Newsgroups: bit.listserv.csg-l
Date:         Tue, 15 Dec 1992 14:03:38 -0600
Comments:     Warning -- original Sender: tag was cziko@UX1.CSO.UIUC.EDU
From:         "Gary A. Cziko" <g-cziko@UIUC.EDU>
Subject:      VOR as Feedforward?

[from Gary Cziko 921215.1956 GMT]

Bill Powers and/or Wayne Hershberger and/or Greg Williams and/or other
interested parties:

I have begun discussion with a neurophysiologist on my campus about PCT.
Since my wife works with and is a good friend of his wife, we keep bumping
into each other and so I thought I would see how he responded to PCT ideas.
 Here is one reaction:

>However, the system I study (the vestibulo-ocular reflex or VOR)
>is feedforward.  Its input (head velocity) is not affected by its
>output (eye velocity).  So this system does not control its input,
>but just responds to it.  I've done some modeling work on this
>system, and in particular, I've tryed to show that other conceptual
>tools can be used to go beyond control theory in attempting to
>understand the neurophysiology of this system.  I'd be happy to
>send you a copy of a relavent paper, if you're interested.

I've heard this quite a few times before (together with the statement that
the eye has no propioceptive sense as to where it is).  But I have trouble
making it mesh with Bill's strong statement of no important relationship
between efferent neural output and behavior.

I can see how the eyeball is a pretty well protected from disturbances, but
nonetheless how can the brain send a computed command to it to exactly
compensate for head motion without on-line feedback?  (Our ability to do
this can be easly demonstrated:  put your finger about a foot in front your
eyes and move your finger side to side about 4 inches at about two cycles
per second.  You will see a blurry finger in spite of your eyes attempting
to track it.  Now move your head side to side at the same speed and keep
the finger still.  You will see it perfectly clear).

Even protected from outside disturbances, there is still the acceleration
to velocity to position problem which Bill claims makes computing muscle
responses highly sensitive to error.  If indeed there is no proprioceptive
feedback about eye position and this is pure feedforward, doesn't that show
that such efferent computations ARE possible?

But then what happens when the muscles tire?  And has anyone ever tried to
disturb the VOR by putting molasses-like eye drops in the eye or resisting
its motion by somehow attaching a rubber and tugging during head movements?

I realize that there is a higher level feedback system which uses vision
here.  But would that be fast enough to make this a real feedback system in
spite of the lack of proprioceptive feedback?

I remember bringing this topic before, but apparently my current confusion
means that either I didn't get answers to these questions or that I didn't
understand them at the time.  Let's try again.--Gary

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Gary A. Cziko                    Telephone: 217.333.8527
Educational Psychology           FAX: 217.244.7620
University of Illinois           E-mail: g-cziko@uiuc.edu
1310 S. Sixth Street             Radio: N9MJZ
210 Education Building
Champaign, Illinois 61820-6990
USA
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