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Colour Vision & Why is Human Colour Blindness so Relatively Common?

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Written by   390
4 months ago

Colour and sight both have to do with light, but what is light? As with most fundamental physical phenomena (gravitation, energy, electron, to mention a few) we do not know. We know something about their properties and behaviour, which we describe in models and learn to handle for certain purposes, but we have no clue what they really are.

To describe the known properties of light, two models are necessary (reconciled by quantum physics). One defines it as undulation, wave motion; the other describes it as discrete particles with a momentum but no mass or electrical charge. Newton considered light as a stream of hard corpuscles. He also said that light and matter could be converted into each other. This hypothesis has never been refuted. It is quite possible that matter is "condensed light". Nothing in modern science contradicts this.

How about colour? Goethe thought that the mixture of darkness and light gave all colours, and he wrote that "light, shade and colour are to be understood as the elements of seeing; even if the colours are represented as the monstrous births of the first two." Newton, on the other hand, concluded that light contained all colours; something he showed with a prism and the visible spectrum.

When we see a colour, it is our subjective optical perception of light of a certain wavelength. Different wavelengths give different colours. For humans, waves longer than 8000Å, or shorter than 3900Å generate no visual perception, although they are chemically active. Long waves, infra-red, are felt as warmth. Short waves, ultra violet, affect chemical and bio-chemical processes. (Å stands for Ångström. 1Å=0.1nm)

The perceptible interval varies slightly between species. Insects cannot see the long waves of red, while at least some of them (butterflies and bees) can perceive ultra violet. Birds, on the other hand, cannot see most blue colours. Insects, amphibians, lizards, most birds and fishes have colour vision. Its exact spread among mammals is a never-ending dispute. Clearly most mammals are nocturnal animals with little use of colours, so we can expect colour vision to be sparingly developed in this class. "Primates", however, to which humans belong, is a mammal order with a well-developed ability to perceive colours.

Human colour-blindness, dyschromatopsia, or defective colour vision, exists in various forms. Most of them are congenital, but some can be a result of pathological processes in the eye. Colour-blindness can be total (achromatopsy), or partial (dichromatism).

When it is total, no colours can be perceived, except the achromatic: black, white, and various tones of grey. This is very rare.

Most colour-blindness is dichromatic; two spectral colours (plus black, white, and grey) are seen. By far the most common form is when yellow and blue can be seen, but not red and green. The opposite, where red and green can be seen while yellow and blue cannot, is very rare.

A completely different problem is when colour perception is quantitatively reduced. That is, when all colours can be seen, but an unduly long time and extra strong light are required in order to recognise the colour.

Almost without exception, colour-blindness is incurable. On the other hand, it is perfectly possible to live with it.

Why is human colour blindness so relatively common?

The explanation is that mammals were originally small night-living creatures. Thus they survived during the whole era of the dinosaurs. While the dinosaurs (at least some of them) had good perception of colour, as they lived in a world were colours mattered - a nocturnal animal has no use for colour vision; in the night all colours are grey. That's why mammals in general still have limited colour vision. Its development in humans and a number of other day-living mammals is a late genetic addition. Consequently, a quite high percentage of genetically based colour blindness is to be expected. In the case of humans, colour blindness is hardly going to diminish in the future, since there is no evolutionary pressure favouring colour vision in modern human life.

What I say is not that colour vision would not be preferable or an advantage in contemporary human life, it certainly is – but it doesn't increase the risk to die or decrease the chance to reproduction. In the past it really did. The most obvious example must be the risk to die by ingesting a poisonous foodstuff instead of an edible one. That risk was quite high when coming to, for instance, berries.

Most non-mammal vertebrates and some invertebrates have more advanced colour vision than humans or mammals in general. As can be expected by animals developing as night-living, the dominant mammalian senses are hearing and smelling. Humans belong to those mammals which have gone ahead as day-living; smelling and hearing have been pushed into the background while vision has gained in importance.

But why should we expect colour blindness to be recurring because our ancestors were colour blind?

We carry within us our whole genetic past and everything from it turns up active now and then; just as an occasional human child is born with a tail. The newer a genetic property is, the more recurrence of what was before it – and, evolutionary seen, our colour vision is a quite recently developed ability.

(This article is based on material previously published in Meriondho Leo and in my e-book “From Vision to Visual Music”, 2017.)

Related articles:

COLOUR & NEGATIVE REALITY: Do we sometimes see what is not?

Blue Light, Blindness, Sleep Disorder & Cancer

Being Blind and Suddenly See: Bliss or Curse?

The Most Powerful Eyes

The Apple of Your Eyes & Too Much Shame Makes the Eyes Drop Out

The Eye As A Metaphysical Symbol

What is an Eye? For What Purpose Do We Have Eyes?

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