From: Marc Bernatchez <mbernat@gel.ulaval.ca>
Subject: Re: TECH: Accommodation vs convergence in stereoscopy
Date: Fri, 8 Mar 1996 13:45:28 -0500
Organization: Universite Laval


From: Marc Bernatchez <mbernat@gel.ulaval.ca>

On Tue, 27 Feb 1996, Jorge Alvoeiro wrote:

> From: Jorge Alvoeiro <J.Alvoeiro@psy.hull.ac.uk>
> 
> Marc Bernatchez wrote:
> > 
> >[...]
> > brain only dispose of the retina image clarity to compensate the
> > crystalline muscles.  This way, the brain proceeds much the way an
> > auto-focus camera will and go by trials and errors to find the
> > operating point.

> Have you forgotten about the 'Corpus Callosum' (CC)? This is the bridge 
> between the two sides of the brain. When information from the eyes crossover 
> (vision from the left side goes to the right and from the right side to the 
> left side of the brain) and reaches the back of the brain it fuses into one 
> and creates a 3d picture. This is called stereopsis. Thus even if you close 
> one eye some information is passed over to the other side by the CC. As this 
> is a very vast area do a keysearch on STEREOPSIS,  RETINAL DISPARITY and 
> OCULAR DOMINANCE COLUMNS.
> 

In fact, from my understanding, it's not the left eye image that goes
to the right side of the brain and vice versa. That's the popular
belief but it's slightly more complicated than that. The leftmost part
of the visual field goes to the right hemisphere and so on. So, each
hemisphere gets information from both eyes even if your two
hemispheres don't communicates together. More than that, a person
which has an hemisphere removed will still see in stereoscopy over a
certain reduced field of view.

Check this picture(netscape news ready):

<img src="http://www.gel.ulaval.ca/~mbernat/stereo/cortexan.gif">

Illustration inspired from the book "Virtual Reality ExCursions with 
programs in C "

You're right saying that even with one eye closed, you still have 
informations from that eye sent to each hemisphere BUT you only have 1/2 
the required info for stereopsis in each hemisphere. So you end up with a 
monoscopic vision over the full field of view. The point is, no matter 
what, you can't achieve stereovision with a single eye.

I still need more feedback on the relation between accommodation and 
convergence, keep it comming :-)

Bye

 Marc Bernatchez             |  E-mail: mbernat@gel.ulaval.ca
 C.O.P.L. (local plt-00307)  |============================================    
 Dep. Genie electrique       |  http://www.gel.ulaval.ca/~mbernat
 Universite LAVAL            |  http://www.imaginative.com/VResources
 Quebec, Canada              |============================================
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