[Avery Andrews 930122.0906]
Bizzi's work seems soundly and closely reasoned, but there's a bit
of evidence that at least in 1976 he had some strange ideas about how
kinesthetic feedback would be expected to work. It appears in
Bizzi, E., A. Polit and P. Morasso (1976) `Mechanisms underlying
achievement of final head position', J. Neurophys. 39:435-444.
The major point of the paper is that when monkeys orient their heads
towards a flash of light, the final head position seems to be determined
whether feedback is available or cut off (by deafferentation), and that
adding either inertial or constant force loads to the head does not
affect the final position (once the constant force load is removed).
The strange idea about kinesethetic feedback arises in discussing
the possiblity of a mechanism comparing actual with desired vs.
actual current head position: "the output of this hypothetical comparator
might provide a signal leading to the cessation of the ongoing motor
pattern." (rather than the cessation of a signal from the comparator
causing a cessation of the motor effort). This idea is attributed
to two earlier papers:
Gibbs, C.B. (1954) `The Continuous Regulation of Skilled Response by
Kinaesthetic Feedback', British Journal of Psycnology 45:24-39.
Eccles, Sabah, Schmidt + Toborikova (1971) `Modes of Operation of
the Cerebellum in the dynamic loop cotrol of movement,"
Brain Research 40:73-80.
Also somewhat suggestive is the conclusion drawn from one of the experiments.
In this experiment intact monkeys orient their heads to a light flash
against a constant force load, which is released soon after the movement
commenses. There is no pre-set position they are trained to achieve,
but what happens is the the head stops for while, and then, when the
load is removed, moves a bit further, as if there were a spring-like
force attracting it to its final position, so that the initial part
of the turn stops when the load balances the spring force.
This is consistent with any number of possiblities (such as a kinesthetic
reference level with a relatively low gain control system), but the
conclusion drawn is:
"the program for final position was maintained during load application
and was not readjusted by proprioceptive signals acting at segmental
or suprasegmental mental levels" (438).
or, restated:
"proprioceptive signals originating from the moving neck fail to
reset the central patterns responsible for final position" (442)
True enough, but what is not so clear is why this would be expected
to happen in the first place: what makes this non-reprogramming
signification? Describing what is basically a fixed setting
of some sort as a `program' also seems a bit odd.
In a later paper,
Bizzi, E., P. Dev, P. Morasso and A. Polit (1978) `The Effect of Load
Disturbances during centrally initiated movements', J. Neurophys.
41:542-556.
they calculate that the feedback loops (spinal and such higher level
kinethetic as may be operating) contribute between 10% and 30%
of the spring-like restoring force.
Something that strikes me about the literature I've seen so far is
that people seem to have the spinal reflex loops reasonably well
in hand, but the show falls apart when it comes to kinesthesis,
arguably, I think, due to the profusion of false ideas about
kinesthetic feedback would actually work.
Avery.Andrews@anu.edu.au