X-Message-Number: 14933
Date: Thu, 16 Nov 2000 07:17:04 -0500
From: Thomas Donaldson <>
Subject: once more on brains and computers

HI everyone!

Various individuals have complained about my posting. Basically they
believe that Turing machines can imitate ...WHAT? One fundamental
issue here is just how far Turing machines can go. (I do want to make
it clear here that I have no objections to the possibility of making
something which works like a human brain. What remains unknown is
whether or not such a device fits the Turing machine picture at all;
and yes, depending on the behavior of the hardware in such an imitation,
it may have to be biological or close to biological because no other
materials match the job that biological ones can do).

First of all, it is NOT enough to simply say that connections generally
are close by. The problem is that we deal with many neurons, and the
connections of one such neuron aren't independent of those of the 
others. The neurons might grow new connections to neurons with which
they did not at first connect. Changing only one neuron won't work.
Second, no one complaining about what I said dealt with the even more
fundamental problem that it looks as if we also grow new neurons 
throughout our brain... many mammals do this to a lesser degree, but
apes and monkeys do it much more. 

Anyone who wishes is welcome to produce a proof that such a creature
(device) follows all the requirements of a Turing machine. I said
originally that I did not consider it obvious that it does... not the
same as saying that it doesn't. Why isn't it obvious? Basically because
by its nature our brain does not in any sense write on a single long
tape. The different neurons with different connections go off and 
write on many tapes, connected as if they were a treelike graph. Or
if you insist on a tape, that tape will have to have the ability to
split into 2 tapes, etc etc. Moreover, one simple feature of our brains
is not matched at all by ANY Turing machine: they work in real time,
while a Turing machine is a single computer with a very long tape
on which it works. If we were only proving math theorems (or any
other purely theoretical act) then this would make no obvious difference.
However our literal survival may depend on whether or not we finish
some computation in a small fixed time. Time is IMPORTANT, and any
attempt to understand how we work must first understand that point. The
creation of new neurons, on top of all of this, means that such a 
system does not have a clear limit on the number of tapes it's
following. (Yes, in practice there is a limit, but it is FUZZY, not
a definite figure). 

So you're welcome to discuss whether or not such a machine can be
truly matched by a Turing machine. But don't forget any of the 
features I have given here: new connections, new neurons, and the need
to do things much faster than any single machine, even the brightest
such machine.

		Best wishes and long long life for all,

			Thomas Donaldson

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=14933