Multi-Coloured Vision

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Colour is our eye’s interpretation of light reflecting off objects. The different colours we see are different wavelengths of light. Colour-specific photoreceptors in the retina of our eyes, cones, translate the wavelengths into what we see as colour.

Humans generally have three types of cone to do this. One responds to shorter waves of light (we see these as blues), one responds to longer waves of light (reds) and one responds to the middle lengths (greens). The cones work together to see other colours, such as purple.

The three cone system is called trichromatic vision, but most other mammals, like dogs and cats, only have two types of cones. This is known as dichromatic vision.

Some people only have dichromatic vision too. We call this colour blindness, somewhat misleadingly, as dichromats still have plenty of colours available to them.

Rather, they struggle to make out some colours. They can be defined asprotanopes (total lack of red cones), deuteranopes (lack of green cones) or tritanopes (lack of blue cones).

There is also another group of people called anomalous trichromats, who have all three cones but make odd judgements compared with the rest of us. The peak sensitivity of one of their cones shifts along the spectrum, so they need more or less of the wavelength to achieve the colour. For instance, people with deuteranomaly (the most common) find their green cone sensitivity shifts towards the red part of the spectrum, so they need fewer long waves of light to see red.

It is rare to be born a tritanope, but around 8% of men and 0.5% of womenare affected by red or green colour blindness.

It is so much more common in men because the gene responsible is carried on the X chromosome. Women (XX) are likely to have a normal copy to override the mutated gene, whereas men (XY), with only one X chromosome, have no other choice.

It is also possible to acquire colour vision defects through problems like disease and injury.

The visual world of the colour blind not only becomes less rich, but simple tasks like detecting sunburn, or determining ripe fruit becomes a struggle. Jay Neitz’s slideshow shows what living in a colour blind world might be like.

There are advantages to dichromacy. Researchers have identified 15 shades of khaki that those with normal vision found almost impossible to tell apart, but those with deuteranomaly were able to easily distinguish.

Their ability to discern camouflage may have provided an evolutionary benefit because a reduction in colour signals makes the differences in texture and brightness more apparent. This would explain why it is so prevalent in society.

But they lack the ability to, say, spot a cherry on a cherry tree, which is possibly why we evolved trichromacy in the first place.

Our world has been designed for trichromatics and living with colour deficiency can be detrimental to many areas of life: education, where colour-coding is extensively used, or pursuing careers, for example, as a firefighter, electrician or pilot.

In the future it may be possible to use gene therapy to correct colour blindness. The idea has already been successfully applied to monkeys.

Adult squirrel monkeys (Saimirisciureus) with red-green colour blindness were trained to touch the location of a coloured patch among grey dots. After treatment that added the missing visual pigment gene, the monkeys passed the test with flying colours.

The success of these nervous systems to respond to a new sensory input indicates that, contrary to popular belief, the capacity for development does not end.

It is not known how the human brain would respond to a new colour channel. Though there was no psychological distress observed in monkeys, the internal experience cannot be discerned. Safety also must be ensured before it can be considered for human use.

There may be another way.

Neil Harbisson hears colours. He is the wearer of an “eyeborg”, an artificial “eye” attached to his head by an antenna that perceives colour and sends it to a chip installed at the back of his skull. He hears the frequency of a colour in real-timesound waves through bone conduction.

He suffers from the rare visual condition of monochromacy, where his single cone forces him to live inside a world of grey, devoid of all colour.

With the eyeborg, he can listen to the rich world of colour. He likens a trip to the supermarket like going to a nightclub. He can eat his favourite song, dress to sound good, listen to a Picasso.

The secondary effect is that normal sounds become colour. With his device, the telephone ring sounds green.

He thinks of the eyeborg like a body part. It has become such a part of him that he dreams in colour.

Some people experience this sensory blend naturally. Synaesthesia is when a stimulated sense evokes a response in a different sense, in this case hearing colours.

But the eyeborg’s technology even extends beyond human vision. Through it, Harbisson can detect infrared and ultraviolet (UV) light.

Although humans have remarkably good sight for mammals, there is only a small section of the light spectrum that we can see. Other animals far surpass us.

The mantis shrimp, for example, has a whopping 16 different photoreceptors.Many birds and insects can see UV. Most snakes can sense radiation in the infrared part of the spectrum, which is produced by warm objects, like the small furry creatures about to become its prey.

There are some humans that may have superior vision. It is possible that there are tetrachromats living among us. These four coned super-sighters could perceive 1004 colours, able to distinguish more subtle shades that the rest of us see as identical. Many scientists believe that there could be many with this mutation lying latent, and perhaps haven’t learnt how to use their extra cone when they haven’t had need of it.

The private perception of colour means that we are unable to imagine what others see, but perhaps it may be possible to experience it ourselves.