From: mcochran@nyx.cs.du.edu (Mark A. Cochran)
Subject: Re: Myelin (Was Re: Spoken Like a True ProLifer)
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Date: Fri, 22 Jan 93 03:58:02 GMT

In article <JBATES.93Jan21035438@pinocchio.encore.com> jbates@encore.com (John W. Bates) writes:
>
>By the way, I just started catching up on this thread, and noticed
>that earlier Steve provided a reference to the same research I did,
>except he did it much earlier than I did. My apologies for posting
>it again. I could cancel the article, but I don't feel like it. Nyah.
>
>In article <1993Jan21.034431.24481@organpipe.uug.arizona.edu> sfm@manduca.neurobio.arizona.edu (Stephen Matheson) writes:
>> From article <1993Jan20.062913.13725@mnemosyne.cs.du.edu>, by
>> mcochran@nyx.cs.du.edu (Mark A. Cochran):
>
>[much deletion.]
>
>>>>>>>>My contention remains: large segments of the nervous system
>>>>>>>>function beautifully without myelin.  Myelin has a very
>>>>>>>>specific purpose: it allows for fast conduction of impulses.
>>>>>>>>It must not be indispensible for function, because it is
>>>>>>>>anything but ubiquitous.  While it is reasonable to guess that
>>>>>>>>speedy conduction would be advantageous to a complex network,
>>>>>>>>it seems reasonable to assume that one can design a network
>>>>>>>>without it.
>
>>>>>>> Kind of like trying to build a hypercube out of a bunch of
>>>>>>> C=64's? I'm sure it can be done, but would it actually be
>>>>>>> capable of performing any resonable work?
>
>>>>>>Huh?  Can you provide some hypercube and C=64 references? :-) Are
>>>>>>they available on request?
>
>>>>> A hypercube (as I understand it, but it's not my field at all) is
>>>>> a bunch of cross-linked multi-processor computers that tries to
>>>>> simulate the neural system by multiple cross-links. The C=64 is
>>>>> the ancient Commodore 64 that people have always laughed about.
>>>>> It would be interesting (funny even) to see what would happen if
>>>>> Commodore got hte idea to try this for real. :)
>
>>>>Wow.  Mark is a mrpmsysp UGR futrvits (Mark's occupation has been
>>>>encrypted at his request; decoder available from T.S.A.K.C. BBS)
>>>>*and* a neural network guru.  That's amazing.
>
>>> Huh UH! No Way! See right up there where I specifically deny any
>>> expertise in this area? I've read a litle bit, and talked to
>>> people, but no *way* are you goin to get me to play neural network
>>> sysadmin here. ;)
>
>> Very well.  You're the one who brought it up.  I wonder if John
>> Bates can get enough free time to comment on the applicability of
>> your analogy.
>
>Sorry, Mark, but you've got things a little mixed up. In networking
>lingo, hypercubes are a network designed for supercomputing, with
>a node at each vertex connecting to each neighboring vertex. It's not
>related to neural networks at all. The closest neural network design
>I can think of is James Anderson's "brain state in a box", in which 
>each output pattern is a vertex of an n-dimensional box. Nice model 
>for associative memory, but n tends to have to be very large (in 
>computational terms) for it to be useful.
>
It's a good thing I made sure to deny any real knowledge of
hyper-cubes then, isn't it? :)
I still bet you can't build one out of C=64's though.... :)

>I've been leary of bringing models into this discussion, since it is
>often hard to relate models of neural networks to actual neural
>networks. But now, let me refer to a model by Stanislas Dehaene and
>Jean-Pierre Changeux, which simulated the performance of human
>infants in Piaget's A not B task. Their results approximated the
>performance of human infants. 
>
>The interesting part of the experiment, though, was that they varied
>the amount of "noise" that the network received. At high levels
>of noise, the network performed at the level of a 7-month old infant,
>but at low levels, it performed at the level of a 12-month old infant.
>Noise levels seemed to correspond to the development of myelin in
>the frontal lobes.
>(from the _Journal of Cognitive Neuroscience_ 1:3, S. Dehaene and J.P.
>Changeux, A simple model of prefontal cortex function: delayed
>response tasks)
>
Interesting. If they were able to jump the noise level around to
approximate various developmental stages, I wonder if they could/did
jump it up to a level that would approximate that development of, say,
a 22 week fetus?
Be interesting to see the results if they did. It could shed some
light on this subject, at least.

[Crabs and Squid deleted, since I've already had lunch]

>>>>> The resonable work in question, though, is thought. Just as you
>>>>> can't use a 4 bit 16K RAM computer as an effective file server, I
>>>>> don't see how you cna use the similarly limited abilities ofthe
>>>>> pre-myelinated neural system as a 'thought server'.
>
>>>>I'm reserving my judgment on the matter for a time when we know
>>>>more about all the issues involved.  The hypomyelinated mice
>>>>discussed later in the post may be our best window into this issue.
>>>>In the meantime, it must be obvious to everyone reading this thread
>>>>(all 3 of us :-) that neither of us has any clue about whether
>>>>myelin is necessary or not.  I think that there is at least a fair
>>>>amount of information on the biological side that suggests that
>>>>myelin is not as central as some claim.  On the other hand, your
>>>>arguments about the presumed complexity of the network are
>>>>certainly thought-provoking (there's that smell again...myelin
>>>>burning or something).
>
>Yes. The major problem that we have in modelling brain processes is
>the complexity of the whole thing. I mean, our supercomputers have
>problems with 2-3000 neuron models. Massively parallel systems
>reach the 16-32000 neuron level. How much of the brain is actually
>dedicated to thought? Maybe what, 10^10 neurons?
>
I recall reading we'd need a computer the size of (something like)
Manhatten to approximate the brain. Does that sound like a resonable
size?


--
                  Mark Cochran          merlin@eddie.ee.vt.edu
These are the views of my employer, your employer, your government, the
Church of your choice, and the Ghost of Elvis. So there.
Member, T.S.A.K.C.