Re: cross,parallel, and wall-eyed viewing

[P3D Jim Crowell <crowell@xxxxxxxxxxxxxxx>]

At 8:21 AM 11/13/96, P3D John W Roberts wrote:
>>>>We certainly have all the nerves and muscles needed to make our eyes
>>move outward, otherwise we'd never be able to look sideways. but we can't
>>simultaneously turn them outward, because that's not on the list of brain-
>>approved activities. Eyes straight forward is the position that offers
>>the maximum field of binocular vision. To see something up close, though,
>>the eyes automatically turn inward a bit-crossing your eyes is merely an
>>extreme tension of that action. but if both eyes were to turn outward,
>>binocular vision would be lost, something the eyes, "central control
>>won't allow," explains Dr.William Power, an instructor in ophthamology
>>at Harvard Medical School.<<

It's definitely true that the fusion mechanism drives vergence (in & out)
eye movements.  You can diverge more (well past infinity) if you're looking
at a stereo pair and the separation between the pair is gradually
increased.
>
>Now for the tentative explanation:
>  It is possible that the medium - or high-level control mechanisms of the
>  brain do not address the sideways motion of the two eyes as "left eye
>  move left-right" and "right eye move left-right", but as "point of view
>  move left-right" and "convergence".

I'm pretty sure this is right (in fact it's something I ought to know, but
it's been a while since I read this stuff).  Eye movements can be
decomposed into an anti-symmetrical "vergence" (in & out) component and a
symmetrical "version" (left & right) component, and the two are believed to
be controlled independently.

-Jim C.

----------------------
Jim Crowell
Division of Biology
216-76
Caltech
Pasadena, CA
(818) 395-8337
jim@xxxxxxxxxxxxxxx



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