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brain/body size and intelligence

John E Anderson jander at unf.edu
Tue Nov 9 23:31:00 EST 1999

JoshCahoon wrote:
> .>within this 'equation', big bodies require comensurately-big nervous systems
> >be-cause they have to signal longer distances within that bigger body, which
> >means that neruons have to be larger so that they can contain larger versions
> >of the energy-transformation stuff that empowers neural activation dynamics.
> >
> So blue whale have huge neurons? And rat neurons are puny? And ours are
> somewhere in between?

If I had to guess, I would say that the size of the cell bodies of whale
neurons, the parts of the neurons that would be in the brain itself,
would not be larger than rat or human neurons in the same proportion as
body size.  Does anybody know?  Wouldn't it be more likely that larger
animals have larger brains because they need more neurons to handle the
greater surface area of muscle and sensory sheets?

> Even if that is the case, it seems to apply significantly only to motor and
> sensory cortex.  The size of the body doesn't result in association cortex
> having to communicate across longer distances.

That's the point.  You would expect brain size to increase roughly in
proportion to body size whether there were association cortex there or
not.  That is, in the absence of association cortex, the ratio of
sensorimotor brain size to body size should be about the same as body
size increases.  However, the addition of association cortex in a more
intelligent animal will increase the ratio over and above that due just
to the sensorimotor cortex.  So more intelligent animals will have
larger brain size/body size ratios.

John E Anderson
Department of Natural Sciences
University of North Florida
Jacksonville FL

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