P3D Animal stereopsis

[aifxtony@xxxxxxx (Tony Alderson)]

First, I should point out I've changed the subject label from the original
thread.

Gabriel Jacob wrote (digest 3235):
>It could have been a survival factor, although not as a prey but rather as
>predator.

I'm no anthropologist, but my impression from my generalized reading is
that our line had stereovision well before we started hunting other
animals. And before we got "smart" too. Many of our primate cousins are
vegetarian. And the earliest homonid hunters were pretty low on the pecking
order--we were prey ourselves pretty often.

Gabriel continues:
>Continuing from the book...

>Why is the visual system designed in such a way as to provide two equally
>sensitive mechanisms (disparity and motion parallax) for seeing depth.
>Wouldn't it be safer to relocate the eyes on the side of the head, thus
>also producing a panoramic view of the world? The loss of disparity could
>be compensated by the equally effective use of motion parallax.

I realize Gabriel is quoting another here, so I don't mean to be
argumentative, just skeptical.  But the author is contradicting his earlier
statement (quoted by Gabriel) that "Relative depth judgments based on
motion parallax are almost as accurate as those made using binocular
disparity (Graham, 1965)."  Well, "almost" is not "equal."  The mere fact
that both have evolved in parallel in the same animals indicates (to me)
that while complementary, the two mechanisms are not equal and have their
own strengths at different tasks. Gabriel's other comments indicate that
the two together are much better at depth perception than either alone.
Which is certainly my experience.

>A possible answer to this puzzle they give is, frontal-eyed animals are
>usually predators.<

Now, I've heard this a lot, and even said it myself, but upon reflection,
is it really true that "prey" animals do not have stereovision? Yes,
herbivores do tend to have eyes on the side, giving them wider peripheral
vision, but I've stood in front of cows, deer and horses, (etc.) and it
sure seems to me that they have some overlap of forward vision, and seem to
be able to coordinate their eyes to look forward. Doesn't this suggest that
they can see in stereo, at least directly in front? (Humans don't see
stereo across our entire visual field either) And wouldn't this be helpful
to them? Sure, the wide field of view helps early detection of stalkers,
but seeing in stereo directly ahead helps when you have to run away, by
making it easier to detect and avoid obstacles. It also seems helpful in
fighting, eating and possibly even breeding. (it sure seems a lot of P3Ders
combine sex and stereo :)  ) So isn't it reasonable that "prey" have
narrowed their field of stereovision in return for a wider peripheral
vision (while predators have done the opposite), rather than given up
stereopsis entirely? Has anybody actually tested "prey" for stereopsis?
Didn't all mammals evolve from a common ancestor with stereopsis?

I'm not just asking rhetorically--if anybody on the list has some knowledge
of this subject, I'd like to hear about it. Because I don't know the
answer!


Tony Alderson




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