Shedding Light On The Evidence
SJAB1958
the electromagnetic spectrum why is it that our eyes can only detect
unaided that tiny part of it that we call 'visible light'?
After all, while the earth's atmosphere blocks out virtually 100% of
the cosmic rays, gamma rays, x-rays, and long wavelength radio waves;
short wavelength radio waves, a small part of the UV range, and various
parts of the infrared range, as well as visible light, also get through
to some extent.
The only medium left on the earth's surface, which blocks out
everything that's left - except for visible light - is water, but
only shallow waters let in the whole of the visible light range, deeper
waters block out all but the red section of visible light before
plunging into utter darkness.
This suggests - to me - that the first light sensitive structures
(and from these all other eyes) must have developed in shallow waters,
and that is why our eyes can only detect this tiny fraction of the
electromagnetic spectrum.
Can anyone suggest an alternative and reasonable explanation that fits
the observable evidence?
Ye Old One
If all eyes we so limited then you could be right. However, eyes work
well outside the human range of vision.
--
Bob.
John Wilkins
Primate eyes are rather skewed, relative to other vertebrate - and even
more to non-vertebrate - eyes. We see mostly in the green and blue
spectrum, possibly because our ancestors needed to distinguish objects
in patchy light in heavily forested environments. Other animals have
more widely distributed colour receptors, and some even see part of the
ultraviolet spectrum. Mammals are rather poor at colour vision. Mantis
shrimps, on the other hand, have spectrally widely spaced colour
receptors, and (if memory serves) 7 of them to our three. Many fishes,
such as cichlids, have UV vision as well, as do many birds.
The evolution of colour vision has happened many times - we do not use
the same pigments as birds do, indicating that we re-evolved colour
vision after losing it or shifting down to bichromatic vision after we
left the sea. Many nocturnal animals, including lemurs and some New
World primates, have monochromatic vision.
http://www.talkorigins.org/faqs/vision.html
--
John S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project
University of Queensland - Blog: scienceblogs.com/evolvingthoughts
"He used... sarcasm. He knew all the tricks, dramatic irony, metaphor,
bathos, puns, parody, litotes and... satire. He was vicious."
LSR
http://www.newscientist.com/channel/life/mg19325855.400-geckos-under-the-colour-of-darkness.html
--
LSR
hbar...@troy.edu
light sensing organs was tied to the ability of organic molecules to
transmit and sense different photon energies. UV needs a UV transparent
substance and IR needs a biosensor that can detect the lower energy IR
photons. Optimum detectability/transparency lies in the range we "see"
today. Of course, it is really the brain's interpretation of the
sensory data, not the light we see.
HB
Sir Frederick
>Colo(u)r vision is discussed in this week's New Scientist. Teaser here
>http://www.newscientist.com/channel/life/mg19325855.400-geckos-under-the-colour-of-darkness.html
Here's the whole thing :
Geckos: Under the colour of darkness
06 January 2007
From New Scientist Print Edition. Subscribe and get 4 free issues.
Sally Palmer
Fade to greyTHERE'S a German expression which translates as "all cats are grey at night". It's certainly true for humans. As night
falls, the colour-detecting cone cells in our eyes switch off, the rod cells take over and the world turns to fuzzy black and white
- until we go indoors and switch on the lights.
It has always been assumed that nocturnal animals also see the world in black and white, albeit far more clearly than us. So when
animal biologist and vision specialist Almut Kelber began studying nocturnal vision in geckos and moths, she was intrigued to
discover that some species were actually seeing in colour.
Kelber and her colleagues at the vision research group at Lund University in Sweden now believe that nocturnal colour vision may be
far more common than anyone imagined and could be found in toads, frogs, bees, wasps, fireflies and creatures of the deepest oceans.
What's more, finding out how they're doing it is helping the Lund group design technologies to make our night life as colourful as
theirs.
Having colour vision at night makes a lot of sense. Mates, food and shelter are all easier to find if you can see them in colour,
and danger is easier to avoid. To see in colour at all, let alone at night, an animal's eye must have at least two different types
of photoreceptor, each sensitive to different wavelengths of light. The eyes of vertebrates like us have two types of
photoreceptors, found on the retina, called rod and cone cells, but only the cones are important in colour vision. Humans, in common
with many primates, are trichromats, which means we have three types of cone capable of detecting long, mid and short wavelengths of
light. We perceive these as red, green and blue respectively. By combining and comparing signals from the cones, our brains
interpret different combinations of these wavelengths, which allows us to distinguish all the colours of the rainbow.
“Mates, food and shelter are all easier to find if you have colour vision at night”Many other mammals, such as horses, are
dichromatic, with just two types of cones in their eyes. They see the world in combinations of two colours, much like a person with
red-green colour blindness.
The other type of photoreceptor, rods, can work at lower light intensities than cones, so they come into play when night falls and
fewer photons of light are available. "It turns out you can discriminate more shades of grey [in low light] if you only use one type
of receptor," Kelber explains. "The signal-to-noise ratio is worse in a colour channel, and rods have been optimised to filter out
noise, whereas cones have not."
The downside is that our rods respond to the different wavelengths of visible light in the same way and so cannot distinguish
colours. This is why we see only in black and white after dark.
If only rods are active in low light, how do nocturnal geckos see in colour at night? The answer goes back to an early stage in the
gecko evolutionary history, when no lizard was active after sunset. "Lizards are very diurnal animals. They have got four different
types of cone in their retina," says Kelber. "They can see all the different types of colour that we can see, plus ultraviolet."
Over millions of years, the diurnal lizards used their rods so little that they simply evolved away, leaving the animals with only
cone cells.
This was fine while they were purely diurnal, but at some point in the past one type of lizard, the geckos, became active at night.
This left them in a bit of a mess, since they couldn't see in the dark without rods. Then, says Kelber, two important things
happened. First they got rid of their red-sensitive cones, which are the first to fail in low light, leaving them with cones
sensitive to blue, green and ultraviolet. Then the outer segment of these remaining cones, the part that absorbs the light, altered
to become longer and highly sensitive - more rod-like.
The cones in diurnal geckos are just five micrometres long, while those in nocturnal geckos measure 10 times that. Scientists
studying nocturnal gecko vision in the 1970s thought the elongated cone cells were actually rods and that the geckos had
black-and-white night vision. The biochemistry of these photoreceptors, however, is that of a cone, and recent studies by Victor
Govardovsky of the I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry in St Petersburg, Russia, and Kristian
Donner of the University of Helsinki in Finland have confirmed that the gecko photoreceptors contain a particular type of opsin - a
light-sensitive protein - that is only found in cones.
This, however, was only half the battle. Just because an animal has the physiological equipment to see in colour doesn't necessarily
mean it uses it. When an animal sees in colour, two separate neural pathways are at work. First, the receptor signals for two cone
types are combined, which provides the brain with a signal about the brightness of whatever it is looking at but not about the
object's colour. For information about colour, as well as hue and saturation, the cone signals have to be compared. "An animal has
to compare or subtract the signals from two or more photoreceptors to see in colour," says Kelber. "This comparison is an important
step, and it is very hard to show with physiological measures."
"Newton pointed out in the 18th century that seeing in colour is something that happens in the brain and is not a property of an
object or light source per se," explains Julian Partridge of the ecology of vision research group at the University of Bristol, UK.
"To demonstrate colour vision requires particular types of colour-mixing experiments."
Knowing that the geckos had the hardware to see in colour at night was one thing, but Kelber needed a behavioural experiment to
prove that they were using it. Borrowing an experimental technique used in the 1930s in studies on diurnal lizards, she presented
her subjects with crickets, some of which had been dipped in salt water and then dried. Crickets are a favourite food of geckos, but
because they live in the desert, where water is scarce, they avoid salt to prevent dehydration.
Kelber and her colleague Lina Roth offered both types of cricket to the nocturnal geckos, always holding the salty ones in a pair of
tweezers with a blue pattern on them and the tasty ones in grey-patterned tweezers. Once the crickets had learned which tweezers to
go for, Kelber and Roth repeated the experiments in various low light levels, including starlight and dim moonlight, which was too
dim for the researchers themselves to tell the colours apart. The geckos, however, unfailingly avoided the blue-tweezered, salty
crickets.
After sunset
This was the first time that a vertebrate had been shown to have nocturnal colour vision, but Kelber thinks the phenomenon is much
more widespread. "I see no reason why all gecko species should not see [colour] in the dark," she says.
There may, however, be a good reason why colour night vision is not universal, and why evolution has left humans in the dark. Seeing
at night is all about how many photons - units of light - your eyes can capture. A moonless night is about 100 million times darker
than a bright sunny day, but unless you live in a cave or deep in the sea there is always some light available. Nocturnal animals
generally make the most of the little light that is available to them by having large pupils to let in as many photons as possible -
but as any photographer knows, large apertures result in shallow depth of focus, so the gain in sensitivity is counterbalanced by
loss in detail.
“Evolution has left us humans in the dark”Seeing colour at night presents eyes with even more of a challenge. Colour vision requires
the output of more than one cone cell, which means that in low light the eye has to share scarce photons between different cones.
Each photon can only be absorbed by one receptor, so sharing them out like this means the eye detects even less detail. "That's why
colour vision in these dim light intensities isn't all that common," says Kelber. It explains, too, why humans can't see in colour
at night - for us, the details are more important than the colours.
For insects, however, colours are often more important. Daytime flower visitors such as honeybees and butterflies have long been
known to use colour to find, recognise and select the flowers that contain the sweetest nectar, and it now appears that some of
their nocturnal counterparts, such as hawkmoths - known as sphinxes in the US - do the same.
Unlike geckos, which have lens eyes with large pupils to maximise the amount of photons able to reach their retina, invertebrates
such as honeybees, butterflies and moths have compound eyes. This type of eye is multifaceted and made up of hundreds of units
called ommatidia containing photoreceptor cells. At the centre of the eye, light-sensing tubes called rhabdoms absorb the incoming
photons and perform the function of both rods and cones in vertebrates.
Hawkmoths prefer to feed on the nectar produced by blue or yellow flowers. The trouble is that after sunset the colour of natural
light changes depending on how far the sun is below the horizon, whether or not the moon is out, whether the night is cloudy or
starry and, increasingly, whether humans have built a light-polluting town nearby.
Discriminating between the colours of flowers in such changing conditions, especially against surrounding green foliage, is
impossible with eyes that only use a brightness scale - a blue flower in very late twilight will look brighter than its green
background, but in starlight it will look darker. Kelber figured that if the moths can pick the right colours at night, they must be
using colour vision rather than brightness.
As with the geckos, Kelber knew that hawkmoths were physiologically capable of detecting colour but she needed to be sure they were
using that ability rather than scent or some other sense to find their preferred blue and yellow flowers. She trained hawkmoths in
twilight using five coloured discs, each with a small hole through which the moth could dip its proboscis. Four of the discs were
different shades of grey and one was blue. The moths quickly learned that there was a sucrose reservoir behind the blue disc. Kelber
then removed the reward, so there was no scent, and performed the experiment in different light levels. Each time, the moths
selected the blue disc in anticipation of a reward.
So far, the geckos and hawkmoths are the only two nocturnal species that have been shown in behavioural experiments to see in
colour. Kelber hopes to study nocturnal bees next: their compound eyes are optically less sensitive than hawkmoths', so having
colour vision would be an even greater challenge for them. She also hopes to establish nocturnal colour vision in toads. Toads have
two different kinds of rod, which some believe might be used for night colour vision, though it has never been consistently shown.
Deep colours
Perhaps the darkest habitat on Earth is the deep ocean. Even here, though, there may be creatures that see in colour. No behavioural
experiments have yet been performed, but Partridge says that several deep-sea creatures have the cellular machinery necessary for
colour vision, and that there are several possible reasons why colour vision might be useful to them. "The spectrum in open seas
varies with depth, especially in the first 500 metres or so, and an animal with colour vision might be able to tell its depth from
the colour of the light," he says. "There are also spectral clues associated with time of day, so perhaps colour vision could be
used to entrain circadian clocks and control vertical migrations - the trips that some species make to feed near the surface each
night."
In addition, many deep-sea species produce bioluminescence, which they use variously to communicate, attract mates, startle
predators or camouflage their silhouettes against downwelling light. Partridge says that for perfect camouflage as seen from below
these creatures need to match both brightness and the spectrum of the light - a match that is rarely perfect. Having colour vision
would be a great way for a predator to break their prey's camouflage.
Interestingly, Lydia Mäthger at the Marine Biological Laboratory in Woods Hole, Massachusetts, says that, except for some deep-sea
species, most squid, octopuses and other cephalopods cannot see in colour, although their eyes are tremendously well developed. This
is true even in species that camouflage themselves by changing their colour. "How colour change is accomplished in a colour-blind
animal remains to be shown," she says. "One idea is that these animals may match their backgrounds by 'intensity matching', for
which wavelength information is not really necessary. We're still trying to figure it out."
Another open question is about the sensitivity of cones in vertebrates other than ourselves. "We know exactly when we humans lose
colour vision when it gets dark," Kelber says. "When half a moon is up, or less, we do not see colour; with more than half a moon,
we can see some faint colour. But this might well be different to what, for instance, a horse can do. It's possible that horses can
see colour under light conditions when we can't, and nobody has ever checked that."
Meanwhile, the knowledge that colour vision at night is theoretically possible is causing a great deal of excitement among
researchers interested in the design of new and better gadgets. Eric Warrant, who works with Kelber at the University of Lund, is
combining information about how nocturnal animals see in colour with mathematics to create algorithms that could allow cameras,
microscopes and night-vision goggles to see colour in the dark. "The hard part is to get the technology to do the job well in black
and white. As soon as that's solved, we can make it do the same thing over three colour channels, red, green and blue, and then
combine them," he says. In the meantime, we'll just have to keep watching the experts in the animal kingdom for more clues on how to
do it.
Sally Palmer is deputy editor of BBC Focus magazine
From issue 2585 of New Scientist magazine, 06 January 2007, page 36-39
r norman
First, it is virtually certain that light sensitive structures
developed in shallow waters. Life evolved in an aquatic environment
and deep water tend to be very dark.
The intensity of solar illumination peaks in the visual region. That
is probably one reason why we our sensors are focussed around that
range. Second, visible light has particular features that are
extremely important to developing light sensors: it is energetic
enough to excite electrons but not so energetic as to cause
significant chemical breakdown. As others already pointed out, many
organisms "see" into the near ultraviolet and near infrared. However
far ultraviolet is very damaging and far infrared is too weak
energetically to do much except stimulate some molecular vibrations.
It is hard to trigger a chemical response from that type of
stimulation.
Lee Jay
> The only medium left on the earth's surface, which blocks out
> everything that's left - except for visible light - is water, but
> only shallow waters let in the whole of the visible light range, deeper
> waters block out all but the red section of visible light before
> plunging into utter darkness.
Water absorbs all but blue, not red. That's why it appears blue under
a few meters of water - the red light from the sun has been absorbed.
http://www.lsbu.ac.uk/water/vibrat.html#blue
Lee Jay
SJAB1958
insect species that pollinate flowering plants are able to see uv
light.
However if you would be so kind as to elcuidtate me, I will take on
board any information you may have at your disposal.
SJAB1958
I stand corrected, but then why do deep sea organisms appear red?
bullpup
"SJAB1958" <bal...@hotmail.com> wrote in message
news:1168090913.7...@51g2000cwl.googlegroups.com...
> I dont know of any organism that can 'see' in infra-red, and only a few
> insect species that pollinate flowering plants are able to see uv
> light.
>
> However if you would be so kind as to elcuidtate me, I will take on
> board any information you may have at your disposal.
Pit Vipers, though they don't use their eyes.
<snip>
Boikat
SJAB1958
If they are not using their eyes to detect the infra-red then they are
not seeing it. That is why I put the word 'see' in quotes in my
previous posting.
>
> <snip>
>
> Boikat
SeppoP
> I dont know of any organism that can 'see' in infra-red, and only a few
> insect species that pollinate flowering plants are able to see uv
> light.
>
> However if you would be so kind as to elcuidtate me, I will take on
> board any information you may have at your disposal.
I seem to remember that there are some species of fish that see in infra red, at least 'near' -infra red.
I don't have any references, but Google might be of help.
<snip>
--
Seppo P.
What's wrong with Theocracy? (a Finnish Taliban, Oct 1, 2005)
John Wilkins
> I dont know of any organism that can 'see' in infra-red, and only a few
> insect species that pollinate flowering plants are able to see uv
> light.
Pit vipers have infra red sensors and can track animals in the dark with
them. I seem to recall they are located in the back of the mouth.
bullpup
Oh, you meant *see*, not 'see'. :}
Boikat
bullpup
"John Wilkins" <j.wil...@uq.edu.au> wrote in message
news:1hrjq4a.ex57jt1hy6hghN%j.wil...@uq.edu.au...
> SJAB1958 <bal...@hotmail.com> wrote:
>
> > I dont know of any organism that can 'see' in infra-red, and only a few
> > insect species that pollinate flowering plants are able to see uv
> > light.
>
> Pit vipers have infra red sensors and can track animals in the dark with
> them. I seem to recall they are located in the back of the mouth.
No, they are the IR detectors are located in little pits on the nose:
http://www.snakesandfrogs.com/scra/ident/images/pitviper.jpg
Boikat
r norman
This is certainly a strange definition of "see".
There are many organisms with photoreceptors -- scallops, flatworms,
some jellyfish, even many protists. Do they not "see" because their
"eyes" are not what you think of as eyes?
Jeffrey Turner
Maybe so they can't be seen? And, I think, they'd lose less heat than
if they were black.
--Jeff
--
The shepherd always tries to persuade
the sheep that their interests and
his own are the same. --Stendhal
Jeffrey Turner
> I dont know of any organism that can 'see' in infra-red, and only a few
> insect species that pollinate flowering plants are able to see uv
> light.
>
> However if you would be so kind as to elcuidtate me, I will take on
> board any information you may have at your disposal.
Even some plants see in the infrared, and adjust their growth accordingly.
SJAB1958
Did I dismiss any reference to photoreceptors? No. After all you can
detect heat, which is created by infra-red radiation, but would you say
you were seeing it? No.
r norman
You made specific reference to "eyes" as a prerequisite for seeing.
The pit viper is capable of determining the direction of the source of
IR radiation; essentially it is capable of forming a crude image. In
fact, the pit structure resembles that of a pinhole camera. There are
IR cameras that function in the same way that the receptors of the
crotalid snakes do -- detecting very small temperature differences and
there are other IR cameras that function more in the way of photon
detection. Both types form images and "see" in the IR spectrum.
Christopher Heiny
> I dont know of any organism that can 'see' in infra-red, and only a few
> insect species that pollinate flowering plants are able to see uv
> light.
>
> However if you would be so kind as to elcuidtate me, I will take on
> board any information you may have at your disposal.
Google is your friend. Try the searches
infrared vision fish
and
infrared vision insects
--
Christopher Heiny
Professor of Bizarre Theories
University of Ediacara
.
SJAB1958
I suggest you re-read my original post, I didnt state that eyes were a
prerequiste for seeing, in the fourth paragraph I referred to other
light sensitive structures and indicated that eyes developed from them.
I didnt say that you have to have eyes to see.
Further to my original post, regarding the sensing of infra-red by such
creatures as the pit viper, the pit is not as you claim like a pin-hole
camera, it is a cavity lined with a delicate skin-like membrane, behind
which are a high concentration of heat-sensitive nerve endings.
The pit behaves more like an optic cup as seen in some invertebrates in
as much as it enables the viper to get some sense which direction the
heat is coming from, imaging actually occurs in the brain, and as yet
no one can confirm if the pit viper actually 'sees' an infra-red image,
or if the directional sense accquired is more like 'hearing' where a
source is located.
I have done a fair amount of googling since placing my original post
and found that there are only four pigments involved in seeing light,
the red, green, and blue light sensitive pigments, and one for
uv-light. I have yet to find any evidence for a pigment sensitive to
infra-red radiation.
r norman
The person I responded to said "Pit Vipers, though they don't use
their eyes. If they are not using their eyes to detect the infra-red
then they aren ot seeing it." I don't know if that was you or not.
But that person definitely did say that eyes are a prerequisite for
seeing.
>Further to my original post, regarding the sensing of infra-red by such
>creatures as the pit viper, the pit is not as you claim like a pin-hole
>camera, it is a cavity lined with a delicate skin-like membrane, behind
>which are a high concentration of heat-sensitive nerve endings.
>
>The pit behaves more like an optic cup as seen in some invertebrates in
>as much as it enables the viper to get some sense which direction the
>heat is coming from, imaging actually occurs in the brain, and as yet
>no one can confirm if the pit viper actually 'sees' an infra-red image,
>or if the directional sense accquired is more like 'hearing' where a
>source is located.
>
You have described a pin-hole camera. Do animals with an optic cup
and use it to determine the direction of an object "see". The pit
viper sees as well.
>I have done a fair amount of googling since placing my original post
>and found that there are only four pigments involved in seeing light,
>the red, green, and blue light sensitive pigments, and one for
>uv-light. I have yet to find any evidence for a pigment sensitive to
>infra-red radiation.
Plants have other pigments include specific ones (phytochromes) for
red vs. far red (660 nm vs. 730 nm). The fish Malacosteus can see far
red (apprx. 650 to 800 nm) using a luminescent "photosensitizer" that
converts far red to yellow, then to blue-green where its more normal
photopigments are.
Still, why do you say you can "see" uv light but still deny the
ability to "see" infrared? The fact is that uv is simply not
"visible".
SJAB1958
A pin-hole camera has an aperture that is very small in relation to the
cavity behind it, if you examine a cross section of a pit viper's heat
sensing pit, you will notice that the aperture is large in relation to
the cavity behind it, which is completely different.
>
> >I have done a fair amount of googling since placing my original post
> >and found that there are only four pigments involved in seeing light,
> >the red, green, and blue light sensitive pigments, and one for
> >uv-light. I have yet to find any evidence for a pigment sensitive to
> >infra-red radiation.
>
> Plants have other pigments include specific ones (phytochromes) for
> red vs. far red (660 nm vs. 730 nm). The fish Malacosteus can see far
> red (apprx. 650 to 800 nm) using a luminescent "photosensitizer" that
> converts far red to yellow, then to blue-green where its more normal
> photopigments are.
>
> Still, why do you say you can "see" uv light but still deny the
> ability to "see" infrared? The fact is that uv is simply not
> "visible".
Seeing is by definition the detection of light by a pigment sensitive
to light of a specific frequency or energy level. The pit of a pit
viper contains no pigment and so therefore it is not seeing the
infra-red 'light'