[From Bill Powers (940813.0715 MDT)]
Bill Leach (940812.2340) --
RE: touch control
Is there a simple diagram of how this transition is wired and
functions?
Sure. You route the touch receptors that would be pressed against an
object by the action of a muscle to the motor neuron that actuates that
muscle, with an inhibitory (negative) effect. That's all. When the
position control system runs the skin into an object, the negative
feedback from the touch receptors looks just like negative position
feedback (all neural signals are alike). So the limb stops moving and
the pressure starts rising.
It's got to be a little more complicated than that, because you can turn
your hand so, for example, the biceps presses the palm (upward) against
an object, or with the wrist rotated 180 degrees, the back of the hand,
and ditto for the triceps pressing the hand downward. The simple hookup
works, though -- I've tried it in simulation. The only real complication
is in the switching network that must exist to decide which touch
receptors will feed back negatively to which motor neurons. You won't
find that arrangement in BCP either.
I've been reading the collected papers of Barbara McClintock, and a
ninetieth-birthday volume in which other people talk about her work. If
you want to know what real PCT research will be like once people can
settle down and do it with official support, read that stuff -- if you
want to feel like a dilettante. Her way of doing things is the way we
will find out how the control hierarchy is actually put together, as
opposed to the way we imagine it now. The simpler we can keep PCT now,
the less embarassment there will be later when the facts come out. I've
been developing a lot of respect for these cell-biology types. They've
had to make up EVERYTHING -- their instruments, their methods, their
notation, their model -- and test it all under the sloppiest imagineable
conditions. It's really quite astonishing. It makes me realize how
impossible it is for us to imagine even a fraction of what will happen
when hordes of real experimental scientists get their hooks into PCT.
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Best,
Bill P.