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Re: Cross-over [HAPP-L]



>	This is a very interesting question.  It certainly makes sense for visual
>information to cross sides (as Ken describes so well), so that the visual
>fields are brought together for steroscopic processing.  But can't think of
>any advantages for general motor and sensory information crossing sides.
>
>At 04:54 PM 11/4/98 -0500, you wrote:
>>>The major question of the day:  Why to the nerve tracks cross over in the
>>>medulla oblongata so that everything to your left is perceived on the right
>>>side of your brain and visa versa?  and of course, why does the left brain
>>>control the right side of the body and visa versa?

Now that James mentions it, I see that I did what I'm forever telling my
students not to do -- I wrote a lot of stuff on the wrong question!  The
question wasn't about the crossing-over of optic nerve fibers but of
somatosensory and motor fibers.  It triggered my recollection of our
on-campus pondering of optic hemidecussation, and I was off and running.
No points for me on that essay question, I guess.

But I think the decussation in the medulla exists only in those animals
with the hemidecussation in the optic chiasma.  It might be stereoscopic
vision that led the way on this evolutionarily.  This would lead to an
advantage in having one hemisphere monitor one side of the body, which
seems to necessitate the visual hemidecussation.  That in turn would confer
an advantage in having that hemisphere control the contralateral side of
the body (hence the medullary decussation).  And finally, that would confer
an advantage in having that hemisphere also receive somatosensory
information from that side.  I'm not sure this is a defensible evolutionary
sequence, but I can see how all of these would tie together.  I agree--I
don't see why there'd be an advantage for decussation of somatosensory and
somatic motor fibers, unless these evolved as consequences of the more
understandable hemidecussation of the optic fibers.

This is a fun place to speculate, isn't it?  I enjoy all the intriguing
questions.  I've also told my students about the listserve and how much
discussion has been provoked by some of their questions.

Ken



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