[Avery Andrews 930130.1100
(mostly Bill Powers (930129.1300))
Feedback too slow:
My understanding was that the famous left jab
*takes* 40ms from start to finish. If this is true, it clearly
can't be done by tracking a step-change in a position reference-level.
I don't think it's true that you necessarily either have a control system
(all the time) or not - just run the positional information through
an inhibitable interneuron on its way to the comparator, & you can
switch off the feedback by turning on the inhibition. & it doesn't
disturb me that you would need a pretty fancy circuit element to
generate the commands for these movements - after all, it take a
tremendous amount of practice to acquire such things. And there's
all this talk about `refractory periods' and the like which could be
evidence for various kinds of inhibitions being switched on and off.
& I'd agree that `for what' is the right thing to say if someone
says `feedback is too slow' & you want to respond brusquely,
but I thought my slightly subtler way of saying the same thing
would be more useful. After all, we don't want PCT to be taken
up by people who just believe what we say, but by people with some
capacity to draw their own conclusions from whatever happens to
be in front of them. You don't have to say it all at once, and often
it's better not to, I think.
Pattern Generators:
My line on them is that they're just irrelevant to the question of
what the role of feedback is. People thought they were a problem
because they confused feedback with peripherally mediated response
chaining. What CPGs normally due is specify perceptual reference
level contours. From what little I've read about gait control in
insects, it seems to involve a complex mix (different from species
to species) of CPGs, response-chaining effects, and actual control.
I expect Adams (1971) to be a very informative source on this
subject, but we shall see.
Output Blunder:
We're using the term in two different ways. Your `output blunder' is
the belief that there are effectors that just produce the results
intended. Mine is a mistaken idea about what feedback means, the assumption
that feedback means monitoring something `directly' produced by the effectors
(an incoherent notion, I would say, but people really do seem to believe in
it). I think this is a critical mistake, which, for example, gives us the
mumbo-jumbo of `coordinative structures' rather than feedback control
of relationships. There may be a better term for the blunder, but
it's got to be a snappy one, since it's so fundamental. Maybe I'm
just getting carried away by enthusiasm, but I'll go so far as
to suggest that it may be the *most important* of the blunders.
Kugler et. al.:
One thing that would be extremely useful is for somebody who really knew
their differential equations to look into Turvey, Kugler et. al. I can't
really be sure whether they are saying profound and useful things (while
being thoroughly dishonest in their portrayal of what other people
are doing), or whether they are just making straightforward things look
deep, dark and difficult by putting them in the most abstract mathematical
setting they can find (e.g. is `limit oscillator' just another way of
talking about an oscillator whose output is a perceptual reference level).
My nose tells me that there's more than a little of the latter in their
story, but my math is too weak to be sure.
Linguistics:
One of the reasons the linguistics discussions are so convoluted is
that people want to talk about the subject before having learned much
of anything about it, and it isn't plausible for Bruce or I to try to
run a basic linguistics course on CSGNet. I don't have any problem with
the stuff that Ray Allis said, or that various other people said later,
but it doesn't have much to do with what linguists are talking about when
they talk about something like `the structure of the lexicon'.
Linguistics is mostly about patterns.
Unfortunately, most of the verbal patter that goes along with linguistics
is nonsense, so that critically minded people without an aptitude for
perceiving the patterns get bogged down in the nonsense, and never
get any sense of what is going on (generative grammarians are perfectly
happy with the idea of distributed representation of roots in the lexicon,
for example, especially because the properties of `strong verb tense stem
formattion (dig vs. dug, sing vs. sang vs. sung) are highly consistent
with what you'd expect from distributed representations). If I thought
that sorting out linguistics was a high priority for PCT, I'd spend more
time trying to make it look sensible, but I just don't think it's the best
use of my time. Something a lot more promising in the intermediate
term would be PCT phonetics and then phonology, since some of the
work on `distributed compensatory responses' is actually about
articulatory phonetics. & I've never seen an introductory phonetics
book with a coherent discussion of the difference between happening
to produce a sound and successfully controlling for producing it
under a variety of circumstances.
Avery.Andrews@anu.edu.au