various topics

[Avery_D_Andrews]

[Avery Andrews 920116.1233]

Central Patterns vs. Feedback:

One aspect of feedback & sequential order that strikes me is this: there
seems to be a bit of a tendency to construe the issue in either-or terms
(central pattern generation or feedback-driven response chaining), but
this seems to me to be completely unmotivated. To get a sequence
of alpha-gamma efferents sent out, presumably the activation of
one population of neurons has to cause the activation of another,
but there is no reason why the causer population can't be a mixed
bag. E.g. the command causing note 4 of the arpeggio to be played
could include the motor neurons that caused note 3, kinesthetic
neurons from note 1 , & auditory from note 2 (guessing random at the relative
speed of the pathways), or more likely, a big sticky mess derived from
all of these sources run through various delays, etc.

So what you would get from cutting any of the feedback paths is a degradation
of performance, but one that could be to some extent fixed by practice by
recruiting more central neurons to do the work of the disabled feedback
path.

Degrees of Freedom:

One respect in which the arm demo is an unfair critique of inverse
kinematics is that real arms have a lot more degrees of freedom that
the current version of the simulated arm, which makes it a lot less
obvious that simple-minded feedback schemes will work. On the other
hand, my impressions of how I actually use my arms suggest that peopple
ordinarily impose constraints that reduce the used degrees of
freedom to something more like those of armdemo. E.g., people
orient their palms towards what they are pointing to or picking up,
don't stick their elbows out too much unless they need to exert lateral
forces, and distribute curvatures between the wrist & finger joints,
etc.

Path Planning:

What about the evidence (discussed by Bizzi in the Posner volume) that
when people point from one place to another, they calculate an ideal
straight path (a virtual path) between the two points. This is supposed
to determine a course of alpha-gamma efferents values, & the elastic
properties of the skeleto-muscular system (including, on my reading,
the spinal reflex loops) take care of the dynamics. There's also
a paper about this: Flash, T. (1987) The Control of hand equilibrium
trajectories in multijoint arm movements, Biol. Cyb. 57:257-274, which
I haven't looked at yet.

Avery.Andrews@anu.edu.au

[_Maler]

[From: Oded Maler 920117]

···

*
* [Re: Avery Andrews 920116.1233]

I agree that it is a matter of levels. The question is what does it
mean (operatinally) to control for a sequence, and what does it mean
to control for a higher level goal by means of setting reference
signals that "encode" sequences to be controlled for. For the lowest
level, although you might see the controller as a program like

  Whenever ref-per \= 0
  do
    o:= (ref-per)*k
  end

it is maybe more natural to see it as a analogue servo-loop.

In the higher-levels, maybe the principles of servoing are more
naturally described as a "program". Note that the more modern
notion of a program is not something that gets input and computes
output in front of a static environment, but as a "reactive" system
consisting of many concurrently running components, having an
ongoing interaction with the envoironment. If you understand
"motor programs" in this sense, the dichotomy between this notion
and feed-back seems (at least to me, without going into the technical
details) less important.

*******

As for Agre's post I interpret is as saying that cultures and
societies (evolve to) provide constructs that allow the individual to
achieve his/her goals in a modular hierarchical manner (PCT-like)
which is of course more computatinally efficient than in the case
where goals interact and instead of many small automata you have to
reason about one big global automaton. It can be seen (maybe with the
help of imaginative creativity as a computational rephrasing of
Bill's principle of evolution toward orthogonal structures.

There is nothing to disagree about in this level of abstraction,
although I personally have recently some troubles in applying the same
"motor program" indepndent of whether I 'm driving a Peugot 309
or a Renault 4.

--Oded

--

Oded Maler, LGI-IMAG (Campus), B.P. 53x, 38041 Grenoble, France
Phone: 76635846 Fax: 76446675 e-mail: maler@vercors.imag.fr

[cam]

[Chris Malcolm]

Oded Maler writes:

I personally have recently some troubles in applying the same
"motor program" indepndent of whether I 'm driving a Peugot 309
or a Renault 4.

It's interesting to write with the wrong hand and try to observe what
carries over, what can, with effort, be moved over, and what is
unavoidably different. A great deal more carries over, for example, than
in the case of throwing with the wrong arm, presumably because writing
has more higher level components.