Re: 3D and peripheral vision

[P3D Jim Crowell <crowell@xxxxxxxxxxxxxxx>]

At 7:47 PM -0700 8/27/97, P3D john bercovitz wrote:
>Norm Goldblatt writes:
>
>> In response to John B's comment about not being able to see an object
>> of interest out of both eyes when looking to the side-
[deleted]
>
>You started by telling us that diverting your eyes made you stereoblind.
>I'd like to hear a little more about these diversions you presented your
>eyes with which made you go stereoblind. Everyone has been cautioned about
>the sorts of non-visual diversions which lead to blindness but I do think
>you've found a previously-undiscovered effect here.
[deleted]

Actually, I think there might be a reason why stereoacuity should be
reduced in near-peripheral vision when you're not looking straight ahead.

Stereo is subserved by a neurons in visual cortext that take inputs from
small regions of the two retinas that are offset from one another--so that
a given neuron will typically respond best when slightly different regions
of the two retinae are stimulated.  To put it in technical language, a
given neuron has a preferred binocular disparity.  Another way to think of
it is that the given neuron will respond best to an object at a particular
location out in space, which you can determine by projecting lines outward
from the retinal regions is responds to in the two eyes in finding the
point out in space (if any) where the lines cross.  The location of this
region changes when you move your eyes.  Looking across the whole visual
cortex, the range of preferred disparities is limited.  Practically, this
means that when you're looking straight ahead, your stereoacuity is
greatest within a particular region of space (because the two "lines of
sight" for most neurons cross there) that changes as you converge or
diverge your eyes.  This region of space is roughly planar, & passes
through the point you're looking at.  When you're looking near, it's
roughly vertical; when you look far, it tips back until it roughly
coincides with a ground plane (handy, no?).

The catch is that when you look off to the side, and even more when you
also look up or down (e.g. up/right or down/left), this arrangement starts
to break down.  By that I mean that if you now take a neuron and project
its "lines of sight" outward, many of them no longer cross anywhere, i.e.
there is no region out in space that will provide input to many binocular
neurons via both eyes.  The correspondence is still good at the fovea
(after all, both foveae are pointed at the same place), but it breaks down
fairly quickly as you move into the near periphery.

Make any sense?  It's hard to explain without diagrams...

-Jim C.


----------------------
Jim Crowell
Caltech Division of Biology
216-76
Pasadena, CA 91125
Tel: (818) 395-8337
Fax: (818) 795-2397
jim@xxxxxxxxxxxxxxx



------------------------------