[From Bill Powers (950518.2020 MDT)]
Bruce Abbott (950518.1145 EST) --
My impression of things like the lobster neural network (there have been
others, such as for Aplysia) is that I'm hearing people say "Here's a
green dot, and right next to it is a red dot at bearing 120 and a blue
dot at bearing 240, and then a row of black dots at approximately equal
intervals, and then crossing that row, a row of green dots next to a row
of black dots ....". I keep wanting to yell, BUT WHAT IS IT? If you
back away just far enough, you see that it's a color half-tone picture
of somebody.
Of course it's not that bad in this case, but there's some of the same
flavor.
The pyloric network in the somatogastric ganglion consists of 14
neurons, most of which are motor neurons, which are interconnected
both via inhibitory synapses _and_ a bidirectional _electrical_
pathway. A sine-wave-like pattern of ion channel openings/closings
on the cell membranes of some of the motor neurons produces a
rhythmic pattern of polarization/depolarization in these cells that
contributes to the production of a regular pattern of impulses to
produce a specific sequence of muscle contractions.
If you think about our models, how much could you convey about how they
work if you just described the connections and didn't specify the
parameters? Do you think that a listener drawing a diagram from your
description would ever tumble to the fact that with a high gain in the
loop and the right slowing factor, this system could control something?
For me, the bare recitation of where "impulses" go is almost totally
uninformative, especially when the report includes details like calcium
channels opening and closing. I want to know the transfer functions --
the relationships of input signals to output signals for each neuron, in
terms of frequencies. Maybe the rhythmic signals are being produced by a
single-neuron oscillator, but maybe they're produced by a feedback
system with parameters set to make it drive itself into oscillations,
with neural signals being looked at in terms of frequencies. When you
look so closely that all you can see are the individual impulses, it's
just like looking at a half-tone picture under a microscope. You can see
what is happening in great detail, but you can't see WHY it is
happening.
The pyloric system has been shown to operate normally after
dissection (when placed in a physiological saline solution without
any sensory connections that might supply feedback; thus the system
is open-loop).
Don't forget that every individual function in a control system runs
open-loop. If the output function is a variable-frequency variable-
amplitude oscillator (something Rick and I are looking into -- now and
then -- as a model of controlled repetitive movements), it will produce
oscillations when driven by a signal. But the consequences of those
oscillations will not be controlled unless there is perceptual feedback
reporting those consequences. If you dissected the integrator out of one
of our control-system models (or the real system) and stuck signals into
its inputs, you would see that the output would be approximately the
integral of the input signal. It would be operating "normally". But it
sure as hell wouldn't be controlling anything.
What I think is of interest to PCTers in the article is not that
the system is an open-loop one, but the way in which its pattern of
output can be changed, effectively reorganizing the circuit. The
pattern is not only influenced by the suppressor-neuron activity,
but also is sensitive to the levels of various neurotransmitters,
which can alter such characteristics as the threshold for
initiating an action potential and whether there is just a single
spike or a sustained depolarization of the neuron that produces a
high-frequency succession of impulses.
The _system_ isn't open-loop; one component of it is.
Control through alteration of system parameters is an interesting
subject, which you will see in the model Rick and I are working on if we
ever get it going. But it's not reorganization in the technical PCT
sense. I am going to be unhappy with reports like the one about altering
characteristics of neurons until I see a study that explores the range
of behaviors of the neurons over a range of input frequencies of
impulses or concentrations of neurotransmitters. I can't believe that
the choice is simply between a single spike and a sustained high-
frequency output. If that's all you test for, of course, you'll see only
those two states. But with the right inputs, you might see that there is
a continuous transition between those extremes, and then you might get
some idea of what this neuron is doing. If you don't do the experiments
in a way that would reveal the continuous transition if it exists,
you'll never know if your observations are real observations or just
samples at the extremes.
The authors do seem to buy into the notion that coordinated
behavior such as walking is produced by "rhythm-generating
circuits" (controlling output rather than input)
I think that a walking-control system is going to need an output
function that is an oscillator -- a rhythm-generating circuit. That will
produce the basic changes in leg position. But that isn't enough to
allow bipedal or other locomotion, because it's open-loop. There's no
provision for correcting unpredictable errors. You need a way of varying
the amplitude and frequency of the oscillations based on their
consequences, such as the effect on foot placement on uneven or tilted
ground, and also varied as a way of maintaining balance and changing
direction.
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Hank Folson (950518) --
I was quite relieved to see on closer examination that Bill Powers
does practice what he preaches (most of the time, anyway). He does
it in a subtle, practiced way, and I simply had not observed what
he was doing.
I hate to disillusion you, but I seldom consciously behave according to
the principles of PCT. I don't have to -- I work according to those
principles whether I know it or not, and so does everybody.
One relevant place where the principles of PCT do come into play is in
helping to resolve conflict with other people. PCT has helped me to
realize why I have had problems with others, and what I can and can't
get others to do. It has shown me options in resolving problems that I
would have had to stumble across otherwise, by chance (if I didn't get
murdered first). Sometimes those options turned out to be simply letting
people go their way and giving up on trying to get them to behave right.
Once I realized that I really couldn't make somebody else shape up, I
had to change my own goals so they didn't include pushing others around
or having power over them.
I think I have gradually given up the wish that I could control other
people: seeing just why it is impossible (in the long run) helps. I
still like to teach and argue and have my own way, but I think I've
learned when to shrug and back off -- most of the time. This didn't
result from always thinking "What should I do as a good PCTer?" It
happened through coming to understand the world and people in a certain
way. Once your understanding changes, you don't have to think much about
what is the right thing to do. Not that you always do it, but on the
average things improve.
And no matter what you do, you're behaving according to PCT.
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Best to all,
Bill P.