[From Bill Powers (940226.1000 MST)]
Bill Leach (940225.2123) --
In reply to your second post (the one to me):
The entire perception signal is considered feedback, yes? A
disturbance then, if perceived becomes a _part_ of the
What we call "feedback" is pretty indefinite. Probably the best
way to think of this is to divide the control loop into two
parts, the forward part and the feedback part. The forward part
is the path from the sensors, through the organism, to the output
effectors. The feedback part is the path from the effectors,
through the environment, to the sensors. By thinking in terms of
path segments instead of signals, you can make the division
We divide the external feedback path into several parts: the
output quantity, which is the immediate physical effect of the
effectors on the environment, the "feedback function" which
converts that output effect into an effect on the controlled
quantity, and the controlled quantity itself which is directly
sensed by the input sensors. The input sensors and the output
effectors sort of straddle the system-environment boundary.
A disturbance comes into the external part of the loop from some
independent source. It can affect any part of the feedback path,
but we always express it as an equivalent disturbance applied
directly to the controlled variable. Multiple disturbances, even
if they affect different parts of the feedback path, are
similarly expressed, as a single equivalent disturbance applied
to the controlled variable.
The perceptual signal, which is the output of the sensor and
which indicates only the state of the controlled variable, is
affected by disturbancess that change the controlled variable,
but it is also affected by the output of the system via the
feedback function, which also can change the controlled variable.
So you can't say that the perceptual signal is a perception of
the disturbance. It is a perception of the state of the
controlled variable, period. There's no way to tell how much of
the perceptual signal is due to the output and how much to the
disturbance, especially since the output opposes the disturbance.
One of the neatest counterintuitive facts about control is that
the state of the controlled quantity (and hence the perceptual
signal in the model) shows a very LOW correlation with both the
output and the disturbance, while the output shows a very HIGH
negative correlation with the disturbance. Of course the state of
the controlled variable shows a very high positive correlation
with changes in the reference signal, in the model.
Note that the disturbance itself is NOT sensed.
Thus, in the skater example: If we assume that the coach's
words are a significant factor in the perception of the
subject, then calling what the coach is doing "feedback" is NOT
really out of line.
If, as I proposed before, the coach is just calling out angles of
bend and the skater knows that the reference angle is 90 degrees,
then the coach is acting as a feedback function, converting the
output of the skater (the angle of bend) into a string of verbal
numbers, 86, 88, 91, 95, 93, 90, 90, etc. that the skater is
perceiving and controlling by varying his angle of bend. The
coach is then part of the skater's feedback loop.
If the coach calls out "86, 88, 101, no I mean 91, 95 ...", the
coach has now injected a disturbance into the external part of
the skater's control loop. The spurious "101" was not generated
by the skater's varying the bend angle, but by a mistake that the
coach made. This mistake caused a sudden change in the controlled
variable which was independent of the skater's actual angle of
bend. The critical thing about a disturbance is that it tends to
alter the controlled variable in a way that is independent of the
control system's own effect on the same variable.
When you understand the basic organization of a behavioral
control system, the words you use to describe it are only mildly
important. A listener who also understands can correctly
interpret even loose usages. If you say the skater is depending
on "feedback from the coach," you know that this means the coach
is responding to what the skater does in a regular way, so the
cause of the coach's feedback is, reliably, the skater's action.
But if the coach spontaneously offers advice like "use your head,
use your head," this provides no regular or understandable
perception to the skater of the skater's own actions, and is just
Now then calling it "postive" or "negative" feedback would be
wrong as far as PCT is concerned BECAUSE the in the sense that
such modifiers are typically used the paradigm is not PCT.
Remember that PCT is based on real control theory, not on verbal
conventions. This means that "positive feedback" already has a
meaning, the meaning defined long before the words were taken up
by laymen to mean "encouragement." Positive feedback results from
a wrong sign in the control loop so that a small error makes the
error larger instead of (negative feedback) smaller. To compute
the sign of feedback, you multiply together all the signs
associated with all the functions (including the comparator)
encountered in one complete trip around the loop, starting
anywhere. This number must be negative if the feedback is to be
negative and therefore error-correcting. Positive and negative
feedback are properties of the WHOLE LOOP, not just the external
The lay usage of the terms came from association of "positive"
with "good" and "negative" with "bad." In control theory, exactly
the opposite connotation exists: for control, positive is bad and
negative is good.
You are feeling depressed and, hoping to hear something that will
make you feel better, you tell me "I'm really feeling terrible
today." I can create a positive feedback situation by saying
something like "Why are you always whining to other people about
your problems?" If I respond to everything you say with words
calculated to make you feel worse, your actions based on your
feelings of error will simply make the error larger. That is
positive feedback. It has nothing to do with what I say, but only
with the _relationship_ between what you feel now and what you
feel next after one trip all the way around the loop.
You can't tell whether a response to someone's words amounts to
positive or negative feedback without knowing how the person is
hooked up inside. If you're talking to a masochist, the above
response to the above complaint will be highly satisfying, being
exactly right for enabling that person to accomplish the goal of
feeling rotten -- you're completing a negative feedback loop.
So the strict usage of positive and negative feedback turns out
to be correct, and the lay usage is wrong, for the lay usage
assumes that encouraging words will necessarily help the other
person in some way. If you really understand what positive and
negative feedback are, you'll wait to find out what kind of
response the other person is trying to get, instead of assuming
that everyone with a complaint wants to be bucked up. The problem
with picking up jargon and free-associating on it is that you are
likely to misunderstand the situation because of your
misinterpretation, and thus behave inappropriately.
Just remember: if a person wants encouragement, and says "I'd
like to be a better person," and you say "I think you are much
better today than yesterday," that is acting to create a NEGATIVE