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From: wcalvin@well.UUCP (William Calvin)
Newsgroups: comp.ai,comp.cog-eng,sci.misc
Subject: Re: Genesis of language (was: Why can't my cat talk, and a bunch of others)
Message-ID: <4627@well.UUCP>
Date: Fri, 4-Dec-87 21:52:39 EST
Article-I.D.: well.4627
Posted: Fri Dec  4 21:52:39 1987
Date-Received: Thu, 10-Dec-87 06:14:10 EST
References: <1987Dec2.182753.622@mntgfx.mentor.com>
Reply-To: wcalvin@well.UUCP (William Calvin)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 19
Xref: mnetor comp.ai:1195 comp.cog-eng:352 sci.misc:669


Apropos cell death in brains, the old saw about losing 10,000 neurons every
day is now being challenged by the people that work on cerebral cortex; they
seem to think that there is little neuron loss there during most of postnatal
life.  Some subcortical areas like substantia nigra do lose 50% of cells by
age 70, while adjacent regions in midbrain may lose less than 2%.
  But there is a LOT of synapse death -- or, as I like to phrase it, withdraw
of axon collaterals, breaking synapses.  Synaptic density in neocortex
peaks at 8 months after birth (in humans; 2 months in monkey) -- and then
drops by 30-50% during childhood.  After puberty, the data gets too noisy
to interpret.  So there is a lot of opportunity for Darwinian editing of
randomly-made synaptic connection, achieving information storage by carving
(rather like photographic development removes unexposed silver grains).
	I review a variety of Darwinian selection stories in my piece
in the 5 November 1987 NATURE 330:33-34, entitled "The brain as a Darwin
Machine."
		William H. Calvin
		University of Washington NJ-15, Seattle WA 98195
		  206/328-1192  wcalvin@well.uucp