[From Bill Powers (950605.1415 MDT)]
Martin Taylor (950605.1100)--
"But if the PIF works only on the current values of the input
signal, then when the input values change dramatically (such as by
vanishing), the perceptual signal will change dramatically,
creating probably large error values, and ineffective but large
changes in output."
Is that a premise or a consequence of talking about the "standard
model," the form of which is shown in the CSG logo?
There is nothing in the "standard model" that defines the input function
to have only a single input. The logo is only a symbolic representation
of the typical control system. In reality, both the input and output
lines should be multiple, with many paths through the environment (as in
the figure in my 1973 Science paper or the cover of LSC I). So yes,
assuming that cutting off visual inputs cuts off all inputs used by a
given perceptual function is an assumption.
>The "use only the current value of the inputs" kind of PIF is
>tuned for all possible unpredictability. It results in immediate
>loss of control when the input is cut off.
If this is a quote of something I wrote, as the single greater-than
signs suggest, I must have been under anaesthesia when I wrote it,
because I don't understand it at all. What does "tuned for all possible
unpredictability" mean?
It's true that if you cut off all lower-level signals of which a
controlled perception is a function, there will be immediate loss of
control. But in suggesting that perceived cursor position might be a
function of kinesthetic inputs as well as visual ones, I was proposing a
way in which loss of _visual_ inputs would not amount to total loss of
all signals on which the perceptual signal standing for cursor position
depends: you would have to lose the kinesthetic inputs, too, to produce
total loss of control. I said, trying to make this clear,
In the case of visual tracking, for example, we tend to assume that the
sense of cursor position is derived entirely from visual information.
But we also know that while we are tracking, we can feel our arms and
hands moving. What kind of input function could use both the visual and
the kinesthetic feedback information to produce a cursor position
signal?
You reply:
This refers NOT to a case in which the input is cut off, but to a
case in which the input comes from a variety of lower-level
perceptions, some of which no longer have access to the real world
data (no telling whether they use prediction, but assume they
don't), and some of which continue to have access.
I think the problem here is that you've got into the habit of thinking
of the standard control system in a certain special way, so when I refer
to a somewhat more general case you don't recognize it as an example of
the standard model. You sometimes write as if you think that for every
perceptual signal there is some real CEV in the environment that
corresponds to it. When you refer to "the" input being cut off, you seem
to mean all contributing signals wherever they come from, which would be
a pretty unusual circumstance under my proposal. And it would probably
result in total loss of control.
In talking with Hans Blom I've been playing his game by his rules. In
offering some ideas about how control might continue even if one
component of the input data is cut off, I was raising a question about a
possible alternative to his model that does not require either
prediction or a world-model to explain what we observe in one specific
example of real behavior. I'm only _suggesting_ this alternative,
because I haven't tried out a model of it.
Of course I agree with the statements in your message, about the
ability to use multiple sources of input. But they are irrelevant
to the question that was at issue: what happens when the input is
cut off, with a "predicting" PIF or a "use current values only"
PIF.
If the currently-perceived cursor position is a function of current
visual inputs and current kinesthetic inputs, then the PIF belongs to
the "use current values only" type of input function. I don't see what
your problem is, unless unconsciously you were thinking of a "use
current values FROM ONE SENSORY MODALITY only" PIF.
My point was that there is a conceptual distinction between "having
a value of zero" and "having an unspecified value," and that the
"standard model" control loop has no way for that distinction to be
represented, either in the PIF or elsewhere.
That's true, and neither does Hans' model. When the input is lost in
Hans' model, the expected variance pvv of the perceptual variable y has
to be set extremely large (10,000), to stop the Kalman filter from
continuing to make adjustments based on a spurious value of y. This is
the equivalent of an external intelligence that realizes that whatever
value y may now have, it no longer represents an environmental variable
and must not be allowed to participate in computations. If you supplied
a standard PCT model with help from a similar external intelligence,
there would be no problem there, either. In fact, in Hans' model, when
the system is instructed to "go blind," not only is the expected
variance of pvv set to a large number, but y itself is arbitrarily set
to 10.0, the average value of xopt. So I suspect that the distinction
you make, which is a valid one, caused problems for the adaptive model,
too.
In fact, I believe, any perceptual input function will produce an output
signal in the absence of all input, and that signal will have a meaning
in terms of the perception (even if the output signal is zero). The only
way to prevent unwanted actions from ensuing would be for a higher
system to disable the control system entirely, in any one of several
ways that have been suggested for doing so. This is as true for the
Kalman filter adaptive model as for the PCT model.
A one degree-of-freedom system in which the single degree of
freedom is dedicated to the value of a variable cannot
simultaneously represent the validity of that variable.
I agree entirely. Something must continually monitor the relationship
between the signal and other signals at the input of the PIF, judging
not the magnitude of the signal but its validity, and taking appropriate
steps to shut down the control process (or the adaptive process).
···
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Your reply to Bill Leach's comment on choosing between unwanted
alternatives used an inappropriate example:
And given free control, I would drive straight from work to home,
if my control systems were not overwhelmed by the difficulty of
driving over kerbs and through houses that intervene. So I control
what I can, and arrive home by a more circuitous route.
Bill's comment was that _neither_ choice presented to the rats was one
it would select by itself. A more appropriate example would be, "Would
you rather be able to blow your car's horn or to have it silent while we
shove the car over a cliff with you in it?" Certainly you have a right
to study the effects of that choice if you want to, but it would be
pretty silly.
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Best,
Bill P.