Cross - Wired Eyes
ghammond at mediaone.net
Thu May 17 01:55:26 EST 2001
Richard Norman wrote:
> "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.
Not so fast. The visual field is split L-R by the medial
fissure of the brain as we all know, however, it is also
split up-down by the Calcahrine fissure. In fact, the
entire visual field IS INVERTED in it's projection on the
brain, exactly opposite to the lens inversion.
> 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.
Someone has suggested that this is another mechanism.
Fact is there are TWO major decussations involved in
any opical-motor response. First the Optic Chiasma,
and second the Pyramidal tract decussation.
> 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.
GH: Yeah. that's what I thought.. no lens, no optic
> 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.
GH: Ahhh.. this isn't true for fishes, and I suspect it
isn't true for squids... dunno, could be wrong...
sure would be interesting to find out.
> Again, the
> inverted image produced by the camera eye is totally
> irrelevant to the contralateral vs ipsilateral wiring.
GH: It'd sure be nice to see someone prove that.
> 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.
GH: I don't think it's unlikely. A noninverting
light sensing, or shadow sensing surface would seem
to be the first step, with the discovery of a "lens"
to produce an actual "image" being the second step.
> 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.
GH: "processing task"? My guess is nature would
choose a simple mechanical (decuss) method
before it would go to computers. Especially with
limited brain power in early animals.
> The imaging has
> absolutely nothing to do with left vs right but
> still they decussate.
GH: If it had a lens, it still had an optical
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George Hammond, M.S. Physics
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