left-right reversal of CNS: Why?

Jerry Larson jlarson at hsc.usc.edu
Sun Sep 8 13:47:16 EST 1996

Robert Steven wrote:
> >Henry... the left-right "reversal" to which you refer occurs
> >ubiquitously within mammalian nervous systems for one reason...
> >
> >The nerve-fiber termination distributions that result align with the
> >totally-inhibitory outputs of the cerebellum so that the problem of
> >activating the muscles to move away from environmental sources of
> >noxious stimulation can be resolved by "simply" minimizing the
> >topologically-distributed ratios of excitation to inhibition (TD E/I)

> What? I didn't realize it was so crystal clear. Doesn't the fact that our
> representation of the environment is reversed on our retinas play any role?

Hmmm.  Retinas.  Hmmm.  I don't know about that, and like somebody else
on this thread, I don't understand how organisms with radial symmetry
can have crossed representation.  Nor do I really know why there is
crossed representation, but I can make a couple of observations.

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. 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.  This is good, because if you lose one eye you don't lose half of
space!  The result is also that the left side of the brain works the
right arm and sees the right visual field, which seems convenient-- but
I don't see why it wouldn't work just as well with ipsi representation. 
btw, most cranial nerves have either ipsilateral or bilateral

I read somewhere that, with the visual fields and somatic pathways
switched like that, you could think of it as though you had a system
with ipsilateral representation, and twisted the top of it around 180º
to face backwards.  I still have no idea why that's better than ipsi
representation, or whether it's just an accident in evolutionary



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