Newsgroups: bit.listserv.csg-l
Date:         Fri, 2 Oct 1992 22:59:58 EDT
From:         "(Martin Taylor)" <mmt@BEN.DCIEM.DND.CA>
Subject: Re: Misc subjects

[Martin Taylor 921002 22:30]
(Bill Powers 921002.0600)

> But the idea of "competing behaviors" inhibiting each
>other doesn't seem very plausible to me. How does one behaving
>subsystem know which other behaviors to inhibit and which to leave
>alone? I don't think you could make a runnable model out of this, but
>you're welcome to prove me wrong. Your idea isn't even self-
>consistent, because you follow the above by saying
>
>>It is the world that stops them all happening at once.  If they
>>could, they would.
>
>But that means that they are NOT mutually inhibiting each other inside
>the system. They're all in a state of perpetual conflict. This seems
>like a lousy design for a control-system hierarchy where you would
>like the loop gains all to be quite high.

Quite right.  I never intended to imply that the competing behaviours had
any connection internal to the hierarchy other than that they all had
references to which a higher-level ECS contributed.
They certainly should not inhibit one another internal to the hierarchy,
unless we are getting into the realm of pre-planning, which was the kind
of choice behaviour I was trying to show was not always required (if ever).

I see you still don't buy the notion that not all controllable percepts are
at any one moment controlled, and that most ECS gains either are very low
or the ECS is in some way disconnected from the physical world.  It still
seems to me inevitable that there exists a very small actively controlling
subset of the ECSs at any one level at any moment, the rest not, at that
moment being in active control of anything except their imaginations.  The
idea of "giving up" is inherent in the normal situation.


>>I guess in the background there is another point I meant to discuss
>>at some time--"giving up."  When there is persistent error, one
>>possible and often used response of an ECS is simply to reduce its
>>gain to zero, to give up on a hopeless situation.
>
>I have already brought up this idea on the net, but not by supposing
>that every ECS can perceive and judge "situations" as being
>"hopeless." My solution was a comparator that has a curve that I drew
>like this:
>
>     ^                  *      |
>     |                *    *   |
>Error|signal      *         *  |
>            *                * |
>*                   -         *|            +
>-------------------------------*-----------------------------------
>         <-- actual error-->   |*                          *
>                               | *                *
>                               |  *          *
>                               |   *     *
>                               |      *
>
>In the central region, feedback is negative. When a disturbance gets
>so large that the peak of the curve is reached (in either direction),
>the error signal begins to fall for further increases in disturbance.
>This results in a drop of output, still more error, still less error
>signal, and so on to the "giving up" regions at the ends.

This is a different situation, in itself quite legitimate.  In the situation
I am considering, the error need not be desperate, but it is consistently
in the same direction.  If the gain depends on the integrated error, I guess
your curve would deal with the same situation, but the way I see it, the
evolved solution should be expected to contain a switch.  It would, as you
say, be a lousy way to build a control hierarchy if every unachievable
reference led to a continuous "kicking against the pricks."  And in your
system, it looks as if what would happen in the car-bicycle-walk scenario
is that when the destination was near (and the higher reference-percept
difference small) the gains of the unselected transport-medium-percept
controllers would increase, giving rise to renewed conflict.

> It seems simpler to me to invoke a higher-level system
>that selects one of several means of locomotion (the kind of
>explanation you used for explaining why the wastebasket wasn't moved
>on the third day). This has the advantage of not pitting the outputs
>of high-gain (competent) control systems against each other, which
>always leads to a lessening of the capacity to control if not its
>complete destruction. I don't think you're taking full advantage of
>the concept of hierarchical control, at least not consistently.

I thought I was pointing out another instance of the power of hierarchic
control.  I am perfectly willing to believe that I'm not taking full advantage
of its power, since I keep finding out new aspects of that power, and I
doubt that discovery process has come to an end.  But putting the onus of
selection onto a combination of the relative insistences of the "competing"
ECSs and the impedances of the relevant (mutually inhibitory) CEVs seems
to me to be consistent with a lot of feelings one has when actually making
that decision, besides not requiring any special mechanism of choice in
the higher-level ECS.  If the bike is handy and the distance not too far,
one might use it, whereas if the bike were hidden behind piles of junk that
had to be moved, one might not.  On the other side of the person-world
interface, if there were a reference level for keeping fit and using the
bike would reduce that error as well as the error of not being at the
destination place, then one might take the bike even if it were hidden
by a pile of junk.

Perhaps it is worth noting that the concept of "impedance" applies not only
to the CEVs of the world, but also to the reference-percept relation of a
single ECS as seen from a higher level ECS that contributes to its reference
signal and receives its perceptual signal.  That impedance will be very low
if the lower ECS has a high gain and the CEV it is controlling has a low
impedance.  And it will be near zero if the lower ECS is imagining.  Conflict
at the lower level may well lead to the higher ECS seeing the impedance of
the lower as being rather high. And for good measure, one should mention the
possibility of negative impedance (which implies positive feedback, and
trouble).

The world may well be much more closely coupled than the perceptual control
hierarchy is.  I think that may be what people mean by "leaning on the world."

Martin
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