[From Bill Powers (931109.0830 MST)]
Martin Taylor (931108.1915) --
... the
_effective_ reference signal is no longer the direct standard
against which the perceptual signal is compared. The engineer,
standing off to one side, has to adjust this function and the
others in the circuit to achieve the desired relationship
between the reference signal and the OUTPUT.
Yes, that's true. But in a biological system, it would not be
an engineer who would make this adjustment. It would be
evolution ...
You miss my point. The L-H design is intended to produce a known
effector output, not a known perceptual signal. Feedback from the
perceptual signal is just a way of adding corrections to the
basic input-output feedforward chain. In the L-H design, the
perceptual signal doesn't even match the reference signal,
showing how unimportant they consider it.
Thus they can say "Two distinct control problems have been
identified: the servo or signal transmission problem and the
regulator or disturbance suppression problem." This amounts to
saying that we are concerned separately about the effector output
and about the error signals resulting from disturbances.
In PCT these are the same problem: if the perceptual signal is
controlled, that is all that the system needs to do. There is no
concern, with respect to any one control system, with producing a
given effector output, too. The only reason a given effector
output would be desired would be that it has OTHER perceptual
effects which are under control at the same time. And those can
be taken care of by another independent control system.
I realized this morning that the L-H design is a solution of a
problem that does not exist in PCT. In an organism, there can be
no control of objective effects of effector outputs, because
there is nobody who knows what those objective effects are. The
only variables that can be controlled are _subjective_ effects of
effector outputs: perceptions, or intrinsic variables. If those
effects are maintained near internally-specified levels, the
organism couldn't care less what its effector outputs are.
The L-H design results from the engineering approach which fails
to distinguish outputs from outcomes. Once it is recognized that
only outcomes matter, then the perceptual signal can be seen as a
direct representation of the outcome to be controlled, and the
output from the effectors can be left free to vary as the means
of accomplishing this end. There is no "transmission" problem
separate from the problem of countering disturbances.
As to "evolution" taking care of the adjustment problem, I object
to using evolution as a catch-all answer. In cases like this it
is no better than assigning responsibility to God's Will. If we
leave the achievement of organized behavior up to evolution we
might as well give up on modeling altogether. Far better to come
up with a model that doesn't need these adjustments. Any system
that depends on accurate calibrations to achieve its functions is
unlikely to be suitable as a model for a biological system. I
count the need for these adjustments as a heavy mark against the
L-H design. Or let me put it this way: a model that requires
accurate calibration to produce accurate control is far inferior
to a model that can produce accurate control without those
calibrations.
···
-----------------------------------------------------------
I think it's significant that the L-H model from 1955 dropped out
of circulation. I expect that what happened was that some other
engineer realized that it could be reduced to a classical model
simply by redefining functions, and thus introduced no new
considerations. Look at the top part of your diagram:
> reference
V
-------------------
V V
----- -----
> fcn | | fcn |
> B() | | A() |
----- -----
> >
V V
---------- ------- -
>comparator>-> |output |->|+|
> > >fcn G()| -
---------- ------ |
The pathway through fcn A() adds to the output of the output
function fcn G(). This effect could be achieved by defining a new
function B'() such that the reference signal produced the same
added effect via the comparator and output function G(). So this
part of the diagram reduces to the canonical diagram, except for
the useless function in the reference path:
> reference
>
>
V
-----
> fcn |
> B'()|
-----
>
V
---------- -------
>comparator>-> |output |->-
> > >fcn G()| |
---------- ------ |
>
Incidentally, your diagram omits an output function for the
direct connection of the reference signal to the CEV.
------------------------------------------------------------
I have an increasing (or is it sinking) feeling that we're up
against unshakeable tradition, here. Some people are just
insisting that the old input-output model is perfectly good, so
why do we need this new-fangled PCT model? No matter how
elaborate the old model has to be, no matter how much it relies
on calibration by some entity who knows the objective properties
of the environment, no matter how complex the mathematics gets,
no matter what problems have to be ignored, the old model is
still preferable.
The PCT model doesn't need special calibrations; all the output
pathways can vary their characteristics over a wide range without
materially affecting the controlled variable. Control is achieved
by an organism that has no knowledge of objective effects in the
external world. PCT redefines the nature of the problem we're
trying to solve: organisms don't produce specified behavioral
outputs at all, which is the problem the control engineers have
been trying to solve. They control a world of perception. The
engineers keep wanting to solve the other problem, which is not
the problem that organisms face.
--------------------------------------------------------------
We should, of course, continue to examine how feedforward might
play a part. Tom Bourbon has introduced a new consideration,
however, which makes the prospects of feedforward dimmer. We do
indeed have many perceptions which provide alternative ways of
controlling. With this in mind, it's perfectly clear that walking
in the dark is not open loop; one searches desperately for SOME
information about position. Walking in the dark is very different
from walking with the lights on. One walks more slowly, feeling
about with the hands, searching for an identifiable glimmer of
light. When no light exists, the mode of control changes
entirely; one controls for feel, not sight. If even feel is
missing, one doesn't control at all. At best, one can take some
steps under kinesthetic control. To view this transition merely
as a continuation of the old mode of control through feedforward
is to distort grossly what actually happens, and the distortion
is in just the direction needed to make feedforward seem still
adequate. We don't just stride on ahead when the lights go out.
We go into a drastically different mode of behavior. Let's
remember that we're trying to explain what actually happens, not
change what happens to make it fit the concept of feedforward.
Note to Rick Marken: you might try making a perceptual signal
depend on more lower-level signals than are needed to define it
unambiguously. Then control (of the perceptual signal) could
continue indefinitely when some lower-level signals are lost. The
actions might change, but the perception would still be
controlled.
---------------------------------------------------------------
Best,
Bill P.