Colour Vision – Richard Jones

Among the vertebrates, fishes, amphibians, reptiles and birds include species with a wide variety of bright colours. Generally, mammals have modest black, brown, fawn, grey or white coats caused by the presence of greater or lesser amounts of the pigment melanin.

Humans have trichromatic colour vision and our eyes have retinal cells responsive to red, green and blue light. Most mammals have poor colour vision and in particular cannot distinguish red and green and so are dichromats. The appearance of most “colourful” mammals like leopards and giraffes to a dichromat and a trichromat are very similar. The commonest form of human colour blindness, deuteranopia, resembles this with the inability to distinguish red and green, for example with traffic lights.

The light sensing cells of our eyes are at the back of the eye in the retina and are of two types: Rods, which give us limited night vision in monochrome and Cone Cells responsible for colour vision in bright light. The wavelength responses of the LW (red), MW (green) and SW (blue) were considered and compared to that of the rods.

We have only cone cells in the centre of the fovea where the light from what we are “looking at” falls and no rods. Looking at a faint star at night is helped by looking away slightly so the light falls on rods. Cone cells may each connect with one nerve cells. Rods show “convergence” where many rods feed into one nerve, increasing the chance of responding to very faint light but giving poor detail. Rods have no sensitivity at all to red light. Where the optic nerve leaves the eye, we have a blind spot with no vision at all, but different in two eyes – if we still have two!

The photochemistry of vision with photopigments consisting of a chromophore – retinal, bound to a membrane protein called an opsin was briefly covered. When light changes the retinal in a fast response, that cell is unable to respond to light again until the slower process of regeneration happens and we become “dark adapted”. Cones recover very quickly, largely in a few seconds and completely in a few minutes. Rods take about 25 minutes of darkness to become fully dark adapted and this adaptation is lost as soon as bright light enters the eye – unless it is red light! An illusion of a stared-at image giving a negative after-image was shown.

Genetic changes through generations tend not to persist if they confer a reduced fitness for life on the individuals who possess them. Where there is no impact, for example in cave-dwellers living for generations in total darkness, the genes for creating functional eyes can be switched off or be lost in mutations. Our distant mammalian ancestors, living at the time of the dinosaurs, were presumed to be nocturnal. So, they lost some of the opsin genes and became dichromats and the great majority of mammals today are dichromatic, many with few cones and poor colour vision. In dogs, the sense of smell is far more important.

Old World primates recovered trichromacy by a process of gene doubling and subsequent mutation. In this way, the LW and MW opsins were created, allowing blue-yellow dichromacy to become red-green-blue trichromacy. This is widely supposed to help monkeys and apes recognise ripe fruit and edible young leaves and improve “survival of the fittest”. Not only did this change bring a third primary colour, it also gave the ability to see “mixed” or secondary colours. This is most pronounced with an equal mix of red and green light which remarkably is seen as yellow. This did not happen in New World primates, where there is a more complex situation. Some primates, like the mandrill, have evolved impressively colourful bodies.

Humans are thought to be able to distinguish about two million colour shades. Birds may do better because some are tetrachromats. Passerine birds possess a fourth, ultraviolet sensitive opsin that may be important in “secret signalling”, in starlings and blue tits notably. Insects can see in the ultraviolet and respond to ultraviolet markings in many flowers. The mantis shrimp has a Guinness World Record fourteen different cone cells.

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