Freethought & Rationalism Archive

CodeMason · 01-10-2002, 06:28 AM

"Live Free Or Die" is an excellent personal philosophy.

Quetzal · 01-10-2002, 07:23 AM

Quote:

Originally posted by LiveFreeOrDie:
Do you have any actual facts to back up this claim about squid eyes or is this just conjecture on your part?

Just because squid are good at catching prey underwater doesn't necessarily imply that this is due to excellent vision.

Of COURSE I have facts to back up my assertion. What do you think I am, a creationist?

In humans (and in fact in all vertebrates) the inner layer of the optic cup forms the retina. This means the retina is actually "reversed", with the light-sensitive portion (the rods and cones) on the outside. IOW, incoming light has to pass through all of the layers before it reaches the rods and cones.

In squid (and octopi and other molluscs), however, the eye is somewhat different. The light sensitive portion is on the inside surface, facing the incoming light. Light strikes the rods and cones before the other parts. This means that squid eyes have substantially greater light sensitivity than humans. The nerve fibers of the squid eye don't have to pass through the retina to enter the visual cortex of the brain, they are already on that side of it. By contrast, the neural elements of the vertebrate eye must pass through, and that's what makes the blind spot. Squid eyes have no blind spot.

In addition, squid eyes have a different focussing mechanism (using the lens rather than the cornea) that allows significantly greater function underwater by reducing refraction and eliminating the problems humans have with astigmatism, etc. Squid don't have to wear glasses...

(Evidently the UBB image function is disabled, otherwise I'd show you the differences.)

Oolon Colluphid · 01-10-2002, 07:42 AM

Quote:

Originally posted by Morpho:
Of COURSE I have facts to back up my assertion. What do you think I am, a creationist?

ROTFLMFAO!

However, I took LFOD to mean evidence to justify a more subjective claim, that they "work substantially better than human eyes". Yeah they do seem to, since squid can catch food under water (and cuttlefish, you really should see one of those blighters going after a crab!) in conditions where we couldn't. But is that due to the retina arrangement (the absence of the vertebrate design flaw), or to the proportional size of the eyes and other, positive, adaptations / design features? (Plenty of creatures are better than us at certain things, due of course to god's great design... but better because we're flawed is harder to show.)

Just wondering...

Cheers, Oolon

Quetzal · 01-10-2002, 09:37 AM

Oolon:

I haven't made a serious study of cephalopod physiology, but I've gleaned a fair bit. To be honest, we are kind of comparing apples and oranges when we talk about "relative efficiency" between completely different phyla.

Still, from a pure engineering standpoint, squid eyes are better "designed". From an efficiency standpoint - well, the squid functions extremely well in its environment. As humans do in theirs. I doubt anybody has made a significant comparative study of actual, environmentally-dependent visual acuity.

I confess to stealing the claim from Zimmer ("Evolution: The Triumph of an Idea", ppg 129-131). He makes a pretty good case. I love the quote he uses to close the section:

Quote:

Meanwhile a few vertebrates on land - most notably birds and primates like ourselves - have evolved extraordinarily powerful vision. They developed a dense patch of photoreceptors in a small region of the retina called fovea; the neurons that would normally block the path of incoming light to the fovea are pushed to the sides. And yet, despite all these innovations, the backward retina endures. Thanks to 530 million years of evolutionary constraints, our children will never be able to see like squid.

Quetzal · 01-10-2002, 10:19 AM

Sorry, Oolon. I just realized I hadn't actually answered your question before my wife booted me off the computer claiming she had "work" to do. <sniff>

Anyway, cephalopod visual acuity (in a water environment) is based on relative size-to-body size (beaucoup more photoreceptors), lens-based focus mechanism (less refraction and distortion), complex refractive lens (cuts down on water refraction and sharpens focus), higher percentage of rod-type photocells than humans have (for ex) for low-light vision, much wider pupil/per size of retina(so even more light can get in), and of course the correct arrangement of optic nerve so there's no blind spot. Imagine how well we would see if we had this type of eye?

Dr Rick · 01-10-2002, 10:24 AM

Quote:

Originally posted by Morpho:
I haven't made a serious study of cephalopod physiology...

'could of fooled me.

tgamble · 01-10-2002, 11:44 AM

posted by a creationist:

[/QUOTE]
Dr George Marshall obtained his B.Sc. (Hons.) in Biology at the University of Strathclyde in l984. He conducted research into bonemarrow cancer at the University of Sheffield for three years until invalided out with a serious, normally incurable illness. He was dramatically healed of this in November 1987 and soon obtained an M.Med.Sci. from Sheffield. He then worked at the University of Manchester before taking up a post at the University of Glasgow in 1988. He obtained his Ph.D. in Ophthalmic Science at Glasgow in 1991 and was elected to chartered biologist C.Biol.) status and to membership of the Institute of Biology (M.l.Biol.) in 1993. He is now Sir Jules Thorn Lecturer in Ophthalmic Science.

Question: Dr Marshall, you wrote to us to comment on the article Seeing back to front which appeared in the March�May 1996 issue of Creation magazine. What was your comment?

Answer: I pointed out that the principal reason as to why the eye cannot be regarded as being wired backward (as some evolutionists claim) was hidden in a footnote in your article.

Would you care to elaborate?

The light-detecting structures within photoreceptor cells are located in the stack of discs. These discs are being continually replaced by the formation of new ones at the cell body end of the stack, thereby pushing older discs down the stack. Those discs at the other end of the stack are �swallowed� by a single layer of retinal pigment epithelial (RPE) cells. RPE cells are highly active, and for this they need a very large blood supply-the choroid. Unlike the retina, which is virtually transparent, the choroid is virtually opaque, because of the vast numbers of red blood cells within it. For the retina to be wired the way that Professor Richard Dawkins suggested, would require the choroid to come between the photoreceptor cells and the light, for RPE cells must be kept in intimate contact with both the choroid and photoreceptor to perform their job. Anybody who has had the misfortune of a hemorrhage in front of the retina will testify as to how well red blood cells block out the light.

Then what do you think of the idea that the eye is wired backward?

The notion that the eye was wired backward occurred to me as a 13 year-old when studying eye anatomy in a school science class. It took me two years of lecturing on human eye anatomy to realize why the eye is wired the way it is. The idea that the eye is wired backward comes from a lack of knowledge of eye function and anatomy.

How do you react to the notion that the human eye is the product of evolution?
The more I study the human eye, the harder it is to believe that it evolved. Most people see the miracle of sight. I see a miracle of complexity on viewing things at 100,000 times magnification. It is the perfection of this complexity that causes me to baulk at evolutionary theory.
[/QUOTE]

Aside from his argument from ignorance and aside from the fact that having such credentials doesn't make him right, any comment on his claims? Thanks.

LiveFreeOrDie · 01-10-2002, 01:20 PM

Quote:

Originally posted by Morpho:
In squid (and octopi and other molluscs), however, the eye is somewhat different. The light sensitive portion is on the inside surface, facing the incoming light. Light strikes the rods and cones before the other parts. This means that squid eyes have substantially greater light sensitivity than humans. The nerve fibers of the squid eye don't have to pass through the retina to enter the visual cortex of the brain, they are already on that side of it. By contrast, the neural elements of the vertebrate eye must pass through, and that's what makes the blind spot. Squid eyes have no blind spot.

Yes yes yes. All very good. But how do we know that squid eyes, as a result of being wired "correctly", don't consequently suffer from some deficiency that vertebrate eyes do not? How do we know, for example, that squid eyes aren't less sensistive due to a metabolic bottleneck caused by a too-distant blood supply?

LiveFreeOrDie · 01-10-2002, 01:28 PM

Quote:

Originally posted by tgamble:
[QB]The light-detecting structures within photoreceptor cells are located in the stack of discs. These discs are being continually replaced by the formation of new ones at the cell body end of the stack, thereby pushing older discs down the stack. Those discs at the other end of the stack are ‘swallowed’ by a single layer of retinal pigment epithelial (RPE) cells. RPE cells are highly active, and for this they need a very large blood supply-the choroid. Unlike the retina, which is virtually transparent, the choroid is virtually opaque, because of the vast numbers of red blood cells within it. For the retina to be wired the way that Professor Richard Dawkins suggested, would require the choroid to come between the photoreceptor cells and the light, for RPE cells must be kept in intimate contact with both the choroid and photoreceptor to perform their job. Anybody who has had the misfortune of a hemorrhage in front of the retina will testify as to how well red blood cells block out the light.

Then what do you think of the idea that the eye is wired backward?

The notion that the eye was wired backward occurred to me as a 13 year-old when studying eye anatomy in a school science class. It took me two years of lecturing on human eye anatomy to realize why the eye is wired the way it is. The idea that the eye is wired backward comes from a lack of knowledge of eye function and anatomy.

The apparent constraints on the wiring of the eye are a direct result of the operation and byproducts of the photoreceptor and RPE cells. Replace those components with something different (better?) and the wiring constraints disappear.

Unless one is foolish enough to think that there's only one way to design a photoreceptor cell.

Oolon Colluphid · 01-11-2002, 01:17 AM

Quote:

Originally posted by LiveFreeOrDie:


Yes yes yes. All very good. But how do we know that squid eyes, as a result of being wired "correctly", don't consequently suffer from some deficiency that vertebrate eyes do not? How do we know, for example, that squid eyes aren't less sensistive due to a metabolic bottleneck caused by a too-distant blood supply?

a) they seem to work pretty darn well, whatever the pros and cons of their ‘design’.

b) are you suggesting that the creator might have made something imperfect? To wit:

Quote:

Unless one is foolish enough to think that there's only one way to design a photoreceptor cell.

Precisely. An omniscient creator, one supposedly rather superior in knowledge to us humans, would be expected to use designs that us mere mortals wouldn’t be able to spot apparent flaws in. The question of flaws should never even occur to us.

And he shouldn’t have to reinvent the wheel -- he would know what the best design is for given circumstances. Want to see in air? Insect ommatidia or vertebrate eye? Focus it by moving the lens in and out (eg in chameleons), or by pulling it into different shapes? Equally, there’s no reason from a design point of view why different, equally efficient, designs should be distributed as they are: horizontal tail flukes in cetaceans, vertical in fish; bat, bird and pterosaur wings being formed in different ways, etc.

TTFN, Oolon

[ January 11, 2002: Message edited by: Oolon Colluphid ]

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01-10-2002, 06:28 AM	#21
CodeMason Regular Member Join Date: Feb 2001 Location: Australia Posts: 226	"Live Free Or Die" is an excellent personal philosophy.

01-10-2002, 10:19 AM	#25
Quetzal Senior Member Join Date: Nov 2001 Location: Ecuador Posts: 738	Sorry, Oolon. I just realized I hadn't actually answered your question before my wife booted me off the computer claiming she had "work" to do. <sniff> Anyway, cephalopod visual acuity (in a water environment) is based on relative size-to-body size (beaucoup more photoreceptors), lens-based focus mechanism (less refraction and distortion), complex refractive lens (cuts down on water refraction and sharpens focus), higher percentage of rod-type photocells than humans have (for ex) for low-light vision, much wider pupil/per size of retina(so even more light can get in), and of course the correct arrangement of optic nerve so there's no blind spot. Imagine how well we would see if we had this type of eye?

01-10-2002, 11:44 AM	#27
tgamble Veteran Member Join Date: Sep 2000 Location: St. John's, Nfld. Canada Posts: 1,652	posted by a creationist: [/QUOTE] Dr George Marshall obtained his B.Sc. (Hons.) in Biology at the University of Strathclyde in l984. He conducted research into bonemarrow cancer at the University of Sheffield for three years until invalided out with a serious, normally incurable illness. He was dramatically healed of this in November 1987 and soon obtained an M.Med.Sci. from Sheffield. He then worked at the University of Manchester before taking up a post at the University of Glasgow in 1988. He obtained his Ph.D. in Ophthalmic Science at Glasgow in 1991 and was elected to chartered biologist C.Biol.) status and to membership of the Institute of Biology (M.l.Biol.) in 1993. He is now Sir Jules Thorn Lecturer in Ophthalmic Science. Question: Dr Marshall, you wrote to us to comment on the article Seeing back to front which appeared in the March�May 1996 issue of Creation magazine. What was your comment? Answer: I pointed out that the principal reason as to why the eye cannot be regarded as being wired backward (as some evolutionists claim) was hidden in a footnote in your article. Would you care to elaborate? The light-detecting structures within photoreceptor cells are located in the stack of discs. These discs are being continually replaced by the formation of new ones at the cell body end of the stack, thereby pushing older discs down the stack. Those discs at the other end of the stack are �swallowed� by a single layer of retinal pigment epithelial (RPE) cells. RPE cells are highly active, and for this they need a very large blood supply-the choroid. Unlike the retina, which is virtually transparent, the choroid is virtually opaque, because of the vast numbers of red blood cells within it. For the retina to be wired the way that Professor Richard Dawkins suggested, would require the choroid to come between the photoreceptor cells and the light, for RPE cells must be kept in intimate contact with both the choroid and photoreceptor to perform their job. Anybody who has had the misfortune of a hemorrhage in front of the retina will testify as to how well red blood cells block out the light. Then what do you think of the idea that the eye is wired backward? The notion that the eye was wired backward occurred to me as a 13 year-old when studying eye anatomy in a school science class. It took me two years of lecturing on human eye anatomy to realize why the eye is wired the way it is. The idea that the eye is wired backward comes from a lack of knowledge of eye function and anatomy. How do you react to the notion that the human eye is the product of evolution? The more I study the human eye, the harder it is to believe that it evolved. Most people see the miracle of sight. I see a miracle of complexity on viewing things at 100,000 times magnification. It is the perfection of this complexity that causes me to baulk at evolutionary theory. [/QUOTE] Aside from his argument from ignorance and aside from the fact that having such credentials doesn't make him right, any comment on his claims? Thanks.

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