of the numbers of neurons, interneuronal connectivity, and axon diameter."

The study was published in the December issue of PLoS Biology and is available >at *Segregation of the Brain into Gray and White Matter: A Design Minimizing Conduction Delays* by Quan Wen and Dmitri B. Chklovskii. PLoS Biology, december 30, 2005. DOI: 10.1371/journal.pcbi.0010078

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