Last November, Max More informed this list of the on-line
article "Ghost in the Machine: What Neuroscientists and
Computer Scientists Can Learn from Each Other" at
http://www.doubletwist.com/news/columns/article.jhtml;$sessionid$WLUGKNIAAA5EBWBCHIVSFEQ?section=weekly01&name=weekly0130
I replied to this with a description of and some excerpts from a book
I had just read, _Darwin Machines and the Nature of Knowledge_ by
Henry C. Plotkin, 1993, Harvard University Press (paperback
edition, 269 pages; see
http://www.amazon.com/exec/obidos/ASIN/0674192818/qid=974465462/sr=1-2/107-6292930-3771726
(<4Q 2000> isn't yet in the Extropians' archive, so unfortunately
I can't give a link to my review). I concluded my post
with the remark:
> Since Plotkin mentions Gerald M. Edelman and Jean-Pierre
> Changeux in the same breath, I ordered a book by the
> latter: _L'Homme Neuronal_ (Fayard, 1983). I have never
> before attempted to read a serious book in French
> (never having gotten much beyond _Le Petit Prince_, and that
> was a long time ago), so I may have bitten off more than I
> can chew. Nevertheless, I still get a thrill from being
> able to use the Web for international shopping -- I ordered
> the book from a Paris on-line bookstore,
> http://www.chapitre.com
Since I've finished this book I thought I'd mention it
briefly. Fortunately, I didn't have to struggle through
it in French; there's an English translation in the
Princeton Science Library entitled _Neuronal Man: The
Biology of Mind_ (Princeton University Press, 1997), see
http://www.amazon.com/exec/obidos/ASIN/0691026661/qid=983820830/sr=1-1/ref=sc_b_1/102-1083400-8739317
I don't know how I managed to miss the existence of the
translation the first time around; I saw it and bought it
at a local Barnes & Noble, and then the French copy I had
ordered too hastily on-line turned out to have been sold,
so I was lucky.
The book is a good introduction to neuroscience for a
layman (like me), with a heavy emphasis on the history
of the field and with a more detailed look at neuranatomy
than Edelman (for example) provides. If Changeux is less
inspiring than Edelman -- he offers no pretense of
constructing the scaffolding for a theory of consciousness --
he's also less susceptible than Edelman to the accusation
of indulging in speculative neuroscience. One is
conscious that this is a translation; one can sense the
creaking of strained idioms from time to time.
Here are a couple of representative passages. From Chapter 2
"The Component Parts of the Brain", section heading "From Mouse
To Man":
"No category of cell, no particular type of circuit is
specific to the human cerebral cortex. The components of our
cerebral machinery derive from a stock very similar, if not
identical, to that of the mouse. The major event in the
evolution of the mammalian brain, as we have seen, is the
expansion of the neocortex... The number of cellular
elements per unit of surface area has not changed... On
average, the cortex of man is only three times thicker than
that of the mouse... The more the surface area of the
cortex expands, the more the number of neurons capable of
establishing association connections increases; the area of
the association cortex thus becomes relatively greater than
that of primary sensory and motor areas. This translates,
finally, into an increase in the mean number of connections
per neuron... Nevertheless, the increase in the mean number
of synapses per neuron is not directly proportional to the
increase in cortical area. Far from it. The density of
synapses per cubic millimeter of cortex is of the same order
in the rat as in man. Because the thickness of the cortex
increases only a maximum of three times, the mean number of
synapses per neuron in man can hardly exceed three times that
of the rat, whereas the area of the cortex is four hundred
times greater. The increase in the mean number of synaptic
contacts is thus not enough to explain entirely the increase
in complexity of the cerebral cortex in the course of
mammalian evolution. Other parameters intervene, such as the
diversification of cortical areas...
At the level of both the macroscopic anatomy of the cortex
and its microscopic architecture, no sudden qualitative
reorganization marks the passage from the "animal" brain to
the human brain. There is, on the contrary, a continuous
**quantitative** evolution in the total number of neurons,
the diversity of areas, the number of possible connections
between neurons, and, therefore, the complexity of the
neuronal networks that make up the cerebral machine."
And from Chapter 9, "The Brain -- Representation of the
World":
"A mental object is by definition a transient event. It is
dynamic and fleeting, lasting only fractions of a second.
The [functional characteristics] of the neurons that form
it, however, are much more stable; they are built up during
development by mechanisms involving internal genetic
expressions and regulations stemming from a chain of
reciprocal interaction with the environment... This imprint
of the physical and sociocultural world remains stable
for many years, even throughout the life of an individual...
The major features of the organization of the brain, which
ensure the unity of humankind and are subject to genetic
constraint, also constitute a representation of the world,
built up over generations by the evolution of the genome
of our fossil ancestors.
Consequently, the human brain contains, or produces, at
least three major categories of representations of the world.
The kinetics of the formation of these representations
and their stability encompass time scales ranging from
tenths of a second to hundreds of millions of years. Each of
these modes of representation widens the horizon of the world
represented. The "rigidity" of a brain determined entirely
genetically would, from the very beginning, impose a limit
on the numbers of possible operations. The capability of
constructing labile representations opens the organization of
the brain to the social and cultural environment. These
"new worlds" can evolve on their own -- limited, however, by
rules that are determined by the overall performance of the
cerebral organization.
If the hypotheses put forward [in this book] are correct,
the formation of each of these representations, although using
different elements and different levels of organization, obeys
a common rule, inspired by Darwin's original hypothesis. A
process of selective stabilization takes over from diversification
by variation. The mechanisms associated with evolution of the
genome[,]... [c]hromosomal reorganization, duplication of genes,
recombinations and mutations, all create genetic diversity, but
only a few of the multiple combinations that appear in each
generation are maintained in natural populations. During
postnatal epigenesis, the "transient redundancy" of cells
and connections and the way in which they grow produce a
diversity not restricted to one dimension like the genome,
but existing in the three dimensions of space. Here again,
only a few of the geometric configurations that appear during
development are stabilized in the adult... Does such a
model apply for the more "creative" aspects of our thought
processes? Is it also valid for the acquisition of knowledge?
...
It is... worth noting that in the history of ideas "directive"
hypotheses have most often preceded selective hypotheses.
When Jean-Baptiste de Lamarck tried to found his theory of
"descendance" on a plausible biological mechanism, he proposed
the "heredity of acquired characteristics", a tenet that
advances in genetics would eventually destroy. One had to
wait almost half a century before the idea of selection was
proposed by Charles Darwin and Alfred Wallace and validated
in principle, if not in all the details of its application.
In the same way the first theories about the production of
antibodies were originally based on directive models before
selective mechanisms replaced them. It could conceivably be
the same for theories of learning."
Jim F.
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