left-right reversal of CNS: Why?

dantso at cris.com dantso at cris.com
Fri Sep 13 21:37:39 EST 1996

In <322E5EEF.73E9 at postoffice.worldnet.att.net>, kenneth paul collins <KPCollins at postoffice.worldnet.att.net> writes:
:>  Cajal put forward a very plausible explanation based on the
:>  fact that images on the retina are left/right inverted.
:>  Without the visual and motor decussations it would need peculiar
:>  and complicated circuitry to make a movement of one side of the
:>  body that related to something in the ipsilateral visual field.
:>  See his "Recollections of my Life" for a concise explanation
:>  with diagrams.
:Cajal was right, but he didn't carry the idea to fruition... the 
:information-processing problems that are addressed by all of the twists 
:and turn that exist within the neuroanatomy are much-more-generalized 
:than hand-eye coordination... although such is included...

	You are confusing image inversion with visual field representation.
	The optics of the eye have nothing to do with crossed CNS
	Humans are among the few species that have an appreciable binocular
overlap, perhaps to provide stereopsis, or whatever.
	Crossed CNS representations are ancient, many species having very
poor or no vision, or no single retinal image. Many species in which other
senses far dominate vision. The visual *field* (not image) representation
crossing followed the previous cross of the rest of the CNS. It did not
drive it.

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