special females

Our colour vision results from the fact that our eyes contain three types of light-sensitive cells or cones that have different wavelength sensitivity. Some people (called anomalous trichromats) are colour blind and this is usually because one of their cones is mutated and has a different wavelength sensitivity compared with those in so-called normal observers. Colour-blind is a misnomer really because anomalous trichromats can still see colour; they just have less ability to discriminate between colours than normals. Some people are missing one of the cone classes altogether and are referred to as dichromats; they have even poorer colour discrimination but can still see colour. Only monochromats are really colour blind and they are extremely rare.

For a long time I have known that some females have four cones classes (this makes them tetrachromats). Dr Gabriele Jordan, a researcher at the Institute of Neuroscience (Newcastle University) has spent the last 20 years working on human colour vision. She has discovered that tetrachromatic females exist and that although this gives them the potential for colour discrimination much better than normal trichromats in practice most have normal colour discrimination. However, in a recent report she has found a tetrachromat who really does have enhanced colour discrimination. This is really exciting news!

The report in the Daily Mail suggest that a functional tetrachromat could be able to see 99 million more hues than the average person. Personally I am skeptical of this claim even if, as I suspect, it means 99 million more hues than the average person. The number of colours that an average person can see is debatable but I think may be about 10 million (see my previous blog post).

2 thoughts on “special females

  1. Hi Stephen

    Great topic.

    I remember reading in an article that our brain combines exponentially the colour discrimination ability of each cone. So it was suggested that if each cone can detect about 100 shades, dichromats can discriminate 100^2 colours, trichromats 100^3, and tetrachromats 100^4, or about 100 million. If it were that simple then a functional tetrachromat could indeed see 99 million more hues than the average person.

    But I do not think that all cones are created alike. Our ability to perceive changes in hue is variable, depending on the wavelength. This is illustrated by the hue discrimination curve, which compares wavelength of light with the smallest observable difference in hue (expressed as wavelength difference). There is a version of the curve by Dawson in Figure 13 of this color perception review, and good explanation:
    From the fact that we perceive changes in green better than red, and even better than blue I draw that we can discriminate less shades of reds an even less of blues, so the number I think would be less than 100 million.

    By the way, do you know what kind of extra colors is the functional tetrachromat able to see?
    I would not want to be the husband when choosing paint colors for new living room.

  2. I like the logic of how you got to 99 million. It’s too much of a coincidence – I am sure that is where the number came from. But like you, I doubt it is that simple.

    The extra colours are in the red-green part of the colour spectrum so it is these colours (especially the greenish yellows) where I would expect the extra discrimination to be.

    I’m sure there are lots of arguments in those homes where one person is a functional tetrachromat!!! πŸ™‚

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