[ExI] Fwd: [tt] A thermodynamic limit on brain size

Tue May 26 15:34:08 UTC 2009

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From: Eugen Leitl <eugen at leitl.org>
Date: Tue, May 26, 2009 at 7:55 AM
Subject: [tt] A thermodynamic limit on brain size
To: tt at postbiota.org, info at postbiota.org

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From: Technology Review Feed - arXiv blog <howdy at arxivblog.com>
Date: Tue, 26 May 2009 12:43:30 +0000
To: eugen at leitl.org
Subject: the physics arXiv blog

[1]the physics arXiv blog
    _________________________________________________________________

  [2]A thermodynamic limit on brain size

  Posted: 25 May 2009 09:10 PM PDT

  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://[3]arxiv.org/abs/0905.3690: Thermodynamic Constraints on
  Neural Dimensions, Firing Rates, Brain Temperature and Size

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