There is more in this world than...

From Greg Williams (930922)

Bill Powers (930921.0830 MDT)

This is a generous view, but I think you give the manual control
experts too much benefit of the doubt. If they recognized that r
is variable, they would make provision for its variations in
their models and specify that they are studying the special case
of r = 0. I have NEVER seen that said. I think that Rick is
right: they are not aware of the role of perception in defining
controlled variables, or of the role of internal reference
signals in selecting a particular state for those variables. They
are naive realists: the error between target and cursor exists in
the objective environment where it is "obvious." They don't see
these as perceptions of their own, so it never occurs to them to
see them as perceptions of the experimental subject, too.

Rick Marken (930921.1030)

I think you are right; the 'manual controllers' would probably see
the difference between spot separtion and cursor-target separation
as the input that causes action: the subject's variations in his
or her own inner purpose (r) would remain politely ignored.

What about the references I showed you at the Durango meeting. If you
don't count the control engineers who authored those works (which
show internal reference signals) as "manual control" biggies, I
don't think you can deny the "manual control" credentials of
Christopher Wickens, whose ENGINEERING PSYCHOLOGY AND HUMAN
PERFORMANCE was recently published in its second edition
(HarperCollins, 1992) and is a standard text for human factors
students. Here are some excerpts from Chapter 11, "Manual Control":

1. "When driving an automobile, the HUMAN OPERATOR perceives a
discrepancy or error between the desired state of the vehicle and its
actual state. The car may have deviated from the center of the lane or
may be pointing in a direction away from the road. The driver wishes
to reduce this error."

2. (regarding tracking of time-varying inputs in general:) "It is
assumed that the human 'intends' to produce a given position."

To me, "a given position" shows that Wickens realizes that the
"intended" position can vary. He does not explicitly say that (for
example, in the auto driving example) he is working with the special
case of r=0; rather, he appears to implicitly note that ALL cases are
"special," that is, r can range over various values, with none of them
"special." So I will, more or less splitting hairs, grant Bill the
first part of his statement. But at least one manual control expert
certainly IS aware of "the role of internal reference signals..." And
Wicken does NOT "politely ignore" variations in reference signals.
Such attention to variations in internal references was shown even
more explicitly in the three engineering text excerpts I showed you in
Durango. I still count Milsum (the authors of one of the excerpts), at
least, as a manual control expert, even if you guys don't; he did
detailed work on modeling human limb control systems, along the lines
of the Little Man.

I am not trying to claim that manual control engineers beat Bill to
figuring out HPCT. I do claim that their (independently derived)
models are similar to those constructed by PCTers, complete with
internal reference signals and comparators. I THINK THIS INDEPENDENT
CONVERGENCE TO THE SAME MODEL SHOULD BE CELEBRATED BY PCTERS -- IT IS
ANOTHER REASON TO BELIEVE THAT IT IS A GOOD MODEL.

Incidentally, for those interested in human motor control, a good way
to learn about the state of the art among nonPCTers would be to read
Mark L. Latash, CONTROL OF HUMAN MOVEMENT, Human Kinetics Publishers,
Champaign, IL, 1993, 380 pp., ISBN 0-87322-455-8.

As ever,

Greg