[From Rick Marken (931117.1330)]
Continuing on the ritual murder theme:
Hans Blom (931117)--
in engineering, feedforward does NOT have a
dubious status. It solves a number of problems that pure feedback cannot
solve, or it enhances the effect of feedback.
Again, this is irrelevant to PCT. We are trying to understand living
systems (including the engineers who successfully use feedforward) and
there is NO EVIDENCE that feedforward mechanisms are needed to explain
any purposeful behavior (the VOR being one POSSIBLE exception -- but
I await the evidence).
Evolution seems, in the long
run, to strongly favor organisms that do "a teensey bit better".
Who is this "evolution", anyway, and why should I care what he (she?)
favors?
Me:
PCT shows that this estimate
of the probable prevalence of feedforward based behavior has been
GREATLY EXAGGERTAED.
Hans:
At the lowest levels of the hierarchy, that is true. As Bill has shown,
however, the higher levels of the hierarchy must necessarily be slower.
Here, feedforward will show its greatest contribution. Most "conventional
models of living systems" study these higher levels of the hierarchy...
How do "models" study anything. As a trendy science "spotter" I find
this a telling slip (if it was a slip). I presume that what you mean is
"conventional TESTS of living systems" are about these higher
levels, the ones that presumably involve feedforward. Unfortunately,
there have NEVER been any conventional tests that show 1) that these
higher level processes are open loop or 2) that they involve ANY
feedforward at all.
Me:
In fact, there is no evidence that ANY behavior
is generated by feedforward processes.
Hans:
Do you have evidence to support this claim?
No, Hans, you're supposed to prove me wrong by pointing to evidence
that behavior IS generated by a feedforward process (the VOR MIGHT
be the one). The evidence for my claim is just that I have never
seen any evidence of a behavior being generated by a feedforward
process. Apparently you HAVE seen evidence of behavior being
generated by a feedforward process; please share it with us.
Bill said:
One point we should be very clear on: the fact that open-loop
control might suffice in a given circumstance does NOT indicate
that the behaving system is operating open-loop. The open-loop
design is a solution looking for a problem.
Hans replies:
For control engineering, this is not true. Why would it be true for
biological systems?
Because there is no god. Engineers are the gods of feedforward systems.
Hans:
In my view, feedforward has an important place in
control, and is most fruitfully combined with feedback control.
Me:
You make this claim even though you have presented NO evidence (other
than the notorius "walk in the dark" anecdotes) that feedforward is
needed to explain any aspect of human behavior.
Hans:
What is "needed"?
You really don't know, do you.
In the past, and now again, I have
given you both examples and theory that show that feedforward can IMPROVE
control, and that the improvement can mainly be found in an increased
speed of response
Well, Bill's analysis of the Lang-Ham system calls that into considerable
question but I'll give it to you anyway. The fact is, IT'S IRRELEVANT.
We are trying to understand living systems -- we are not trying to
build optimal control systems. You have presented no behavioral data
that requires a feedforward system for its production. So all of this
feedforward talk is just so much hot air (as far as living systems
are concerned) until we see the evidence.
At higher -- slower -- levels of the hierarchy you will start to
NEED feedforward.
I am feeling the breeze from waving hands. Some evidence, PLEASE.
And what is "explain"? When you "explain" a system, you reduce it to
simpler concepts/components and their interactions. If you allow feedback
components only, feedforward will never be able to "explain" anything.
"Explain" is "build a model that behaves like the real thing". You
apparently think that "explain" is "build a model that behaves
optimally" (in some sense). We don't care about optimal; we care about
ACTUAL. You have presented no credible evidence of feedback processes
in behavior. We can point to TONS of evidence of feedback control
processes in behavior. So, when it comes to engineering, feedforward
may be the greatest thing since sliced bread (I doubt it -- but as you
noted, sometimes it's the only thing that will do). But when it comes to
living systems it seems to be feedback all the way, not because of our
bias but because of what we have OBSERVED and TESTED. If you have observed
and tested to determine the existence of feedforward processes in behavior
then, please, share the results with us.
Thanks for sharing,
Rick