Vision is one of the most complicated of the senses, and how the eyes perceive colour is broken down by ocular cells called cones. Most people have three types of cones and are described as being 'trichromatic'. Those who are colour blind have only two types of cones, making them dichromatic. And individuals with tetrachromacy have four types of cones, allowing them to see up to 9 million more colours than everyone else!

The theory is that tetrachromats have an extra cone type as a result of a genetic mutation on the X chromosome. Women designated female at birth inherit two X chromosomes, so if a mutation occurs on both chromosomes, they have the potential of being a tetrachromat. As biological males only inherit one X chromosome, any mutations that occur would result in them having anomalous trichromacy rather than tetrachromacy.

Differentiating between many shades with a variety of names from vermilion and cerulean to periwinkle and chartreuse, the average human can perceive around a million different colours. That already sounds impressive, but when we consider that a tetrachromat can see 9 million more than that, it’s hard to comprehend.

Concetta Antico is a tetrachromat and fine artist who used to take pupils to the park for art lessons and question them about the many shades she saw flashing before her eyes. “I’d say, ‘Look at the light on the water – can you see the pink shimmering across that rock? Can you see the red on the edge of that leaf there?’” The students would all nod in agreement. It was only years later that she realised they were just too polite to tell the truth: the colours she saw so vividly were invisible to them.

It is estimated that four-coned women make up as many as 12% of the female population, but it is not known what factors come into play for someone to become functionally tetrachromatic, as most four-coned women aren’t. Jay Neitz, a vision researcher at the University of Washington, thinks that potential tetrachromats may need practice to awaken their abilities. “Most of the things that we see as coloured are manufactured by people who are trying to make colours that work for trichromats,” he says. “It could be that our whole world is tuned to the world of the trichromat.” He also suspects the natural world may not have enough variation in colour for the brain to learn to use a fourth cone, meaning tetrachromats might never need to draw on their full capacity.

What would it be like to see through a tetrachromat’s eyes? Unfortunately, it’s no more possible to describe than a trichromat describing the colour red to a dichromat. We may just have to accept that a tetrachromat’s world is filled with a kaleidoscope of colours beyond our comprehension.

Sources:

1. BBC.
2. Medical Health Humanities.
3. Discover Magazine.