IUBio

Cross - Wired Eyes

Richard Norman rsnorman at mediaone.net
Wed May 16 09:07:59 EST 2001


"George Hammond" <ghammond at mediaone.net> wrote in message
news:3B01EA45.DECB4BE3 at mediaone.net...
> Richard Norman wrote:
> > > [Hammond]
> > > Is it plausible that decussation originated in
> > > the fact that a lens "reverses" it's image, and the
> > > easiest way to correct this unwanted optical circumstance
> > > would be to simply "reverse" the wiring somewhere else
> > > in the system.  To wit: do animals without eye lenses normally
> > > exhibit a major decussation in the CNS, or is this
> > > reserved only for animals with lenses?

> > [Norman]
> > Maybe it sounds plausible, but unfortunately there is no
> > validity to it.  Nothing in the nervous system "cares" whether the
> > image is inverted or reversed or upside down or whatever.
> > The mapping of the visual field to the cortex is highly
> > distorted over a highly folded surface.  Still we see "up"
> > and "down".  Besides, the auditory system and the
> > somatosensory systems, not to mention the motor
> > systems, are also decussated.

This is getting very far out of hand.

First, a lens not only reverses left and right, it
reverses up and down.  It is silly to argue that somehow
the nervous system knows how to "correct" for up/down
but requires decussation to "correct" left/right.  The
lens inversion is a simple change of coordinate systems.
If you send pixel values from a digital camera to a CPU,
but then say "whoops, I really scanned from bottom to top
and right to left" it is a trivial operation to reinterpret the
data and properly sort out exactly where a detected
"object" is in the image to respond appropriately.
In the evolution of an image forming eye, that interpretation
would happen automatically.

Second, the decussation applies to far more than vision.
The left cerebral hemisphere relates to the right side
of the body, the right hemisphere to the left side (in
general, there are specific cases of ipsilateral function).
Since the motor, auditory or lateral line, and somatosensory
systems do not involve a lens and inversion, there is no
reason to believe that an inverting lens has anything at
all to do with vision also being crossed.

Third, the evolution of complex eyes is not too
relevant.  There do seem to be several alternative
paths both originating in simple light-sensitive patches.
One goes through a single focusing mechanism, from
a "pinhole" type eye to the development of a lens
and the other through the development of
separate light gathering facets to a compound eye.
The compound eyes of arthropods connect ipsilaterally
to the CNS.  However, each facet does have some
imaging properties and the individual retinula cells are
wired specifically to handle the "inversion" that does
occur.  However, I believe that the camera eye of the
cephalopod molluscs (which forms an inverting image)
is connected ipsilaterally to the CNS.  Again, the
inverted image produced by the camera eye is totally
irrelevant to the contralateral vs ipsilateral wiring.
In any event, it is unlikely that there was a sudden
evolutionary "switch" from a non-inverting compound
type of eye to an inverting camera eye.


Fourth,  the early vertebrates did not have binocular
vision.  Their laterally placed eyes had complete
decussation to the opposite visual system (optic
tectum rather than cerebral cortex, but that doesn't
change the story. No, I was not there to examine
them, but I argue from modern chordate types).
In this case, the inversion causes reversal of up
vs. down and anterior vs. posterior.  They can
easily handle both inversions which is a trivial
information processing task.  The imaging has
absolutely nothing to do with left vs right but
still they decussate.











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