[from Gary Cziko 931217.0322 GMT]
A while ago on CSGnet I had been pushing for acceptance of the
vestibulo-ocular reflex (VOR) as an open-loop behavior so that PCTers
wouldn't appear to reject out of hand all open-loop explanations of
behavior (even if the VOR is the ONLY such behavior for you and me and
fellow vertebrates).
However, I have been told of an experiment which now opens up some doubt as
to whether the VOR or saccades are open loop. Here is Joe Malpeli of my
campus describing this study (which I have not yet been able to read for
myself):
Note: SC apparently stands for "saccade."
The monkey SC stimulation
experiments were done by David Sparks. What he showed was that if you
force a saccade by electrical stimulation of the SC just prior to a
natural saccade to a briefly flashed target, the eye still winds up on
the target position (in total darkness). However, this has nothing to
do with visual feedback, since the target is turned off prior to the
electrical stimulation. Rather, the brain keeps track of the eye
position by integrating the velocity signal initiated by the electrical
stimulation to lower oculomotor control centers. What this experiment
shows is the below the superior colliculus, eye position is kept track
of in head-centered, rather than retinotopic coordinates.This reference is:
Sparks, D.L and Mays, L.E. Spatial localization of saccade targets. I.
Compensation for simulation-induced perturbations in eye position. J.
Neurophysiol. 49: 45-63, 1983.
So the its seems that some control is going on here since in spite of
disturbances to the eyeball, the saccade winds up with the eye oriented in
the proper position.
Malpeli's explanation (I suppose it's really the authors') is that the
brain keeps track of where the eyeball is by integrating the velocity
command signal. But I don't understand this here since the disturbing
signal was not initiated by the brain itself, but came from outside the
brain. So perhaps what is really going on is that there is some
kinesthetic feedback (or should it be called proprioceptive feedback--I
don't know what the difference is) after all concerning eyeball position
and all "commands" to move the eyeball whether natural saccades or VORs are
really reference signals supplied to a control system for eyeball position.
As I said above, I have not read this article, and I'm afraid that even
when I do I may not understand the technical details (I'm only an education
psychologist). But I will follow it up and let you all know what I find.
In the meantime, perhaps some other CSGnetters with more expertise in this
area (e.g., current CSG President, Wayne Hershberger) will also take a
look.--Gary