What is feed-back too slow for?

[From Oded Maler (930129)]

* [From Rick Marken (930128.2200)]


* Avery.Andrews (930129.1000)
* >People who make generic claims that feedback is too slow are probably (a)
* >thinking of highly practiced rapid movements (b) thinking of the 200-150
* >ms reaction time for visually presented stimuli.
* Before starting the article (or whatever it is) here, why not spend
* a moment explaining what the hell people might mean by the idea that
* "feedback is too slow". I think the whole concept is ridiculous
* because it is based on a sequential state, cause effect concept of
* a control loop -- one which leaves out time (and, hence, a large hunk of
* reality). How can feedback be "too slow" in a closed loop where feedback
* is present (as a perception) at the same time that the cause of that
* feedback (error) is present. There are phase relationships between the
* continuous variables in the loop --the result of transport lags and
* slowing factors around the loop. Lags that are too long or slowing
* factors that are too great can create instabilities in the loop (they
* can also create stabilities). But it would be necessary to know what
* is meant by "feedback too slow" and then to test it in a model before
* one could say whether "feedback too slow" (what ever that is) would
* be a problem. In fact, without a definition of what "feedback too slow"
* means in term of closed loop control, it is difficult to know what
* is being measured in these reaction time experiments. If you apply an
* impulse disturbance to a controlled variable, there will be a change in
* the output variable; what is the "reaction time" here? The time to
* maximum output, the time until this output has some other effect
* (like pressing a button), the time until the derivative of the output
* is maximum, minimum? And when you decide what reaction time is, has it
* been measured in the same way by all these experimenters? And if it
* has, what is the reason for this reaction time - is it a transport lag,
* slowing factor, some of both? Without a model, how can they even tell
* what the reason for the reaction time might be?

I think this exchange clarifies some important points and shows which
parts of the elephant's body (to use the the by-now-classical
metaphor) are observed by "blind" non-PCTers and which by "blind" PCTer.

The answer to the question "what is feed-back slow for" you must
invoke some "objective" performance criterion independent of the
internal perceptual coordinates of the acting individual. You must
assume that "playing a piano trill correctly at some speed" or
"knocking out a boxing champion" has some more or less agreed-upon
meaning. Then you can build a mathemtical model of that act and
the component involved (muscles, nerves and their reaction time)
and show WITHOUT USING THE CONTROL MODEL that it is impossible
for information to travel and affect the muscle at the time scale
between the intitiation of the action and its outcome. The nature
of these impossibility/lower-bound arguments is that they consider
ideal situations and thus apply as well to the "correct situation".
By showing that an ideal pianist or boxer with ideal "objective sensors"
and "objective effectors" cannot achieve something because of timing
constraints you show a-forteriori (?) the a realistic (i.e. PCT-based)
pianist/boxer with the same timing constraints cannot do it either.
It is the same like proving, based upon bio-chemical and physical
reasoning that no human is capable of, say jumping above 10m.
This argument is true, regardless of whether he is commanding the
muscles via hierarchical servoing, inverse-dynamics calculations
or coin tossing. (I think that some of Martin's attempts explain
information-theoretic constraints were along a similar line, and
maybe he was right in the statement he made back then concerning
the "real" understanding of PCT :slight_smile:

The emphasis of the important PCT insight that within the individual
"it's all perception" in contrast to the naive objectivism of, say,
cognitive psychology, should not be exaggated into a solipsist neglect
of the external environment. The question of how and under what
conditions people can achieve certain "objective" performance, in
other words, what guarantees that a system organized in a certain way
survives ("objectively") in a given environment, deserves more
attention and better answers than "otherwise, reorganization will

*The point is that most of the data presented in these papers is
*probabaly useless because it is not collected in the context of a
*working control model. There are lags and slowing factors in
*control systems so people are bound to find response latencies when
*they apply sudden disturbances to things people are (or are expected
*to be) controlling; the observed results in these studies (16 ms, 150 ms)
*may look very scientific and all but they are almost certainly useless
*for modelling behavior. And there is no question that measures of
*"response latency" that are collected in this way (with no understand-
*ing of the behavior of closed loop systems) say nothing about what
*variables people can and cannot control. Using these reaction times
*as a basis for showing the limitations of feedback control is just silly
*-- and an impediment to real research on control.

Maybe Rick is right about that, but still some neurophysiological
evidence about basic properties of nerves and muscles can replace
this data and prove the uselessness of feedback for certain kinds
of actions.



Oded Maler, LGI-IMAG (Campus), B.P. 53x, 38041 Grenoble, France
Phone: 76635846 Fax: 76446675 e-mail: maler@vercors.imag.fr