How is the perception of colour achieved in the visual system?

The perception of colour is achieved in the visual system through the interaction of light with photoreceptor cells in the retina.

The human eye perceives colour through a complex process that begins when light enters the eye and strikes the retina, a thin layer of tissue at the back of the eye. The retina contains millions of photoreceptor cells, specifically rods and cones, which are responsible for detecting light and colour. Rods are more sensitive to light and dark changes, shape and movement and are not involved in colour vision. Cones, on the other hand, are responsible for colour perception and are activated when exposed to bright light.

There are three types of cone cells, each sensitive to different wavelengths of light: short (S), medium (M), and long (L). These correspond to blue, green, and red light sensitivity respectively. When light strikes these cones, it triggers a chemical reaction that sends a signal to the brain. The brain then interprets these signals to form our perception of colour. This is known as the trichromatic theory of colour vision.

However, the trichromatic theory alone cannot explain all aspects of colour perception. For instance, it doesn't explain why some people can see more colours than others or why we can see colours that are not present in the light spectrum, like pink. To explain these phenomena, Ewald Hering proposed the opponent-process theory. According to this theory, the cones send signals to the brain that are then processed by opponent neurons. These neurons work in pairs of colours: red-green, blue-yellow, and black-white. When one colour in the pair is activated, the other is inhibited, allowing us to perceive a wide range of colours.

In summary, the perception of colour in the visual system is a complex process involving the interaction of light with photoreceptor cells in the retina, and the subsequent interpretation of these signals by the brain. Both the trichromatic theory and the opponent-process theory play crucial roles in our understanding of this fascinating process.