[From Rick Marken (970223.1540)]
Bruce Abbott (970223.1545 EST) --
Your description of the Little Man is fascinating. Thank you.
It is certainly a correct description. What's interesting is
what is emphasized and what is played down. What is emphasized, it seems
to me, is what the model does (the objective results of its output);
what is played doen is what the model tries to perceive.
You have described the behavior of the model from the observer's rather
than from the model's perspective.
For example, you say:
there was a system to point the eyes at a target
Actually, there is a system that controls the perception of the
retinal location of the target. This system controls it's perception by
varying the pointing angle of the eyes relative to the target.
An observer would see this as "a system to point the eyes at a
target" but, of course, it's really a system to bring a percpetion
(of retinal location) to a reference state.
You go on:
Another set of control systems was used to set the vertical
"finger tip" to shoulder angle, the horizontal angle of the
arm (pivoting at the shoulder), and the distance between the
shoulder and finger tip
This description is fine. But, since it is essential to the operation of
the model, I would have tried to make it clear that it is a perceptual
measure of each of these variables that is "set" (to a
specified reference value).
References for these three control systems were derived from
higher level systems
Again, though this is certainly correct, it would be nice to
emphasize that these references are specifications for the
perceptions that these control systems should have. This would emphasize
the difference between the organization of the Little Man model and that
of other, more popular, control models in which the
references are specifications for the outputs that the control
system should produce.
Because of this organization, as you moved the target around via
the mouse, the head turned to keep the eyes facing the target
and the finger tip followed, so that it continued to touch the
target no matter where in the permitted 3-D space you put the
target.
Yes. This is what one sees from the observer's point of view. But I
think it would be worth it to mention that what this organization
does is control a hierarchy of perceptual variables (perceived joint
angles, visual distances. etc), keeping them at their fixed and varying
reference specifications in the context of a realistic physical model of
the environment and despite disturbances (such
as changes in the amount and direction of forces acting on the
Little Man).
That about it?
I think it's a good description of the behavior of an HPCT model --
as the behaviorist views it;-)
Best
Rick