[From Bill Powers (941002.0830 MDT)]
Bill Leach (941001.1851 EDT) reminds me of a comment I had meant to make
on
[Martin Taylor 940929 10:30]
When an electronic engineer wants to examine an unknown control system,
one way is to break the feedback connection and examine the S-R
behaviour of the broken loop.
It's very difficult to break a control loop that a person normally uses,
without disrupting the control system. As soon as the loop is broken,
the person loses control and furthermore knows that control is lost, for
actions no longer affect perceptions. The normal consequence of breaking
the external part of a control loop is for the person to switch to some
alternate method of control as soon as loss of control is detected, or
to give up controlling completely until the outer part of the loop is
restored.
The electronic engineer doesn't have this problem because the system
being investigated is usually a one-level system, or at least has no
ability to adapt to loss of control. When the external feedback path is
opened, the test signals applied to the input have to be made much
smaller (by a factor equivalent to the output error sensitivity) if the
result is not to be simply driving the output from limit to limit as
fast as possible with every excursion of the test signal. If that does
NOT happen when the external path is broken, you can be pretty sure that
you aren't looking at the same control system -- something inside has
reduced the output gain to prevent such wild output fluctuations, or has
turned off the former control system entirely.
Investigations of passive perception usually require that the person
perform an action which tells the experimenter something about the
subject's experience of the perception but doesn't affect the perception
directly. So there is no control loop to be broken -- not one that
involves the perception in question as a controlled variable. You can't
assume in that case that the response to the test stimulus is the same
response that would exist if the perception were under control.
The nearest you can get to seeing open-loop behavior of a living control
system is when the external path is very briefly interrupted and then
restored before the loss of control is detected by a higher-level
system. This requires special quantitative experimental techniques and
real-time instrumentation. Rick Marken did something like that in his
"reversals" experiment.
Investigations of open-loop passive perception are peculiar in that they
make the test subject a partner, and even a substitute, for the nominal
experimenter. If the test subject is not an acute and eloquent observer
of subjective phenomena, the experimenter may be unable to understand
what the test subject is reporting, or to know the meaning of a report,
or a response, in terms of the perception actually being experienced.
Worst of all, the experimenter may assume that the test subject is
seeing and reporting on the "objective stimulus," meaning the stimulus
as the experimenter sees it in his own subjective world of perception.
What is generally done, as I understand it, is for the experimenter to
tell the test subject to make some standard indication when some feature
of the perceptual field is experienced -- but there is no way to know
what that feature is, in the test subject's world. Thus perceptual
experiments tend to be cast as S-R experiments, with occurrance of a
response being assumed to indicate occurance of the perceptual situation
that the experimenter has in mind. In very, very simple (low-order)
situations this may not be a bad guess, but it is always a guess, and it
will become progressively worse as more complex perceptions are
investigated.
In any case in which the experimenter can independently determine the
stimulus inputs on which perceptions are based, the situation is by
definition open-loop. If any control is involved, it cannot be control
of the same variable that the experimenter is manipulating. This is why
I insist, against considerable resistance from some directions, that the
standard PCT diagram include an explicit disturbing variable separate
from the controlled variable. Effects on the variable actually under
control are achieved not by arbitrarily altering that variable (which
would break the loop) but by altering some other physical variable, the
state of which _contributes to_ but does not _determine_ the state of
the controlled variable. The action of the control system must be able
to produce independent effects on the controlled variable regardless of
the state of the disturbing variable. If the experimenter can
arbitrarily set the state of the variable being sensed and controlled,
then while that arbitrary state exists the loop is no longer closed, for
the system's action can have no effect on the variable.
Even in cases where the experimenter-applied manipulation of the
controlled variable is very brief, such as an impulse disturbance, the
response that is observed is not the open-loop response. It is true that
for the first delay-time the loop is effectively open, but the ensuing
changes in action are feeding back to affect the input in every
subsequent instant; the observed response curve is the closed-loop
impulse response, not the open-loop response. If the loop were truly
open, it would be seen that the output response develops over a period
equivalent to many delay-times, is very much larger, and has a form very
different from (and more protracted than) the form of the closed-loop
response.
In summary, "breaking the loop" to observe open-loop responses to
arbitrary changes in the input is not a practical method for
characterizing human control systems. Nor are tests of passive
perception that do not involve control likely to tell us about the
characteristics of control systems.
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Best,
Bill P.