Re: stereovision question

[P3D Jim Crowell <crowell@xxxxxxxxxxxxxxxxx>]

At 5:13 PM 4/8/96, P3D Josh Rubin wrote:
>
>I recall from my undergrad days that there are
>feature detectors, including motion detectors,
>at the retina, or at least in the optic nerve,
>at least in frogs.

Frogs are quite neat, actually.  They have neurons early in
their visual pathways (perhaps even in the retina) that look for
small, dark, moving objects overhead.  The outputs of these
neurons bypass most of the brain and feed directly to the
tongue; they're "fly detectors".  In people motion extraction is
a much more complicated process that happens in several stages.

>But I betcha that the depth cues
>from motion parallax, even though monocular,
>occur "after" the convergence of the two
>optic nerves.  That being the case (if it is),
>why wouldn't it be adaptive for the binocular
>mechanisms to be available to parse and
>interpret monocular stimuli?
>

There may be a single stage that computes depth using both kinds
of signals, but some motion calculations happen before the two
eyes' inputs merge.  Also, at the front end the two have rather
different properties.  For instance, your sensitivity to stereo
is greatest when the scene is stationary or moves very slowly,
whereas your sensitivity to motion parallax peaks at much higher
speeds.  Motion parallax is also an evolutionarily older depth
cue; stereo is relatively recent.

So anyway, while it's true that the sensation of depth you're
talking about may be mediated by some of the same brain areas,
it ain't called stereo unless it comes in via the stereo pathway
(using the differences in the two eyes' images).

-Jim C.


------------------------------------------------
Jim Crowell
School of Optometry
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