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Is thinking more like a thunderstorm or like a calculation?

Arthur T. Murray uj797 at victoria.tc.ca
Sat Mar 20 11:57:21 EST 1999

Bill Modlin modlin at concentric.net wrote a clear and fascinating post:

[ First, the title was excerpted from a post by Aaron Sloman. - Mentifex]

>Thomas Gigear:
>Also, I just realized, isn't the brain somewhat digital on at least
>one level? [SNIP]
>Gary Forbis:
>I could be wrong about this.  It seems to me too much is made of
>the "digital" firing of neurons.  There is another domain, time, in
>which which they are very analog. [SNIP]
>Kin Hoong:
>Anybody recall the Perceptron?  It was designed on the assumption that
>neurons were digital devices... then somebody whose name escapes me
>proved that they could not do all that much.  Neurons are analogue
>devices.  I seem to recall that even if they fired, the amount of
>chemicals released into the synapse is not always the same (dependent
>on quite a few things) and even if they were, reception is not always
>the same depending on how much stimulation the receptor had just
>received etc.  Sorry I can't remember the details, but they should be
>available somewhere.
>  +++++
>Two things:       [ Now begins Modlin's treatise.  - ATM/Mentifex ]
>First, about the supposed limitations of Perceptrons.
>Minsky and Papert showed that a SINGLE STAGE (or "layer) of
>perceptrons is limited to computing linearly-separable functions.
>It is a trivial result, not nearly worth the attention paid it.
>Unwarranted emphasis on the point retarded progress in neural
>modelling for 20 years, and even now people who quote it without
>understanding continue to propagate the damage it has done.
>The same limitation applies to ANY set of primitive functions, whether
>Perceptrons, arithmetic operations, or things like AND/OR/NOT boolean
>logic gates.  In one stage you can only compute those primitive
>functions.  To compute more functions you have to combine the outputs
>of the first steps with more of the primitives, producing a
>multi-stage computation.
>Adding more devices lets you compute more complex functions.
>Perceptrons with non-linear or thresholding outputs are boolean
>complete, so you can compute any computable function with a network of
>them, just as you can with NAND, or with AND/OR/NOT, or with many
>other sets of primitives.
>Technically, one extra layer of intermediate functions between inputs
>and outputs is enough to compute anything.  This cannonical form is
>inneficient, and practical circuits use many stages.

You mean a little sandwich of three layers can compute *anything*?
(Would that be like the ones and zeros on a Turing machine tape?)
The bang-bang cannonical (sorry!) form must be reiterable, so
that you may simply concatenate whatever it is you want to
compute -- is that how it works?

>Second, about the analog/digital confusion:
>All devices, including the logic gates of a computer, are "analog" in
>basic construction.  The class of digital devices is distinguished by
>having discontinuities in its mapping between inputs and outputs, so
>that the output switches states abruptly as the computed function
>crosses decision boundaries.  Typical computer logic gates and neurons
>all share this property of having discrete decision boundaries, and so
>all are properly classified as digital devices.

In a face-to-face discussion I had for six hours with a netgod of
comp.ai.philosophy on Sun.7.Mar.1999, we discussed the absence of
Bill Modlin from the newsgroup, so I made sure to read this quoted
post -- and it turns out to be one of the most informative Usenet
post that I have ever read.  Thank you, Bill Modlin.  - Arthur M.

>Dynamic digital circuitry depends on the timing of signals for its
>In a computer, time is broken into "clock cycles" and functions are
>computed based on signals presented together in a single cycle.  For
>neural circuitry each cell provides its own asynchronous timing, but
>functions are still based on the coincidence of signals in some
>definite interval of time.  Adjustment of the interval to suit
>conditions encountered is part of the learning process implemented by
>a cell, and this is an analog process.
>The variable releases of chemicals you mention are the variable
>connection weights, and their variability is also important to
>learning.  The weight adjustments are also reasonably considered
>But all these analog processes feed into a decision surface with
>discontinuous decision boundaries for output, so the overall function
>is still properly called digital.
>Bill Modlin

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