left-right reversal of CNS: Why?

Jerry Larson jlarson at hsc.usc.edu
Wed Sep 11 13:25:38 EST 1996


Henry M. Wieman wrote:
> 
> In article <3233149B.75CC at hsc.usc.edu>,
>    Jerry Larson <jlarson at hsc.usc.edu> 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.

I don't find this very compelling.  You can describe the sensory and
motor maps as "upside down" in a human being because of our upright
posture, but in a quadruped those maps are not "upside down", but at 90
degrees to the axis.  Further, there are a number of motor and sensory
and motor homunculi, in thalamus and basal ganglia, and in cerebellum,
and I'm not sure that "upside down" describes them especially well;
again, they're more orthogonal.The cerebellar homunculi, in fact,  are
not even contiguous.


There are not, after all, upper and lower hemispheres to the brain!  The
left-right reversal actually involves fibers decussating from the left
side of the brain to the right side of the body.  There is nothing so
clearcut for "upside down".  

> 
> 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.

That works a little better, except, as I said, the virtual lens you are
talking about is divided between the two eyes!  That, of course is not
true for the up-down reversal.
> 
Jerry



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