IUBio

Cross - Wired Eyes

Marielle Fois im99_foa at nada.kth.se
Sun May 13 12:35:40 EST 2001


On 13 May 2001, Manni Heumann wrote:

> "Karl Self" <karl.self at gmx.net> wrote on 13 Mai 2001:
>
> >    I know that in humans, the eyes are cross wired to the
> >    brain, i. e. the
> >left eye is connected to the right brain hemisphere and vice
> >versa.
>
> This is not quite correct: It's not the eyes that are cross
> wired, it's the two halfs of the visual field that are. Thus, if
> you are looking straight ahead, anything left of your nose will
> end up in the right hemisphere and vice versa.

Exactly. This means that two optical nerves, one from the left eye
carrying information of the left hemifield, and the other from the
right eye carrying information of the right hemifield, cross at some
point to meet the other nerves.

> > My questions:
> >-  why is this so?
>
> Good question. IIRC, this has been discussed here a couple of
> weeks (months?) ago. Although, with no definitive answer.

I can think of some reasons. Given the position and structure of our
eyes, we perceive information from both hemifields with both eyes. It
seems reasonable that the optical nerves that perceive information
from the same hemifield will finally meet so that this information is
processed in the same part of the brain. If this is accepted, the
next to know is which nerves should cross.

It seems also reasonable that the nerves to cross are the ones
closest to the nose. First, because in other case it should happen an
early cross between the two nerves of each eye, and what for to do
that. Second, for mercy. Well, not really, but it could be. Suppose
you suffer a damage in the part of the brain where the two optical
nerves cross. Just imagine the field of vision it would remain to you
if the nerves to cross were the farthest from the nose.


Marielle




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