[Martin Taylor 931222 1800]
(Rick Marken 931222.1430) and later (Bill Powers 931221.1345)
Bill's statement nicely summerizes my impression of this whole
thread on dynamical systems and vortices. If nothing else, the
dynamcial systems approach to behavior manages to make obscure
and arcane what is elegant and simple. The phenomenon of control
is elegant and simple; it is not stability, it is not equilibrium.
Control is active compensation for disturbances to a VARIABLE.
That's fine. If you happened to read my postings on vortices, you
will note that most of them carried the disclaimer that the topic had
nothing to do with PCT. But people wanted to follow that thread, so OK,
I didn't mind too much writing what I thought about it. If it didn't
interest you, no problem.
As for the most recent interchange on dynamics, if you read it you would
note that it isn't about how control systems work, but about one way
of looking at the physical variables that MIGHT be acted upon as part
of a control loop. However, in the future you might find it useful
to keep some of this latter interchange in mind if you want to participate
in a future possible thread on information in control. I previewed a
bit of that, in showing an independent way of determining from outside
whether a variable was being controlled. I expect there will be more,
but I don't mind if it doesn't interest you.
The main goal of PCT is to discover
WHAT variables living systems control and HOW they control
them.
Did you notice that pull-only control systems work differently? In a
structure of pull-only systems, you can't make unique determinations
of what variables living systems control. You can only determine
within what perceptual spaces they control (unless the pull-only
systems are arranged in discrete opposed pair sets). I don't think
that this changes the objectives of PCT, though, since I never really
thought it as important to find what variables are controlled (it changes
by the minute, or faster) as to find out how the controlled variables
are controlled and how the system shifts its control around among the
variables possible to control.
Pull-only control systems do have other interesting aspects besides
being more plausible that push-pull systems. They work naturally as
coarse-coding systems, that allow precise and robust perception in
continuous spaces; they seem likely to allow the kind of reorganization
that involves relinkage, without the problem of large transients; they
allow virtual push-pull control systems with continuously variable gain;
and they seem to fit naturally into what Bill yesterday cited as a
puzzle (Bill Powers 931221.1345)--
A few years ago, in _Science_, I
saw an article on brain-mapping in which a visual target was
shown (measured) as a hump of activation in the visual map, and
when something was moved to that target position, another hump of
activation actually moved across the map to the target position.
Then I lost the reference and I can't find it now.This, of course, suggests a completely different mechanism for
control of spatial perception from the one I use. Instead of
having a simple scalar signal the magnitude of which indicates
position, we have actual movement of activation regions from one
place to another in a geometrical map in the brain.
Bill says:
I think there's some principle buried in there that would make this
method of perceptual control _simpler_ than the way I'm trying to
model it.
Maybe it can be found. That seems to me to be one of the objectives
of PCT, much more interesting than filling in a catalogue of controlled
variables.
ยทยทยท
===============
Continuing with the same posting of Bill Powers:
I'm beginning to think that even the earliest true control
systems must have incorporated a catalyst of some sort, so that a
small input effect could produce a large output effect. While I
haven't been able to elucidate the idea yet, I think that
amplification is the key: anything that gives a "signal" an
effect on a macro variable. This creates the necessary
unidirectionality, so that sensing something has -- all right --
onlyt a minute effect on what is sensed. This is what makes it
possible for a control system to affect physical processes in an
arbitrary way, a way that is not dependent on the process itself.
Yes, that seems eminently plausible. It fits well with what I have
been saying about amplification, but I hadn't thought about catalysts,
which probably are very important.
In living control systems, there is no
energy expended but the modulation energy. You seem to be
visualizing the output energy as a fluctuation in a continuous
energy flow through the muscles.
Since we were talking about self-organizing system in which this would
normally be true, I can see why you would get that impression. But it
would be a waste of food for any evolved system to keep supplying energy
that was only passed into the sink (other than to keep warm, which we
sometimes do by shivering).
Half-wave rectified modulation works fine.
But if there is no error signal,
the energy flow through the muscles is zero. There is no
background energy flow being increased and decreased by the error
signal around some mean value. The error signal is simply
translated into output forces, with a drain on the power supply
that depends on the amount of force. If the required force is
zero, the drain is zero: there is no energy flow to modulate.
But if the power supply is inadequate to supply the modulation energy,
half-wave rectified or not, you won't be able to generate the force.
Look at the difference between aerobic and anaerobic use of muscles.
When you speak of modulations on the main energy flow in the
vortex, you're simply partitioning the only energy flow there is
into conceptually different parts.
Oh yes. That's the critical part. The greater the proportion that goes
into the controlled degrees of freedom, the better. I don't know what
our efficiency is, but I think it's around 10%, isn't it? That's not
too bad at all.
(The following is cited out of context, but I agree with it here as well
as in its original context.)
No point in guessing. It works the way it works.
Let's work on that other example.
Any good ideas?
Martin