[from Mary Powers (980929)]
To Bruce G:
I have no way to get hold of R's book since interlibrary loans on this
years' book are no-no's - have to wait until '99. What I'd like to know is
how the circuitry runs between the motor supplement, parietal lobe and
cerebellum areas. R says copies of a command signal are being sent to where
comparing will take place - is there no comparing at the supplementary motor
level? Also, are these areas arranged hierarchically or are they parallel?
"Having received a command to move, the muscles execute the movement. In
turn, signals from the spinal muscles and joints are sent back to the brain,
via the spinal cord, informing the cerebellum and the parietal that 'yes,
the command is being performed correctly'. These two structures help you
compare your intention with your actual performance...and modifying the
motor commands as needed (applying brakes if they are too fast and
increasing the motor outflow if it is too slow). Thus intentions are
translated into smoothly coordinated movements."
The trouble with this is in the concept of "motor commands". The ability to
speed up commands or slow them down does not sufficiently address the
problem of correcting for environmental disturbances, beginning with
continuous recalculation of commands as individual muscle fibers fatigue and
recover, and continuing on to the exact motions involved as joints shift
relative to one another, and the environment outside of the organism changes
(such as the weight of a coffee cup before and after a sip is taken, and
similar minutiae as well as larger disturbances). The phrase "yes, the
command is being performed correctly" in no way resembles the concept of
perceiving that the intention is being carried out. An action/command sytem
is simply not capable of specifying movement to the detail required. A
controlled-perception system does not have to specify, it varies as it must
in order to realize the intended movement.
I sympathize with your belief that R is saying the same thing PCT is. I
made this mistake often myself. It isn't easy to recognize the difference,
which is subtle but enormous. The whole ball game, in fact.
Mary P.