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Four examples of the Ishihara plates. Each plate is made up of small circles of differing size and lightness. The colour of some of the circles differs so as to define a target number that the person must read. The changes in the lightness and size of the circles make cues other than colour impossible to use—hence this is a good test of colour perception. People with ‘normal’ colour vision give the responses of 57, 42, 45, and ‘nothing there’. People who lack red cones (protan-opes) respond ‘35, 2, ‘nothing there’, and 73’, and people who lack green cones (deutran(protan-opes) respond ‘35, 4, ‘nothing there’, and 73’. Because of limitations in reproduction you shouldn’t use this example as a definitive test of your vision.
CHAPTER OVERVIEW
The world would be a lot more boring if it weren’t for colour. It would be a mistake, though, to think that colour vision has evolved just to make things look more fun. But we shouldn’t take colour vision for granted, as if it must be there for all (including other animals) to see.
This is simply not the case, and many of the colours that we take for granted are not seen by many other creatures, or indeed by some fellow members of the human species. So why does any animal have colour vision, and why does our own human colour vision differ from that of others? In this chapter we consider the advantages that having colour vision gives us, and what happens in humans when it goes wrong. Remarkably, we conclude that our own colour vision is really two colour vision systems—one that we share with many other mammals, and one that evolved far more recently which we share only with some of our primate cousins.
Introduction
Most of us live in a world full of colours. We use colour to signal danger, to tell us to stop, to ‘make a statement’, and to notice the embarrassment or illness of our friends and loved ones. However, it’s also clear that we can watch a black and white movie and still laugh at Laurel and Hardy, and be frightened by Jimmy Cagney (or the events in the wedding chapel in Kill Bill). So why do we need colour vision?
We shall start by asking ‘what is colour?’ and what kind of information it may con-vey that is useful over and above the luminance (black and white) information that we have considered thus far. We then go on to consider how the retina encodes colour, and what can go wrong when it does not do so in a normal fashion—the case of colour blindness. Finally, we will talk about colour processing beyond the retina and some interesting, but rare, conditions in which colour vision is affected as a result of brain damage.
What is colour, and why would you want to see it?
Colour vision (like all aspects of our vision) has evolved over millions of years. We shall take the old-fashioned view that the colour system that we have today is the result of evolutionary pressures and natural selection, but if you prefer you can call this ‘God’.
Imagine the world a long time ago. In this long-forgotten world there lives a pond creature—we’ll call him Percy—who has already evolved a vision system that can dis-tinguish light from dark, but doesn’t have colour vision. What useful information would Percy get from evolving colour vision? Why have so many other creatures evolved this ability?
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The nature of light
Before we can answer this question of what colour is good for, we need to think a bit about light. This sounds like a job for a physicist, a proper scientist whom you might expect to give you a straight answer. But no, physicists tell us that light comes along in little packets, called photons, and the number of packets arriving at any one time determines the intensity of the light. Light is just one form of electromagnetic radia-tion, and occupies only a small portion of the electromagnetic spectrum (Figure 5.1).
Notice that as the wavelength changes we move from radio waves through micro-waves to X-rays and gamma rays. But what is the wavelength of this radiation if it comes along in little packets, we ask our physicist. At this point the physicist mumbles, avoids eye contact and runs hard-bitten fingernails through lank greasy hair. In fact, most physicists will do this even if you don’t ask them a hard question. However, it turns out that they have two ways of considering light—it can be thought of as a stream of photons (photons are the light version of more general energy packets called quanta) but it can also be considered as a wave energy (Figure 5.2). The distance between the peaks of the waves is termed the wavelength of the radiation. The wave-lengths emitted can vary massively, from fractions of a millionth of a millimetre to
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Figure 5.1 The electromagnetic spectrum and visible light.
Figure 5.2 Light can be thought of either as particles, or as a wave.
many kilometres. There is only a difference in wavelength between radio waves (have you ever listened to a programme on long-wave radio, where the waves are around 1500 m long?) and X-rays. Other parts of the electromagnetic spectrum can be used to heat up our baked beans (microwaves) and give us a sun tan and skin cancer (ultra-violet). Look carefully at Figure 5.1 and you’ll notice that nestling between infrared and ultraviolet is a little strip called ‘visible light’. This is the bit we are interested in.
From the physicist’s point of view, there is nothing particularly special about this visible spectrum that makes it visible—it is visible because of our physiology. If we had a different physiology we would ‘see’ a different part of the spectrum. So we are not being very politically correct to call the visible part of the spectrum the visible spec-trum. We might be accused of being horribly species-ist. One could imagine a creature on some distant planet Zarg whose visual system is excited by the part of the spectrum we call gamma rays. Our Zargian equivalents writing their textbook on Zargian vision would presumably have a diagram where the ‘visible spectrum’ is over to the left side of the electromagnetic spectrum and the region we call visible would be invisible to them. Indeed, we do not have to travel as far as Zarg to find creatures that can see parts of the spectrum that we cannot. Many snakes (Figure 5.3) can sense radiation in the infrared part of the spectrum, which is produced by warm objects (such as small furry creatures). One example is the sidewinder snake, which has given its name to a type of missile that also senses infrared emissions. To the snake, the infrared radiation makes warm objects seem to ‘glow’ in the dark. Figure 5.4 shows just one such small furry creature glowing in the infrared. Thus, the snake can see its prey in the dark. So why can’t we see in the infrared, if it’s such a good idea? The answer is that we have warm blood, and we have blood vessels in our retina. These would glow in the dark too, and all we would see would be the glow from our own eyes. The military make night-sight devices that transform infrared radiation into visible radiation, thus allow-ing human bodies or hot engines to be seen in the dark.
Figure 5.3 The green tree python—a snake that uses infrared vision. Note the pit organs on its upper mandible.
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Many birds and insects can see in the ultraviolet (UV) part of the spectrum. For exam-ple, zebra finches seem to use UV coloration in mate choice (Bennett et al., 1996). UV information allows insects and birds to see patterns, for example in flowers, which are invisible to humans and to mammals in general. Figure 5.5 shows some common flow-ers which, when photographed with film that is sensitive to UV light, have a striking pattern. This pattern is visible only to those birds and insects that see in the UV, and
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Figure 5.4 A dog photographed in the infrared part of the spectrum. This is what a snake might see.
The ‘hot’ bits will be the bits not covered in fur (e.g. the eyes and mouth). Note that this dog has a cold nose, a sure sign of good health.
Figure 5.5 The flowers on the left are photographed in visible light, while those on the right show the UV markings. These flowers have petally bits round the outside that reflect UV while the bits in the middle, known to biologists as the naughty bits, absorb the UV and so stand out prominently, allowing them to be detected easily by the insects they rely on for their pollination.
consequently they are attracted to the flowers. This is why you may have noticed UV lights in butchers’ shops—they attract the flies, which are then electrocuted. The com-mon blue tit (Figure 5.6) should really be called the ‘ultraviolet tit’ as its blue coloration is far more striking in the UV region of the spectrum (Figure 5.7) (Hunt et al., 1998). So why can’t we see in the UV? The answer is that it is damaging to our eyes and is filtered out by them. This filtering out is only partial, and therefore if we go into high-UV envi-ronments such as high mountains or tropical sunshine, we need to wear sunglasses that block UV. Birds and insects generally have shorter lives than we do, so their eyes are damaged less by UV light.
Let’s look a bit closer at the bit of the spectrum that we, in our species-ist way, call the visible spectrum (we shall call it ‘light’ for short). Figure 5.8 shows what happens when white light (such as sunlight) is passed through a prism which splits the light so that the different wavelengths now have slightly different paths. What we see is the visible spectrum spread out before us. When this happens in nature, when sunlight is split up by little prisms called raindrops, we see a ‘rainbow’ of colours (Figure 5.9). At the long-wavelength end, the light appears red (not unreasonable, as it is next to infra-red—infra is the Latin for below) and then as wavelength gets shorter, the light colour appears to go through the colours of the rainbow. Remember Richard Of York Gave
Figure 5.6 A pair of tits.
Figure 5.7 A blue tit photographed in UV light. Notice how its distinctive coloration is still clearly visible in this part of the spectrum, which is invisible to us but visible to other birds.
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Figure 5.9 A rainbow over Sir Isaac Newton’s house, Woolsthorpe Manor (Bishop, 1981–82). In 1665, Cambridge University was closed because of the plague and Newton returned to his family
farmhouse. It was here that he conducted investigations into the nature of light.
Figure 5.8 ‘White’ light is split by a prism into many different colours.
Battle In Vain: red, orange, yellow, green, blue, indigo, and violet (or ROYGBIV which kind of rhymes with roast beef). Beyond violet is ultraviolet and, comfortingly, ultra is the Latin for beyond. Have you ever heard of indigo, though? Isaac Newton, who first showed that white light is composed of these different colours, wanted to show that there are seven basic colours (like the seven notes on the piano) and needed to include indigo to make the arithmetic work.
Our sun emits enormous quantities of electromagnetic radiation, including lots in the visible spectrum. When all these wavelengths come along jumbled up together the light looks white, but remember that it is really a mixture of all the colours of the rain-bow (including indigo). A ‘green’ leaf will look green to us because the leaf absorbs all the red and blue wavelengths and just reflects the green stuff. A red bus absorbs green and blue (and some of the orange and violet as well) and reflects red. So this colour business seems pretty easy; light of a particular wavelength enters our eye and we translate it into its appropriate colour. Er, well, not exactly. Remember Chapter 2—
brightness turned out to be a bit more than just the intensity of the light, and it’s going to be the same again with colour. The key thing to remember now is that, as Sir Isaac Newton famously said ‘the rays are not colour’d’ (Newton, 1704)—colour is an inven-tion of your brain. We will see later in this chapter just how the brain invents this col-our. But, for now, let us return to our primitive creature (Percy) sploshing around the primordial swamp with the most rudimentary visual system.
➾See Chapter 2
Baikal fish spectral sensitivity
Lake Baikal is an amazing body of water in Siberia—the huge empty bit in the middle of Russia. It is over 600 km long and about 80 km wide, and has an average depth of over 700 m, and a maxi-mum depth of 1642 m, which is just over a mile. It contains one-fifth of the fresh (i.e. not salty) water on the planet, and is home to more than 1700 species of plants and animals, of which two-thirds are found in no other place on earth. If you ever get the chance to travel on the Trans-Siberian Railway, get off at Irkutsk and spend a few days on the shores of the lake—but be warned, the water is icy cold even in summer. In winter, the whole lake is frozen.
Lake Baikal is interesting to vision scientists precisely because it is so deep, and has so many unique fish in it. The eyes of the fish have been studied and reveal an elegant relationship between the depth at which the fish lives, and the wavelengths of light which its rods and cones respond to.
If you look at clean water, such as in a swimming pool, it looks blue. That’s because it absorbs long wavelengths. The deeper you go down, the more blue it gets; it also gets more dim as you go down and by the time you are at a depth of 1000 m there is virtually no light left—the water is pitch black.
So, a body of water such as Lake Baikal lets you look at the relationship between the wave-lengths which are most prevalent in the environment and the wavewave-lengths to which the retinal photoreceptors have the highest sensitivity. The water near the surface contains most
wave-Box 5.1
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The nature of light 139
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lengths, while deep water contains only light around a wavelength of 470 nm, which looks blue to us. So, you would expect the rods and cones of fish to have sensitivities that get ever closer to 470 nm as the fish live deeper in the lake.
Jim Bowmaker studied Lake Baikal fish. He pulled out fish from different depths, took out the retina of each fish, and studied the light absorbed by the photoreceptors (Bowmaker et al., 1994). The deeper the fish live in the lake, the shorter (more blue) the wavelength to which their rods and cones responded most.
So what does this mean for our own retina? The human peak sensitivity to light occurs around 550 nm, which is green. John Endler has studied the amount of light present in forest environ-ments in which primate vision evolved and has shown a peak at around the same wavelength (Figure 5.1.2) (Endler, 1993).
Note that the amount of light in the infrared part of the spectrum is huge—the so-called ‘green’
chlorophyll in plants is more infrared than it is green, but we can’t see infrared and it would be silly to say that the ‘the grass is always more infrared on the other side of the fence’. Perhaps unsur-prisingly, the peak for the human sensitivity function is around the same place—close to 550 nm.
So that means that, just like the Baikal fish, we have a retina which is matched to the amount of light available in the environment in which we evolved.
There are exceptions to this story. Some deep-water fish give off their own light because there is none to be found in their environment. They really have headlights! But some fish have red headlights. Given that there is no red light in deep water, other fish are unlikely to have cones that respond to red light. So the red headlights are a secret signal that only conspecifics can see.
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Box continues . . .Figure 5.1.1 An animal emerging from Lake Baikal.
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Box continues . . .Other fish use light to lure others to them. Figure 5.1.3 shows a female angler fish. She uses her light to draw food to her. Interestingly, the male fish (which is tiny in comparison to the female) lives solely to reproduce with the female. He locks onto one and then slowly is digested—brain, then heart, and then eyes—until he is nothing more than a pair of gonads.
Nothing like humans, eh?
Figure 5.1.3 The female deep sea angler fish (Melanocetus johnsonii).
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Figure 5.1.2 The amount of light in the forest canopy. Note that the peak is at around 550 nm. There is a second peak in the near infrared which we don’t see.
The nature of light 141
A single-cone system—monochromatic vision
Our pond creature, Percy, is going to have to build his colour visual system from scratch.
The first thing he will need to do is to detect the light, and we know from Chapter 1 that we can do this by having a photoreceptor. Percy is going to rely on sunlight, and although the sun emits light of a huge range of wavelengths, most of the surface of the Earth reflects light with a peak around 550–560 nm, as chlorophyll, the stuff that makes plants look green, reflects most light at that wavelength. Thus, it will make sense if Percy has a photoreceptor that responds best to wavelengths around 550–560 nm. For con-venience, we’ll call this our ‘yellow’ detector, and we’ll put the peak of its sensitivity at 550 nm. Incidentally, to our eyes, 550 nm is more a limey-green, but never mind because Percy wouldn’t call it yellow or limey-green, and not just because he has no linguistic ability. Figure 5.10 illustrates to what extent a single receptor can detect the different wavelengths of light. If you look back to Chapter 1 you’ll see that the photoreceptors we’re talking about here are the cones, so we’ll call this single receptor type our ‘yellow cone’. Now this yellow cone doesn’t just respond to one wavelength of light; it is sensi-tive to light over much of the spectrum, but is less and less sensisensi-tive as the wavelength
The first thing he will need to do is to detect the light, and we know from Chapter 1 that we can do this by having a photoreceptor. Percy is going to rely on sunlight, and although the sun emits light of a huge range of wavelengths, most of the surface of the Earth reflects light with a peak around 550–560 nm, as chlorophyll, the stuff that makes plants look green, reflects most light at that wavelength. Thus, it will make sense if Percy has a photoreceptor that responds best to wavelengths around 550–560 nm. For con-venience, we’ll call this our ‘yellow’ detector, and we’ll put the peak of its sensitivity at 550 nm. Incidentally, to our eyes, 550 nm is more a limey-green, but never mind because Percy wouldn’t call it yellow or limey-green, and not just because he has no linguistic ability. Figure 5.10 illustrates to what extent a single receptor can detect the different wavelengths of light. If you look back to Chapter 1 you’ll see that the photoreceptors we’re talking about here are the cones, so we’ll call this single receptor type our ‘yellow cone’. Now this yellow cone doesn’t just respond to one wavelength of light; it is sensi-tive to light over much of the spectrum, but is less and less sensisensi-tive as the wavelength