[Avery Andrews 930123.1350]
Schmidt, R.A. (1980) `On the Theoretical Status of Time in Motor-
Program Representations', in Stelmach & Requin, _Tutorials in
Motor Behavior_, North-Holland, pp. 145-166.
Discusses experiments in which levers are moved quickly, in
opposition to various sorts of disturbing forces. The results
suggest that any positional control has fairly low gain, but the
article suggests unawareness of the role of gain in the functioning of
feedback systems:
"A feedback view would, of course, predic tthat the limb would reach
the target (against a constant-force disturbance) ..." (p. 159).
"The effects of added or subtracted spring tension on the movement
endpoint support the mass-spring notion, and provide additional
evidence against the idea that the terminal position is achieved by
some sort of feedback process." (p. 160).
Also betrays possible confusion of continuous control with proprioceptively
mediated response chaining:
"This argument was strengthened by human evidence that the processing
of information leading to a new movement was slow, requiring 150
to 200 msec for the new action to begin. This kind of feedback
processing, if it were to be emplyed in the ongoing control of
a rapid motor act like throwing, would be too slow to be effective
until that act is completed." (p. 148)
I am frankly not at all sure what's being addressed here: throwing is
a *continuous* act, not one that is cleanly segmentable into discrete
subacts, so it's not clear how response chaining would work; furthermore
initiating a voluntary act is presumably different from ongoing modification
of a `program' (and, Gary Cziko has a demo to the effect that accurate
throwing is possible in the face of disturbing forces - a quantitative
study would seem called for).
An important point is that PCT does not challenge the existence of CPGs,
rather, it simply claims that they will normally produce reference levels
for perceptions (and will therefore in general be able to produces
error signals and drive behavior when the afferent pathways or cut, although
considerable retuning will be necessary to get passably effective behavior).
On a more positive note, the article contains a lot of interesting material,
and does refute the impulse-timing view of movement control, which is
more counter-PCT than the mass-spring view, and is open to the idea that
afferent information modifies the outputs of Central Pattern Generators
(p. 147).
It is proposed that programs have a variety of parameters, some of which
seem more plausible than others. E.g. Movement time seems plausible
(the pattern runs faster or slower), while Force and Muscle Selection
seem very dubious. I suspect that Gary Cziko's throwing demo can
be turned into a total refutation of the Force parameter (if you
can throw accurately against variable disturbing forces, you
can't do it with a preset Force parameter).
There is perhaps an avenue of empirical investigation into Muscle Selection
as well. People appear to have a fixed handwriting
style that is invariant over a substantial size range, from blackboard
writing done with arm muscles, to ordinary writing done with fingers.
This suggests that a size-scalable perceptual target is involved,
perhaps involving kinesthetic effort perceptions, etc. If so, then
*deafferented* people would not be expected to have a size-scalable
handwriting style, at least when their eyes were closed. Writing
on the blackboard with your eyes closed and rubber bands attached to
your arms might reveal things as well.
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