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A Thermodynamic Limit on Brain Size

If our brains have to be cooled like computer chips, is there a limit on how big they can be?

In recent years, chip makers have conlcuded that the race to produce ever faster circuits is a fool’s game. As the clock speed increases, the amount of energy lost as heat becomes too large to dissipate efficiently and in any case, the waste is unjustifiable.

That raises some interesting questions about the human brain, says Jan Karbowski at the Sloan-Swartz Center for Theoretical Neurobiology at the California Institute of Technology. Karbowski points out that the problem of heat transfer could be a serious factor shaping brain evolution and so has embarked on a program to determine the relationship between brain temperature, its size, cerebral power generated and neural activity.

The question on Karbowski’s mind is whether there is any thermodynamic limit on brain size. And if so, does 5 kg, which Karbowski says is the mass of the largest mammalian brain, approach that limit?

Karbowski points out that brain cooling is not a classic problem of surface-area to volume. Instead, brain cooling is more closely comparable to that in a combustion heat engine where a liquid coolant removes heat.

“In the brain, the role of the coolant is played by the cerebral blood, but only in the deep region because there blood has a slightly lower temperature than the brain tissue,” says Karbowski.

But in the regions closer to the surface, it is the oter way round: brain tissue is colder than the cerebral blood which warms the brain.

This implies that the thermodynamics of heat balance does not restrict the brain size. And this in turn suggests that brains could be heavier than 5 kg, says Karbowski.

(And of course they do get bigger than this. The sperm whale’s brain can be 9 kilograms).

That leaves plenty of growing room for humans which have brains of only 1.5 kilograms on average.

Ref: http://arxiv.org/abs/0905.3690: Thermodynamic Constraints on Neural Dimensions, Firing Rates, Brain Temperature and Size

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