left-right reversal of CNS: Why?

Henry M. Wieman wiemanh at ma.ultranet.com
Mon Sep 9 18:16:22 EST 1996


In article <3233149B.75CC at hsc.usc.edu>,
   Jerry Larson <jlarson at hsc.usc.edu> wrote:
>Robert Steven wrote:

>
>Hmmm.  Retinas.  Hmmm.  
>I don't know if the crossed image on the retina has anything to do with
>it.  After all, the lens also reverses things up-down as well as
>left-right.
(snip)
Ah, but the sensory AND visual, and motor mappings ARE upside down as well as 
right-left reversed. 
> Anyway, contralateral representation works differently in
>the visual system.  The somatomotor and somatosensory pathways cross to
>the opposite side of the body, at the level of the medulla.  The visual
>pathways cross, not from one eye to the contra hemisphere, but from the
>left visual field of EACH eye to the right visual cortex and vice
>versa.
Well, the visual "map" in the occipital lobe is, just like the sensory map in 
the parietal, as though there were one upside down and backward retina. Just 
like the slides you put in the slide projector. It looks like you rotated the 
image around an axis running front to back 180 degrees. So the retinal 
hypotheses has some merit. If you had a lens that projected onto the back of 
the brain it would project onto the map of the visual image just right.

> most cranial nerves have either ipsilateral or bilateral
>representation.
No, most cranial nerves map onto the regular old senory and motor maps. Smell 
is, as far as I know, non lateral and hearing is mixed.

I personally am of the opitical connection persuasion, but was looking for 
some info from a comparitive neuroanatomists to bolster or damage the 
hypothesis. I would like to see more than the "same hands." :)


   	Henry M. Wieman          | "Can you answer? Yes I can!
        wiemanh at ma.ultranet.com  |  But what would be the answer,
        Call me Hank             |  To the Answer Man?"
                                 |       -Robert Hunter in St. Stephen



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