A colourless world

As fascinating and beautiful as it can be, the perception of colours is to many of us something completely ordinary and obvious. It would seem as a necessary component after observing what is around us. Yet… Do colours exist? This may seem a silly question. Of course they do! In fact, you may even be tempted to describe colours as an objective physical reality by refering to specific measurement on properties of light. As often is the case, however, things are a little bit more complicated than they seem. By understanding better how colour vision works we can find important insight into the nature of reality, and how surpisingly much of it is an illusion. Sure, we are aware of people where such perception partly «fails»: that is, people with an inability to perceive some colours or to discriminate between them.

The most dramatic and obvious case is of course complete blindness, but in some cases this can happen also in people with an otherwise functional vision. This is called colour blindness, and there are many different ways this can manifest, the most common of which is the inability to tell red from green.

We often think of conditions like that to be a «defect» in vision, and we make a false distinction between «proper» vision (being complete in perceiving whatever external stimuli) and different kinds of deficits. Reality, however, is subtler than that. Human vision is not like a camera meant to photograph the external world. Or, to better say, our eye is somehow similar to a camera, but the process of vision mostly occurs in the brain, not in the eye*.

Neuropsychologist Richard Gregory compares vision to what usually do the like of Sherlock Holmes: he doesn’t have the objective perception of what happened at a crime scene, but after noticing few crucial details he is capable of reconstructing the events in a way that is likely to be very close to what happened in reality. In similar fashion, our senses do not capture the external world as it is. Instead, they catch few aspects of it, upon which they generate an interpretation we then mistake for objective reality. With this in mind, we can now get back to the point that colours do not really exists. What does that mean? Well, it means that the world does have certain properties, some of which are used to create what we perceive as colour. Specifically, light is an electromagnetic radiation, which consists of waves, synchronized oscillations of electric and magnetic fields that propagate at high speed. Don’t worry about the technical details though. Just imagine an ordinary wave, with water oscillating up and down. The distance between two following crests (the upper peak) of these waves, is called wavelength. Among the objective properties of the world, there is a wavelength associated to each light ray, and each colour is associated to a specific wavelength, from red to blue as it gets shorter. You may say, then, that colour is a property of the world. Well, yes and no. The wavelength is, but the reason we perceive colours the way we do is that our eyes have certain specific ways to interact with this objective property. Our vision is trichromatic (tri- meaning «three» and chroma meaning «colour»), which means that it is based on three types of cells that interact differently with specific stimuli: each cell has a peak of activity associated with a given wavelength, and vision depends on the combination of these activity levels. Therefore, the «green» cell will be mostly active at the wavelength for «green», and will progressively become more inactive as the wavelength differs from that point. And so on. There is an easy way to visually understand this. Simply open whatever computer graphics software and try to select colours. You’ll be able to see as they are «made» of different levels of three components RGB (red, green, blue). These levels are usually expressed as number from 0 to 255, with white colours being the combination of the maximal level for each (255,255,255), black being the lowest level for all three (0,0,0), red being the highest value for the first one with 0 for the others, and so on. In this way you can obtain all colours the computer is capable of visualizing (and that you can see, basically). Notice that this is not just a fancy way to describe how you perceive what objectively exists. Just take a delicious big red apple. Or imagine it, for what it matters. It’s not inherently red, even to your own eyes. Try to look at it in the middle of the night without turning the light on, keeping it enough close to your face so you can distinguish it in such a dim light. It will not be red. It will not be a poorly illuminated red either. It will be some kind of a dirty grayish black. Not red for sure. That’s because your three types of colour cells (called «cones») do not work if there is not enough light, and your vision switches to an other type of cells called «rods». As you only have one type of rods instead of three, you can’t see colours.

But that’s not all. What your eyes detect as «colours» is nothing but a very small portion of the whole electromagnetic spectrum, the range of all possible wavelengths that electromagnetic radiation can have. Just look at the image below: all you can see is that very very small region in the middle, while you are totally blind to all the radiation below and before that. Now, think once more about what I mentioned at the beginning of this segment, about our usual perception of colour blind people having a «defective» vision, while others have a «proper» one. Colour blindness usually is due the lack of one of the three cells we usually have. Although there are different types, the most common is the inability to tell green from red, which is because the person only have two «colours» cells, while it is lacking the third that allows for that differentiation. This is called dichromacy (di- meaning «two»). A similar thing happens, by the way, in some other animals. There are actually many popular myths about animal vision you probably have heard at some time, such as that dog can only see in black and white, while other animals like sharks or bulls can only see the red colour *.

In reality, most mammals have two different cells, so their vision is somehow close to that of colour blind people. Few animal do have what we could say «black and white» vision (that is, a visual system dependent only on intensity of light, or monochromacy), but this usually is seen in marine mammals. Which is what we too can experience somehow, like when I suggested to you to try looking at a red apple in the night. The «cones», the cells responsible for colour vision, work only above a certain level of illumination. When the light is too dim (such as during the night), we switch to a second type of cell (called «rods»). In such dim light condition we can’t see colour either, so we are effectively monochromats.

I mentioned all of this to point out how our «usual» vision (that is, colour vision obtained by the combination of three different colour cells) has nothing really special. Sure, you can say colour blind vision is «defective», in the sense it is lacking certain properties other have. In our society it is considered as a mild disability, the reason being that colour blind people aren’t capable of doing some things others can, such as responding to messages conveyed by colour signals (which can occur while operating some machinery, or more obviously understanding traffic lights). However, this is a bit a circular argument. Their disability is really there because our society was arbitrarily designed to rely on certain stimuli that they can’t see (such as the conventional meanings of red and green conveying «stop» and «go»). In a society designed by colour blind people, this wouldn’t be an issue. You could argue that yes, that would solve practical problems, but still it wouldn’t change the fact that colour blind people can’t see certain things other can (differentiating red and green, in the most common manifestation). That’s undeniable. But it turns out, on the other side, colour blind people have an advantage in certain other tasks. Some researchers, perplexed by the surprising high frequency of congenital red-green colour blindness, suggested there may be an evolutionary pressure for preserving this «condition». To test this hypothesis researchers compared the performance of colour blind and normal observers in a task in which texture is camouflaged by colour. The texture elements in a target area differed in either orientation or size from the background elements. While those with a «full» sight had difficulties in certain of the conditions being tested, colour blind people had not, and indeed performed better. In addition, surprisingly, dichromats seem to be able to compensate for their reduced information when viewing complex natural scenes because their visual memory is not impaired, as compared with that of trichromats (who fare worse when dealing with images with a limited range of colour). [xyz-ihs snippet=”evil-demon-excerpt”]Once more we had to face the themes discussed in the chapter A distorted reality: perception is the result of the evolutionary need to respond to some properties of the external world, not an objective capturing of what is out there. That’s what happens in other animals and that’s what happens with ourselves. Our trichromatic vision is just one of the many possible configurations. We have it simply because it is was what made sense in our evolutionary history. According to a lot of research the specific reason why trichromacy is useful is that it allows animals to distinguish reddish (ripe) fruits or young leaves (which frequently flush red in the tropics) from other vegetation that is not beneficial to their survival (which happens to be green). This would explain why we are trichromats (along with other primates who, just as we used to do, mostly rely on vegetables as the primary source of food), while most carnivore mammals fare well enough with dichromacy. It’s not that our sight is «correct» while colour blind people are defective. Neither of us can see «reality». We see those aspects of it that were meaningful in an evolutionary perspective. Since our ancestors happened to rely on distinguishing reddish fruits and leaves from green foliage to survive, that’s how we see. And there are many «real» things we don’t happen to see. We don’t have night vision, for example. How cool would that be? It’s not that when it is dark there is nothing out there. We simply didn’t need that much to be able to see it. Notoriously, other animals do. Or what about infrared and ultraviolet? Those two are also electromagnetic radiations, corresponding to wavelength just before and beyond the visible spectre. They are «invisible», but that too is not a property of reality or a property of vision in general. It’s just how our eye evolved to work. In reality, infrared or ultraviolet are not that different from, say, a ray of green light. Some other animals can see those. We can’t because of the specific configuration of our senses, not because they are inherently invisible. The same applies, by the way, for things farther on the extremes of the electromagnetic spectrum, such as x-rays or radio waves.

More importantly, trichromatic vision isn’t by any mean a «maximum peak» of information available to animals. We tend to think of ourselves as the «top» of evolution, an ultimate state of near perfection after many steps of «less evolved» trials. That’s not how evolution works, however (in fact the term itself is a bit misleading, as thing do not evolve from a lesser to a improved state, they merely change accordingly to the necessities of a changing environment). Thanks to a pretty interesting history spanning from dinosaurs to humans *, most birds, fishes and reptiles have tetrachromacy (from Greek tetra- meaning «four»), possessing four independent channels for conveying colour informations. What such animals (or people, see below) actually see? Well, they see things that are invisible to us, although they are in the «visible» spectre. For instance, they may be able to tell the difference between what looks to you as the sample of the very same «yellow», while for them they would be something different entirely. I’m not really talking about «shades», though. That’s pretty much the same thing that happen with colour blind people not seeing red and green. To us trichromats, red and green are not just «different shades» of the same things which dichromatcs can’t notice, they are as different colours as they can be. Similarly, two identical (to us) «yellow» boxes may be as different as it gets for a tetrachromat. But well, why stop with four colour cells either? Is that, then, some kind of «objective» upper limit? Do those animals see objective reality? Nope. Some animals are believed to posses pentachromacy. As penta- is the Greek for five, I bet you guessed what that is. For example some research has shown pigeon vision involves the active participation of five different primary mechanisms.

If you are struggling to image what the visual experience of a pigeon or any other bird or animal with tetrachromacy may be (my bet: you should), think of the mantis shrimp: up to a dozen cones for different kind of colours along with a few more dedicate to UV light alone, and one of the most complex and advanced visual systems in the animal kingdom. In case you’re disappointed of being beaten by a pigeon and outmatched by a shrimp, you didn’t hear it all yet. Plants too can not only see, but they do so better than you do with their multiple photoreceptors! But wait, if you are a woman, you may still partially console yourself: although rare, different types of tetrachromacy have been reported in humans, allowing for a far richer colour experience. Such unusual condition may happen as two cone cell pigment genes are on the sex X chromosome. As women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby possibly being born as full tetrachromats and having four simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum. Concetta Antico is a famous Australian artists who has been reported to have a fully functional tetrachromacy.