[From Bill Powers (2002.05.21. 0957 MDT)]
Bruce Nevin (2001.05.20 16:07 EDT) --
Why two control loops? Please specify. Is there a duplication of the
hunger/eat loop? Is there a duplication of the see/smell/locate loop that
serves it, or of the walk over to hay loop that also serves it?
You bring up a relevant question, to which I have only a conjectural answer.
I see one level at which the food locations are perceived and controlled,
and a lower level in charge of moving to and being in different spatial
positions. If two independent systems are controlling for food at two
locations, they both would operate by sending reference signals to a
location-controlling system (if we assume that that is where the conflict
would be expressed). The reference signals would be added, or perhaps be
averaged, at the reference input of the lower system, where the virtual
reference level would be established. At that level we have normal control,
but the final goal position would not be at the location of either food source.
The basic question is, why two control systems at the higher level? This is
a result of a basic assumption I made long ago. Rather than thinking of one
perceptual signal that can convey many different kinds of perceptual
information (the "encoding" concept), I opted for a model in which each
perceptual channel conveys only one dimension of experience, with
variations in signal magnitude only, so a given signal never means any
different _kind_ of perception. This may seem wasteful, until you look at
the other problems it does away with, such as decoding that can recognize
one signal pattern out of many different patterns that might be present in
the same complex signal. Instead of having, for example, a
phoneme-recognizer that can put out one signal pattern for "a" and another
pattern, in the same physical channel, for "e", we would have separate
_simple_ signal channels for different phonemes, with one channel always
representing the same phoneme. If we go for the "encoding" idea, we then
have to provide, at the input of every system that receives complex phoneme
signals, a device that can pick one particular pattern out of a stream of
many possible (or even overlapping) patterns -- so we have simply moved the
pattern-recognition problem into the receiving system without solving it.
If each perceptual channel always carries a signal representing a specific
dimension or attribute of perception, then the receiving system needs no
decoder. However, the control of any given perceptual signal requires its
own control system, which might seem like an undesirable tradeoff. We get
it all back, with interest, when we look at how complex the control systems
must be. The comparator consists of a simple magnitude-subtractor: it
receives two signals and outputs a signal having a magnitude proportional
to the difference between the input magnitudes (magnitude == frequency).
The output function may or may not be complex, but it doesn't need to
recognize different _kinds_ of error signals, so it's far less complex than
the output system needed with the "encoding" concept. In fact, the minimum
control system,. at any level, consists of just two neurons: the input
neuron that creates a signal that is a function of multiple inputs, and the
comparator, which receives a reference signal and the perceptual signal and
outputs the difference signal. These functions could even be combined into
a single neuron. The complexity of the whole system is then brought about
by having a large number of cheap and simple control systems rather than a
small number of very complicated and neurally expensive ones.
This is basically how I can visualize two control systems, one for
controlling the relative position of each food source (by moving either the
body or the food source). However, since this implies a huge conflict, we
need yet another level that activates the reference signals for just one of
the food-source controllers at a time. If that level exists, the donkey
will simply walk up to one haystack (the biggest? The closest? The
greenest? The first one noticed?) and eat it first, and then if it's still
hungry, will move to the other and eat it (sequence control).
If, somehow, the donkey failed to use a higher level of control to resolve
the conflict, then both control systems would turn on at the same time and
would try to move the body to two different locations at once. The rest
follows.
Am I satisfied with this? Well, no. There must be some compromise idea that
retains the simplicity of the one-perception-one-channel approach, but
without the awkwardness of supposing that there is a control system for
every possible perceptual signal. On trhe other hand, we must recognize
that there are many situations in which we actually notice multiple
attractive things, and for some time at least dither about which one to
pick first (assuming, of course, that we can't have all of them at once).
Why do we dither instead of just picking one and being done with it? That's
really the situation we're trying to analyze, even if it's short-lived.
I quite agree with you about actual donkeys, and would agree if you said
that if any real donkeys behaved like the apocryphal donkey between the
haystacks, there would be no real donkeys. We have probably wasted enough
time on this invented and imaginary problem which psychologists, but no
real donkeys, have had.
>As Martin (2002.05.20 09:38) also observed, this is about making a decision.
Well yes, that names the problem but doesn't illuminate much. Exactly what
is a decision? How is a decision made? I think that what we usually call
decision-making situations are simply conflicts, where, for example two
desirable goals exist but we can't achieve either without precluding the
other, or where picking the desirable one means we get the undesirable
consequence, too, and so forth.
To say that a decision is needed doesn't tell us how we make it. We've
already considered the main possibilities: reorganization, or the
application of some learned algorithm that automatically makes the
selection of one goal for us. Either way, flipping a coin or turning a
logical crank, we have avoided the need to "make a decision." I don't
really think there is any special human capacity for "deciding." There are
just various miscellaneous tricks for resolving conflicts that we have
learned, or that are built in to us. When there are no conflicts, no
decisions are needed.
Indecision does happen, but usually when to gain A I lose B and vice
versa. In Martin's example, do I go by car, bike, or bus? Each choice has
some benefits and some detriments. These values (e.g. exercise vs.
possibly picking up a package at the post office vs. wife possibly needing
the car) are not commensurate and matched up like pairs of haystacks. The
monkey with its fist caught in a jar is another example. Two controlled
variables at a higher level (food vs. freedom) each depending upon the
employment of one effector (the monkey's paw). When no matter which way
the ass turns it gains hay to eat, the choice lacks this poignancy. It
doesn't matter which one it goes to, there's hay there.
That's a good discussion of conflict, which is the same thing I'm talking
about. But no particular "poignancy" is needed to create a conflict. When
no matter which theater you go to, you will see a good movie, you don't
generally just turn at random or go into the nearest one. You stop and
discuss it for a moment, looking for some "reason" for picking one over the
other -- for example, you want to see both, and could see either one, but
one will be playing tomorrow while this is the last day for the other one.
Ah, no decision required. When you're in conflict between equally desirable
alternatives, almost any reason will do, however, superficial (better popcorn).
>Again, why do you need two control loops?
One reason is to explain why we feel a pull in two different directions at
once, even though we know that any such "pull" is really our own doing
(unless we prefer the S-R explanation). Somehow it seems that we are really
trying to go two ways at once, if not with our muscles, then at a higher
level, in picking where we want to go. Such situations don't last long,
normally, but when they do we can be in a real pickle. Stay with the wife
and kids, or fly off to Tahiti with the gorgeous bimbo? I have that problem
all the time.
>Why turn toward one haystack rather than the other? Well, why does the
vortex over >the drain start one way or the other?
If you mean that any slight disturbance could tip the balance one way or
the other, you're at least acknowledging that there are simultaneous
opposing tendendies or balanced forces. But the crucial question is what
you assume about the consequences of a perturbation. If you assume that the
equilibrium is unstable, then anything that creates a tiny unbalance
determines the outcome. But if the equilibrium is stable, like a marble in
a bowl, then perturbations will be resisted and the equilibrium will be
restored. So your argument tends to get circular. If, as you say, the
imaginary donkey will go increasingly in whichever way it is perturbed,
then it follows that the donkey's system is of the type that is in unstable
equilibrium, which is the kind it has to be if it is to behave that way.
But if it's the other kind, it will return to the equilibriuum point after
the perturbation. Whatever you assume determines the outcome. It's not
valid to make the assumption, deduce how the donkey would behave, and then
use that behavior to prove that your assumption is correct.
For example:
No, that's backward. Something has to happen (and keep on happening) to
preserve the stability of the situation. The ass lives in the same
disturbance-filled universe as we do. Unless something is controlling to
resist disturbances, something will happen to disturb whatever transient
stability you may have set up.
That assumes that the ass's behavior is not determined by a pair of
conflicting control systems (which keep on acting to preserve stability),
but is in an unstable equilibrium such that a push in any direction will
cause it to go further in that direction. Fine. But you could have assumed
the other thing, too.
As soon as you drop from the ethereal realm of logic to an actual ass in
an actual disturbance-filled environment, including factors incorporated
into the ass's memory and imagination from its history of experience, any
perceptual differentiation can become the basis for preference. Which way
is the breeze blowing? Which way is the sun shining? is there a fly
buzzing in that ear? Is the ass right-hoofed or left-hoofed? Does it smell
water too?
All this starts with the assumption that there are no control systems in
conflict, operating at the same time in different directions. Instead, the
proposal is that our actions are caused by the sum of stimuli acting on our
nerve-endings, which trigger our behavior to go one way or another. Is
that really what you mean to propose?
I don't know about you, but when I'm faced with an embarrassment of riches
I look for differentiators. Those tomatoes in the grocery store may look
all alike, but are they really?
That's what I've been saying here. When there's conflict, we look for some
way to resolve it -- rejecting one tomato that has a micosopic blemish that
the other doesn't have, or one more blemish than the other has. The last
thing we want is to have to make an actual arbitrary decision. But of
course no such problem would arise if we didn't have any conflict. We'd
just pick up the first tomato to hand and go on our way. We create our own
conflict by setting up multiple goals; if we can't see any basis for
choice, we make up some basis. It's as though we have to suffer at least
some conflict before we can feel that action is "justified." If we happen
to be trying to remember where the bread aisle is, we just pick up the
tomato at hand and press on, forgetting to make the decision.
So the ass between two haystacks is not stuck in indecision which way to
go because something, it matters not what, tips the balance. And no matter
how carefully the investigator has balanced the choice, it cannot stay
evenly balanced in a world filled with disturbances. And there's hay
either way. In just the same way, the ass facing a stack of more than one
mouthful is not stuck in indecision which mouthful to eat, because it's
all hay. All it has to do is pay attention and move toward the hay it's
looking at.
I think that's about enough of that. You opt for unstable equilibrium and
the impossibility of conflict, as well as the deciding influence of
stimuli. I opt differently. And we must remember that no such asses,
organized in any way mentioned here, have actually been observed to behave
in either way, whether on the head or the point of a pin.
The only way out of arguments like these is continued modeling. Every set
of assumptions implies its own set of predicted behaviors. The best we can
do, lacking data, is to set up models covering the various possibilities,
and wait for a time when we can put them all to the test.
Best,
Bill P.