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Date: Thu, 8 Jan 1998 02:36:24 -0700
Reply-To: “Control Systems Group Network (CSGnet)”

Sender: “Control Systems Group Network (CSGnet)”

From: Bill Powers
Subject: Re: Tensegrity
To: Multiple recipients of list CSGNET

X-UIDL: 9609e32fd5264b23dffd94efb649101b
[From Bill Powers (980108.0200 MST)]
Found Ingber’s article on “tensegrity” on the Web, thanks to Rick’s reference.
What Ingber seems to have discovered is Hooke’s Law. Well, give him credit;
he’s discovered that mechanical systems, under applied forces, tend to
distort, and that the distortions produce restoring forces. Thus “tensions”
appear within the distorted system that, when the disturbance is removed,
restore the “integrity” of the system: therefore, I assume, “tensegrity.”
As Rick pointed out, this has nothing to do with control, and therefore
misses the main point of living systems. Even tennis balls have tensegrity
– heck, so does every material object that doesn’t fall into powder at the
first touch. The effect Ingber is trying to describe, or course, is
active resistance to disturbances, in which there is far more resistance
than can be accounted for by elasticity. He doesn’t, however, know that
this is what he is trying to describe; there’s no evidence in the article
that he knows the difference between passive elasticity and active
resistance to disturbance.
I recommend a dose of “A Bucket of Beans,” in LCS II. For those not
familiar with the demo:
The wire handle (bail) of a toy bucket is unhooked from the bucket and
re-attached through two rubber bands. The subject is told to grasp the
handle and hold the rim of the bucket next to and level with some object,
like the seat of a chair (but not over the seat). The experimenter then
slowly pours a bag of pinto beans into the bucket until it is full (less
messy than water if a rubber band breaks). You know the effect is getting
across if the audience gasps. What happens, of course, is that the rubber
bands stretch as the beans are poured in, but the wrong end of the rubber
bands moves
. The bucket stays more or less stationary, and the handle rises.
You can see the same effect happening if you hold a pot under a faucet and
run water into it. Instead of the pot sagging as the increasing weight
stretches the muscles, the pot stays still and the muscles shorten.
Another nice demo is to sit in front of a table and press down on it with
one hand, using the other hand to feel your triceps. The arm doesn’t move,
but you can feel the triceps tensing. The same thing will happen if you
hold the arm out and ask someone else to push up on it while you hold it
These effects do not demonstrate tensegrity; they demonstrate active control.
If any of you haven’t seen or tried the Bucket of Beans demo, I highly
recommend it. It’s a great way to understand the difference between passive
and active resistance to disturbances.

On another subject, I have a new demo of the effect Gary Cziko discovered,
in which you try to put your finger on a dim light-source in a dark room. I
noticed this one when I lost track of the text cursor (a short vertical
bar) on a cluttered Windows screen. Since there is no absolute relationship
between the mouse’s physical position and the position of the little line
on the screen, you can’t control the cursor until you can see it. You move
the mouse around at random until the cursor finally moves back onto the
screen; then you can control it again.

One final subject. A question that has arisen with respect to Bruce
Abbott’s experiments is that if rats don’t vary their rate of pressing
under either fixed ratio or variable-ratio schedules (as is beginning to
look like the case), what the heck are they controlling, if anything, and
how? My thought is that these experiments are far too messy and
complicated, and it would be nice to find something simpler and clearer.
There is a precedent; about a million years ago, someone studied
“approach-avoidance gradients” by attaching harnesses to rats and seeing
how hard they pulled to get to a food dish. Of course they pulled hard
enough to get to the dish, if they could. But a variant on this experiment
would be easy to set up, in which a rat already located at a food dish is
pulled away from it by a string attached to a harness. If the floor of the
cage were mounted on strain gages, we could measure how hard the rat pulled
back against a force applied to the string. We would find, of course, that
the rat’s effort was equal and opposite to the pull on the string. Of
course we could deduce how hard it was pulling back from the fact that it
remained at the food dish, but just for completeness it would be good to
demonstrate the equality of the applied force to the muscle-generated
resistive force.
This may be a very simple experiment, but at least we could demonstrate one
variable that a rat controls with high accuracy: its own body position.
Bill P.