This red is a figment of your imagination

José Manuel Gavilán Ibáñez
Researcher at the Department of Psychology
jm.gavilan(ELIMINAR)@urv.cat

We live in a vividly coloured world. The blue of the sea, the green of the trees or the reddish hue of the sunset. We think that colour is a fundamental property of the objects in the world, but in reality, it is an 'illusion' that occurs solely in our minds; it is not a primary property of objective reality.

The stimulus that allows us to see colours is light, an electromagnetic radiation that can be produced by a natural source, such as the sun, or an artificial one, such as a light bulb. Light consists of tiny energy particles called photons that travel through space at a very high speed. During their journey, these particles generate waves; that is, they emit certain vibrations or oscillations per second which are called frequency or wavelength.

When light is projected onto objects, they absorb some wavelengths and reflect others. The wavelengths reflected by the objects are the ones that reach our retina. Once there, the different wavelengths strike receptors called cones, of which we have three types, each composed of a different chemical substance that reacts specifically to different wavelengths (short, medium or long). When the light reflected from objects reaches the cones, they convert the wavelengths into electrical signals that are interpreted by the cortical areas of the visual system.

When a tomato is illuminated by a light source, it absorbs the short and medium wavelengths and selectively reflects the long wavelengths. Therefore, the only wavelengths that reach the retina are the long ones, which, once converted into electrical signals, are interpreted by the brain as "red" through a series of complex computations. The tomato appears red to us not because it is red in the real world but because our visual system works by psychologically differentiating between the wavelengths that reach the retina.

In the animal kingdom, not all species see in colour. Animals such as whales have only one type of cone cell and see the world in black and white. Dogs or cats have two types of cone cells and perceive colours in a way similar to a colour-blind person. There are also animals such as birds of prey that see colours more richly, because they have four different types of cone cells.

Therefore, colour is a special effect that our psychology adds to the stimuli of the world. It is not some random adaptation. In our evolutionary history, colour vision has provided us with advantages when searching for food or a mate or building social relationships. The general development of colour vision and perception should be understood as the most appropriate way that nature has found to adapt us to planet Earth. That is to say, in natural or evolutionary terms, it is not so important to perceive reality exactly as it is, but rather to perceive it in a way that is useful for an organism's adaptation and survival.