Ain't Miss-bee-havin'; Flyus Goldbergus

[From Bruce Abbott (950526.0930 EST)]

Rick Marken (950525.1045)

Bruce Abbott (950525.1100 EST)--

I don't think you got my point. You said:

The signal to stop flying comes from sensors on the foot pads.

This is an S-R description because it takes account of only one half of the
actual relationship between variables. If it is true that sensor signals from
the foot pad have an effect on the wing movements that produce flying, then
it is also true that the wing movements that produce flying have an effect on
the sensor signals from the foot pad. There is a closed loop of cause and
effect that goes through the environment.

One could actually argue for a _positive_ feedback relationship here, if one
were so disposed. Footpad contact with a surface increases the output of
footpad pressure sensors, shutting down the motor, eliminating the lift,
placing even more pressure on the footpad sensors. [This is not a serious
proposal, just an illustration.] In complex systems like this, nearly every
action has perceptual consequences. A closed loop does not necessarily
indicate the presence of a control system. What you have to ask is, does
the system actually control a perceptual variable.

The existance of this closed loop
could not possibly be determined by looking at just the wiring diagram of the
fly.

True enough, but the control systems analyst is not interpreting the diagram
in a vacuum and knows what to look for. If it's a control system, somewhere
in that diagram there has to be a sensor, pathway for the perceptual signal,
comparator, pathway for the error signal, and an effector whose actions feed
back through the environment to affect the sensor. As you say:

In order to
determine whether a system is a closed loop control system (by inspection
rather than by doing The Test) you would have to determine whether the
efferent outputs of the wiring diagram have an effect, via the environment,
on the afferent inputs to the circuit; so you have to inspect the circuit
AND its relationship to the envionrment.

No closed loop, no control system. No problem! We ain't miss-bee-havin'.

Rick Marken (950525.1215)]

Well. I was going to let this go but I just can't.

Why am I not surprised? (;->

Bruce Abbott (950525.1100 EST) --

Ever wondered how a fly lands on the ceiling? If the optical signals
indicating a looming surface come from the upper portion of the eyes the fly
begins to extend its legs while increasing its angle of attack. The
forelegs make contact with the ceiling and the forward momentum gets
converted to rotational momentum, pivoting the fly's body around the contact
point, bringing the second set of legs into contact with the ceiling and
shutting down the flight motor. The legs are then adjusted to bring the
body level and the hind legs in contact with the ceiling, all accomplished
by means of a small set of perceptual control systems.

This is a very strange statement. You have described a sequence of causes and
effects:

optical signals from upper portion of eye-->leg extension & increasing angle
of attack--> foreleg contact-->rotational momentum-->pivoting around
contact--> second set of legs in contact--> shut down flight motor.

Then you say "...all accomplished by means of a small set of perceptual
control systems" as though it should be obvious from you description that
perceptual control is involved. But you never say what variables the fly
is controlling or what means it uses to control those variables.

Gosh, Rick, do I have to do _everything_ for you? (;->

You describe the mechanism of fly landing as though it were a Rube Goldberg
device rather than perceptual control system.

I described no mechanism at all, only a sequence of events. I considered
providing a (highly speculative) PCT analysis but decided that a brief
description of the events would communicate the fly's landing strategy well
enough. Based on this description, one could develop a PCT model that would
behave as the fly does, but that exercise was left for the reader.

As seen from the outside, most behavior consists of sequences such as I
described for the fly. Consider the directions for making coffee: Remove
the filter basket and check it for contents. If it is full of old grounds,
empty it. If it is empty, place a new filter-cup in the basket. If it
contains a new filter-cup, add three measuring spoons-full of fresh ground
coffee to the cup. Replace the basket into the coffee maker. . . . and so
on. Each step describes the relevant sensory conditions and the behaviors
that should occur under each condition.

The sequences are there; what matters is how you explain them: as an S-R
chain or as, for example, a program-level perceptual control system. I
think I made clear in my brief description of fly landing which view I
prefer. If you want to spend the time to develop and describe a PCT model
that will account for the observed sequence, be my guest. I thought it was
more trouble than it was worth.

Regards,

Bruce