[From Rick Marken (960307.0900}
Scott Graham (960306) --
I am looking for a metaphor ro help me understand the concept of
positive feedback.
Why a metaphor? How about the real thing?
The most familiar example of positive feedback is probably the microphone
"feedback" we occasionally hear as a very loud "wow". This happens when
the speaker that emits the amlified mic output is pointed toward the mic
itself. So when you speak into the mic, the amplified version of what you
say into the mic comes out of the speaker and goes right back into the mic,
adding to your own contribution to the input to the mic.
Here's the positive feedback loop:
v-->i-->|amp|-->o
^ |
···
>
> >
-----------
The mic input, i, to the amplifier (amp) is your voice, v, plus any output,
o, from the speaker. The amplified speaker output, o, feeds back additively
to the input to the amp, i. Thus, the fed back output, o, continuously causes
_itself_ to increase; this is positive feedback. The runaway increase in such
a loop continues until the limits of the amp are reached and/ or the
influences on the input are removed (you stop talking into the mic and/or
protect the mic from the output of the speaker).
positive feedback... opposes negative feedback
Not really. Positive feedback doesn't "oppose" anything. What "opposes"
negative feedback is another negative feedback system controlling the same or
a similar perception. This is called _conflict_.
There is conflict when one negative feedback loop opposes the efforts of
another; what typically results is that instead of having negative feedback
loops that control you have negative feedback loops that (in their effort to
control) paradoxically _prevent_ control. This seems like what is, indeed,
going on when there is addiction; the inability to control a perception that
you want to control.
Conflict is what Hans Blom (960307) descibed in his "hungry donkey" example.
Hans gave this as an example of positive feedback but it's not; it's an
example of conflict between negative feedback control systems.
In Hans' example, a donkey is confronted with two haystacks; there is a
conflict because the donkey wants to eat from both haystacks. But the donkey
cannot be in two places at the same time so there is a conflict between two
control systems; the one that wants to be near haystack 1 to eat it and
the one that wants to be near haystack 2 to eat it.
These two "haystack eating" control systems can only control their
perceptions by setting a different references for the same lower level
control system -- the one that controls the perceived position of the donkey
relative to the haystacks . If the gains of the two haystack eating control
systems are nearly equal, the reference for the donkey's position control
system will be stuck between haystacks 1 and 2; the result is that neither
haystack eating control system can control eating the haystack it wants to
eat. The control systems are working against each other (trying to set
opposite references for the position of the donkey) and it will look, to an
observer, like the donkey is "addicted" to being in one position -- between
the haystacks.
The only way out of this conflict is for the donkey to revise it's haystack
eating goals; it can only do this by "going up a level" and resetting the
reference for the systems that want to eat haystacks 1 and 2. I doubt that
donkeys can do this. But people can -- at least, they can be taught to do
this if they are willing to learn PCT.
Best
Rick