IUBio

Cross - Wired Eyes

Richard Norman rsnorman at mediaone.net
Mon May 14 19:02:39 EST 2001


"Brian" <zhil at online.no> wrote in message
news:AVVL6.7887$Ty6.170980 at news1.oke.nextra.no...
> "Marielle Fois" <im99_foa at nada.kth.se> skrev i melding
> news:Pine.SOL.4.30.0105132056520.22994-100000 at my.nada.kth.se...
> > On Sun, 13 May 2001, Marielle Fois wrote:
> >
> > > In
> > > addition, what is perceived by the left half of the retina of the
> > > left eye and the right half of the retina of the right eye, is the
> > > same.
> >
> > I am sorry, this is completely false.
> >
> > Marielle
>
> No, you were almost right.
> The difference in both visual fields are calculated as the 'depth'.
> Why it's devided on both eyes are to make sure that if you lose
> one eye, you'll still have depth-vision.
> Mvh
> Brian
>

How much misinformation can we really tolerate here?

The original post referred to the fact that most sensory
and motor information pertaining to the left side of the
body is dealt with by the right hemisphere and vice-versa
in the vertebrate world.

It then became a discussion of the details of the
decussation at the optic chiasm, which is expressed
incorrectly above.  The temporal half of each retina
is uncrossed, the nasal half is crossed (at least in
animals like primates with primarily binocular vision
and omitting the double representation of the fovea).
This pattern of crossing ensures that the left visual
field from whichever eye goes to the right side of
the brain and the right visual field to the left side of
the brain. An earlier poster correctly indicated that
many animals with laterally placed eyes (little or
no binocular overlap) show complete decussation.

Now it has to do with cues for depth perception.
Binocular vision deals with the fact that information
from both eyes shows up on each side of the brain.
The disparity in the two images produces very strong
cues about depth.  If you lose one eye you will lose
all binocular disparity.  However there are remain
other cues for depth (which you can verify by closing
one eye and looking around).  The fact that each eye
is divided into nasal vs. temporal halves which separate
at the chiasm is totally unrelated to making "sure that
if you lose one eye, you'll still have depth-vision."





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