Newsgroups: bit.listserv.csg-l
Date:         Tue, 22 Dec 1992 18:41:31 EST
From:         mmt@BEN.DCIEM.DND.CA
Subject: Re: Misc replies

[Martin Taylor 921222 18:15]
(Bill Powers 921222.0800)

> As long as the list of people actually
>devising and carrying out experiments and modeling is limited to
>Rick Marken, Tom Bourbon, and me, the queue of possible
>experiments with HPCT is going to grow while the actual work done
>trudges along at a slow pace.
>
>When I last looked, there were 132 subscribers to this list.
>Permit me a moment of impatience: when are some of you people
>going to get out of your armchairs?

Well, we are trying the Little Baby experiments, and are doing preliminary
work for Genetic Algorithm experiments, both to study reorganization.
Does that count?
=============
--------------------------------------------------------------
>Martin Taylor (921221.2000) --
>
>>If a generative model does predict reality well, without
>>excessive use of parameters, then it produces strong evidence
>>of the plausibility of the theory that underlies it.
>
>The control system model we use predicts detailed handle
>movements within 5 to 10 percent with a single adjustable
>parameter, the integration constant. By adding a transport lag,
>we can approximately halve that error. So at most we need two
>parameters to reduce prediction error to 2.5 to 5 percent. If we
>introduce nonlinearities, we might get that down to 2 to 4
>percent. But diminishing returns will set in pretty quickly. I
>figure that we are pretty near the noise level set by the
>discrete nature of neural impulses.

I intended to imply that PCT had that kind of requisite support.  It's
one reason why I think it worthwhile working on the foundations.  But
I'm equally impressed with the argument from evolution and other
more abstract arguments of necessity and possibility.  They all support
one another.
============

>I think you're going to have to take us back to a more elementary
>level. I still don't understand why a perceptual signal that is
>maintained in a match with a changing reference signal is said to
>have a LOW information content.

...

>>In the 2Hz case, the presumption is that all these
>>probabilities are flat (actually Gaussian) distributions,
>>maximizing the information that could be transmitted.
>
>Who is doing this presuming in the system? I asked why the
>perceptual and reference signals should not both be considered to
>carry an information flow appropriate to a signal varying within
>a 2Hz bandwidth. You didn't answer that.

The presumption was one that you (Bill Powers) would have to make
in order to assert that the perceptual and reference signals would
convey to you (Bill Powers) information at a rate appropriate to a
2Hz bandwidth.  To anyone else, the information rates might be different.

I'm working on a more considered presentation of the information-theory
stuff, as requested.  But it isn't going to be immediate.  To tighten
it up and make it didactically useful is going to take work, especially
since I have to go back to basics, as these questions suggest.  It could
be that I will make up some document to be deposited with Bill Silvert
or to be circulated on paper (lots of pictures required).
==================

(Tom Bourbon 921222  10:10)

Tom,
Thanks for your considered (and considerate) posting.

> I offer this suggestion:  Demonstrate that you can work from
>first principles in information theory and (necessarily?) arrive at
>an architecture identical to that in the PCT model.  If the model
>you derive is identical to the PCT model, you are right; there is
>no need to simulate it -- to run it.  But if it differs in any
>details, run it, to confirm that it behaves as you think it will.
>That step should satisfy any questions, doubts or criticisms I have
>seen directed toward your posts about information theory and PCT.

Yes, I started working on it after yesterday's postings.  See above.
I would very much like to see it actually run as a model of the type
you like, but as a practical matter I spend much more time on CSG
matters than I should (as well as having contractors work on PCT-related
issues).  I am not a strong programmer, though I sometimes find it fun.
Quite probably I will write the document that Bill asks for, and then
see whether it opens fruitful lines of discussion that might induce me
or someone else to do the actual generative modelling.  Producing the
document has obvious benefits for me, to make the ideas more precise,
rather than intuitive, as in the posting to you.  And it would seem
necessary if the ideas are to be propagated (assuming that they have
value).

>I believe a major question that is unresolved for some of the
>modelers is whether you would necessarily arrive at the PCT
>structure.  Couldn't you just as easily arrive at other, sometimes
>implausible, structures?  I have seen information theory used to
>justify or explain many varieties of theory in behavioral and
>cognitive science.  Why should one person arrive at a PCT
>structure, when so many others did not?  I am not saying that you
>will not, just that I do not see the necessity that you will.

You are quite right about the (mis)uses to which information theory
has been put, and this paragraph will make me look much harder at
my assumptions than I otherwise might have done.  I do not want to
provide a circular argument "PCT, therefore PCT."  What I believe
should come out is "chaotic world, therefore PCT."

>> It's hard for me
>>to get back to basics (or even to exact formulae, since I don't use
>>them much), but it will be a good exercise for me to try.
>
>Try it!  You might like it!  Even should the task prove daunting
>(which, for you, I doubt), the least you can expect is that your
>personal insights about PCT and information theory will be more
>easily digested by readers on the net -- the discussion will be
>much more likely to remain on a technical level.

I think I would like it eventually.  But as with getting into physical
shape, the muscles hurt during the build-up period, so one tends to
avoid the exercise.

One problem I have is that I always feel that if I understand something,
then any moderately competent person with whom I wish to communicate
will also have understood it.  Intellectually, I know this not to be
true, but it always feels to me to be patronizing when I start to explain
something that the listener presumably knows.  This problem causes great
confusion when I start speculating on the basis of things that are not
known to the other parties.  That's obviously what is happening here.

Anyway, thans to you both.

Martin
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