[From Bill Powers (930806.1945)]
Rick Marken (930805.1900) --
It seems to me that the French derivation of "counter-roll"
doesn't quite capture the meaning we now use. Which set of
figures is the reference set? When I get the bank statement, I
have two representations of the state of my account: the bank's
and mine. While I'll admit that the bank probably makes fewer
errors, bank errors are not unknown. The reference condition I'm
after isn't patterned after either account: it's for the two to
agree.
ยทยทยท
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Allan Randall (930806.1520) --
Just to make sure I understand, is the following a proper
paraphrase?:
Disturbance(1) (Taylor & Randall): that part of the CEV
fluctuation that is not caused by the control system itself,
and thus is due to an external cause.
Disturbance(2) (Powers & Marken): the external cause of that
part of the CEV fluctuation that is not caused by the control
system itself.
Yes, that's the difference.
It sounds like you agree that the percept contains information
about disturbance(1), but not about disturbance(2)? Is this
right?
Disturbance (1) would seem to be the difference between the state
of the perceptual signal in the absence of an external disturbing
influence and the state with that influence present. If the
reference signal is set to 10, with an integrating output
function the perceptual signal will also be at 10 with a constant
disturbing influence (zero or nonzero). Departures of the
perceptual signal from a value of 10 then indicate the presence
of some transient disturbing influence, and in the first instant
after a disturbance commences directly indicate the amount of
fluctutation due to the external influence. From then on the
contribution from external sources can be found only by solving
the system equations. Is that what you're after?
Disturbance (2) is not deducible by this means because so many
different external events could produce the same transient
fluctuation in the CEV and the perceptual signal.
However, I do not think I see why anything changes when we
switch to definition (2). Perhaps the reason we have been
talking past each other is that you are making a distinction
that I do not really see. Is there any *meaningful* way in
which disturbance(1) and disturbance(2) are different?
Yes. Behavior seen from outside the behaving system appears to
consist of responses to stimuli, where stimuli are customarily
defined not as changes in a CEV, but as changes in environmental
variables remote from the organism. These remote variables may
contribute to the state of a CEV, but they do not determine it.
In our tracking experiments, we define the disturbing variable
not as a change in the cursor position or cursor-target distance,
but as the magnitude of an independent variable that is added to
the existing cursor position, contributing to the cursor position
but not determining it (the control handle position also
contributes to it). The actual state of the CEV is not
predetermined; it arises from the operation of the system and the
influences from disturbances. It is a dependent variable.
Further, disturbance (2) is different from disturbance (1) in
that it may be multiple. In the example of steering a car, there
may be simultaneous disturbances (2) consisting of soft tires,
misaligned wheels, tilts in the roadbed, crosswinds, and so on.
The controlling system can't distinguish among these disturbing
quantities, and need not. It's necessary to make this point in
distinguishing PCT from SR psychology. The controlling system can
control by paying attention ONLY to the state of the CEV. It
needs to know only that the CEV -- that is, the perceptual signal
-- changed; it does not need to know what caused the change.
Isn't "that part of the fluctuation caused externally" simply a
way of describing "the external cause of the fluctuation"?
No. Think of steering the car. If the car veers to the left, the
CEV is experienced as changing and the system exerts a steering
effort to the right. But that veering to the left does not
indicate whether the road tilted to the left, a crosswind from
the right appeared, or the left front tire went flat. Or all
three.
How can something be said to have information about (1) and not
about (2)?
Y = a+b+c. Y is observed to change. Does this give any
information about the behavior of a, b, or c? Is there not an
infinite number of behaviors of a, b, and c that would produce
exactly the same change in Y?
If there *is* a meaningful distinction here, I'm not sure how
definition (2) fits into the arguments Rick Marken has been
making. Rick and I have been talking about reconstruction of
the disturbance in a computer simulation. What are the values
Rick was looking for me to reconstruct if he was using (2) as
opposed to (1)?
Rick was looking for you to reconstruct a, b, and c from
variations in Y (or, equivalently, variations in a perceptual
signal representing Y). This is clearly impossible using only
information in Y or the representing signal. Perhaps all of this
would have been clearer if Rick had said that there is no
information IN THE CEV about the disturbance -- that is, there is
no information in Y from which you could deduce the states of a,
b, and c -- disturbance (2). Or even more clearly, now that you
have supplied the language: there is no information in
disturbance (1) from which you can deduce disturbance (2).
If they both (two different light sources) really do give
identical fluctuations, then there is no point in making any
distinction at all between them. Certainly calling this extra
aspect (which doesn't even affect the CEV) part of the
"disturbance" seems a tad bizaare, considering that it's
actually the part that doesn't disturb the CEV at all!
From the standpoint of the control system, there is no point. But
to an external observer, the "equipotentiality" of two different
stimuli may be puzzling indeed. Why do opening a window and
letting the fire in the fireplace go out both cause the response
of putting on a sweater? Why does opening the window today cause
that response when opening it yesterday caused the person to
build a fire in the fireplace instead? Psychology has been trying
to search the environment for clues about such similar and
different effects of stimuli, without knowing about controlled
variables.
I have already made the point that a CEV can be caused to
fluctuate in many different ways. The fluctations are perceived.
Their causes, which may be continually changing, are not.
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Best to all,
Bill P.