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Author
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Topic: Seemingly Unevolvabe Biochemical Systems
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Rex Kerr
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Member # 632
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posted 11. May 2004 04:12
The blind spot could be distributed as very tiny peripheral speckles. This would reduce the probability of any important information being missed. Or the photoreceptor cells could send processes down a short bent pigmented process through the layer of pigment cells, and the rest of the cells could be underneath. Then there wouldn't be any blind spot at all.
We don't know how to build these things biochemically, but then we don't know how to build the normal eye biochemically. It doesn't seem drastically harder, just different, in terms of developmental processes.
Edited to fix a typo.
[ 11. May 2004, 19:16: Message edited by: Rex Kerr ]
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zenheadache
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Member # 1233
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posted 11. May 2004 19:23
Rex, are the photoreceptors sensitive to light only across the surface areas of the rods and cones perpendicular to the light striking them?
Would the photoreceptors send a different type of electrical signal if they were oriented differently?
The image cast on the retina is inverted, and the design of the retina is itself inverted. Do we know if retinal inversion is in any way connected to processing the inverted image projected on the retina so that it is inverted again and we perceive things right side up?
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Rex Kerr
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posted 11. May 2004 23:32
Rods and cones are parallel to the light striking them. Flipping them (parallel vs. antiparallel) doesn't matter. Inversion is a property of lenses, not the order of components in the detection layer (retina). Electrical signals depend on ion channels and second-messenger cascades, not orientation.
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zenheadache
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Member # 1233
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posted 11. May 2004 23:42
Yes, but what I was getting at is that each data point (I don't know what else to call them) on the retina is upside down and inverted relative to its corresponding point on the object being resolved, and this data has to be turned upside down and around for us to perceive the object as it is.
Probably this is done entirely in the brain, but it seems that the order in which each rod and cone was "wired" and "routed" into the brain would also be quite important.
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E&M
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Member # 1186
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posted 12. May 2004 03:36
Zenheadache: quote:
Yes, but what I was getting at is that each data point (I don't know what else to call them) on the retina is upside down and inverted relative to its corresponding point on the object being resolved, and this data has to be turned upside down and around for us to perceive the object as it is.
Probably this is done entirely in the brain, but it seems that the order in which each rod and cone was "wired" and "routed" into the brain would also be quite important
The experiments have already been done with inverting prism "eyeglasses". After a couple of days the brain reinverts the images to correspond to the "right way up". How it is done I don't know but there is a need for consistency between proprioceptive, vestibular and visual field correlates. Most perception involves a bringing together of distinct, frequently incongrous material and the generation of a perceptual order that combines them into coherent version of things, a typical example is depth perception i.e. the creation of a single valued with added depth visual field from diaparate retinal binocular images. Inicidently the binocularity of vision renders the question of "efficiency" of the inverted retina largely irrelevant as the blind spots do not coincide between the two eyes in primates or other animals with coincident binocular fields of view.
cheers
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Scott
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Member # 1222
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posted 12. May 2004 16:54
quote:
Inicidently the binocularity of vision renders the question of "efficiency" of the inverted retina largely irrelevant as the blind spots do not coincide between the two eyes in primates or other animals with coincident binocular fields of view.
Are you saying that there is no "blind spot" unless one chooses to consider only a single eye in isolation?
regards
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RBH
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Member # 380
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posted 12. May 2004 17:53
Scott asked quote:
Are you saying that there is no "blind spot" unless one chooses to consider only a single eye in isolation?
There's a blind spot in each eye. Close one eye. See a blank place? Nope. Brain 'fills in' the missing chunk of the visual field. The perceptual apparatus running from the retina to the lateral geniculate nucleus of the thalamus to occipital cortex and further into cortex does a whole lot of processing on the already retinally processed signal. The brain uses information from the neighboring area of the visual field to 'fill in' the blind spot, so if there's something small whose image falls just on the blind spot, you won't see it.
How about looking at a good intro sensation and perception book rather than trying to learn this stuff on the fly on a Web board? Here's a place to start. Here's another introduction. From there work up to more complete explanations.
RBH
[ 12. May 2004, 18:18: Message edited by: RBH ]
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zenheadache
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posted 12. May 2004 18:29
RBH, thanks for the refs.
I am puzzled by claims of the eye's imperfection. If I were forced to put my reputation on the line before uttering something like "the design of the eye is stupid," I would certainly keep my mouth shut, if only out of fear of being used as an example of a fallacious ad ignorantium in some logic textbook 20 years from now.
I would say that having all those various cells interposed between the receptors and the light is odd, as Rex said, but I would stop short of saying it is imperfect, or could be improved, even with Rex's conception of how it might be improved. The only way to show this is actually to engineer varying eye designs and test their functionality against the eye as it has evolved.
I don't find the Miller/Dawkins criticism of the eye as evidence of unguided evolution persuasive, although it seems to be popular amongst the evolutionist faithful.
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Scott
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posted 12. May 2004 20:26
quote:
How about looking at a good intro sensation and perception book rather than trying to learn this stuff on the fly on a Web board?
RBH,
I got my intro from the following book, not off the web, but thanks for the web links.
An Introduction to the Biology of Vision
[ 12. May 2004, 20:27: Message edited by: Scott ]
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zenheadache
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Member # 1233
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posted 14. May 2004 21:20
Would someone who knows please point me to a literary source where the idea of random mutation was FIRST proposed and defined?
I need to know the origins of the idea, along with the definition(s) by which random mutations are understood in science.
I have been pointed to Haldane, but I am unsure if he was its first proponent.
Any help would be appreciated.
Thanks.
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