Imagine two young children are being taught the names of the primary colours. They are each presented with a tomato. One child sees a blue object and the other sees a yellow object. Both, however, are told that tomatoes are red. Once the children have internalised this label, if you present them with a series of objects of different colours, they will both be able to pick out which objects are red, even if they don’t see the same thing.
So how do you know that the red you see is the same as the red I see? This is an old philosophical question. Descartes, the ‘father of modern philosophy’, denied that colours were objective qualities of objects. He thought that when we perceive an object as being red, we are not given evidence that the object is red, but only that it produces a particular sensation in us. This is by no means just a question for dead philosophers. To this day, scientists are still attempting to find an answer.
Broadly speaking, the scientific view of colour perception relies on three key factors: the wavelength of light reflected, the apparatus whereby we gather a signal from light, and the processing of that signal by the brain. Different wavelengths of light are said to correspond to different colours. At the high energy end of the spectrum we have red light (short wavelength) and at the low energy end, we have blue light (long wavelength). This, of course, is not the full electromagnetic spectrum but only the tiny section that we can see. Different objects appear to be different colours because their surfaces reflect different wavelengths of light into our eyes. However, this does not tell us how our bodies perceive these differences.
Rods and cones have long been known to affect how we see colour. Each cone contains a different photopigment, proteins which change shape in response to light, triggering chemical reactions which send a signal to the brain. Humans have three different types of cones which pick up red, green and blue light. Rods allow us to see in low-light but since there is only one type, we cannot distinguish between colours as easily when it is dark. This is why we see in ‘black and white’ at night but in glorious technicolour during the day.
The final stage of colour perception is the processing of information sent by rods and cones to the brain. Since our cones can only pick up three colours of light, the brain must do most of the work when it comes to distinguishing between colours. When we perceive something as yellow, for example, it is because our cones have picked up red and green light simultaneously and our brain has processed this information to give us the sensation of perceiving yellow.
Colour blindness occurs either when there is a fault in one of the cones or in the pathway to the brain itself. The most common form is a fault in the ‘red’ cone, which leaves sufferers unable to effectively distinguish between red and green. On the other end of the spectrum are ‘tetrachromats’ who have four different types of cone. Birds are tetrachromats. They are sensitive to ultraviolet light which is invisible to the human eye. Some studies have suggested that up to 3% of women are tetrachromats, with their fourth type of cone cell allowing them to differentiate between red and green better than the rest of us. The reason there are more tetrachromat women than men is that the cone cell genes are carried on the X chromosome.
Male squirrel monkeys lack a cone containing red photopigments, meaning that they can’t see red light. In 2009, however, researchers at the University of Washington experimented with using gene therapy to imbue existing cones in the eyes of two monkeys with red photopigment. After the therapy, the monkeys were able to see red, showing how versatile colour perception is, and how crucial our individual biology is to how we see the world.
Now that we’ve had a brief run-through of the mechanisms which allow us to see colour difference, we can begin to ask how the scientific view helps us to answer our original question: how can I know that my red is the same as your red? The short answer, in my view, is that science provides little help in this regard. It can confirm that we all consistently identify the same objects with the same labels, but we knew this already. Though science describes and explains the physical process which allows colour perception to take place, it does not tell us anything about what it feels like to perceive colour. If I show you a tomato, you will tell me that it is red because the colour you see is the colour you call red. I will call it red because the colour I see is the colour that I call red. Even if the colour we see is completely different, this won’t cause us confusion when identifying objects. All that matters is that we use the same label.
It seems to me that we are likely never to find an empirical answer to this question. No amount of science can allow us to see through someone else’s eyes. We will always be confined to our own perception of the world and our own alone. This sensory isolation requires that we can never know the quality of someone else’s experience. Even if a scientist could hook someone’s perceptions up to a computer screen, the colours seen by the scientist on the screen would be processed by the scientist’s eyes and brain in a way that could be radically different to the way the subject is processing them. The only way we could truly know if people perceive colours the same way is if we could become someone else. Unfortunately, outside of Freaky Friday and Quantum Leap, this is not an option.
By Adam Boland – Science Writer