Science Sunday

Science Sunday: Color blindness

What exactly does it mean to be color blind? People have varying misconceptions about this–the most common forms of color blindness doesn’t mean that you can’t perceive any color, or that you see the entire world in grayscale. What it really means that their total color space is skewed.

That is, the color blind are incapable of seeing the entire rainbow as everybody else sees the rainbow. I’ve given a couple examples of what this set of color pencils might look like if you were color blind. What’s the biology behind this?

What a rainbow of pencils would look like with normal vision. Images from Colour Blind Awareness.

What the rainbow of pencils would look like if you had a defect in one type of cone cell (the M cell).

What the rainbow of pencils would look like if you had a defect in one type of cone cell (the S cell).

What the rainbow of pencils would look like if you had a defect in one type of cone cell (the L cell).







Well, we have three types of cells in our eyes that respond to different wavelengths of the light spectrum. The most common types of colorblindness involve a defect or absence of one or more of these cones. Since they still have other cones intact, they can perceive some colors–just not all of them.

The response curves of the three types of cone cells. Adapted from Stockman, MacLeod & Johnson (1993). See here.

In this figure, you can see the response curves for each type of cone cell, which have been dubbed S (short), M (medium), and L (long) cells, for the wavelengths that they prefer. These cone cells, like many other sensory cells, respond maximally when they are hit with the wavelength that they prefer. That is, the response of an S cell peaks somewhere in the blue part of the color spectrum, and the response of an L cell peaks somewhere in the red part of the color spectrum.

Why are there only three cones that, roughly, perceived just blue, green, and red? Well, that’s because you can create the entire spectrum of visible colors from just these few. For example, purple is created from mixing blue and red together. Someone with a defect in their S or blue cone cell would also have trouble seeing purples because they involve seeing blue, as well.

There are a few different types of defects that might lead to altered color vision, or defects in color vision. For a person who has trouble seeing blue, it might be the case that they have fewer blue cones, or that the response curves of your blue cone cells is shifted to produce a great overlap in response between the blue and green cones. It might also be the case that they have no blue cones at all. However, their red and green cones are still intact and normally responsive.

You might be able to see now that “color blind” seems like a misnomer. Most people with color vision defects are simply “color challenged.”

So are there color blind people who see the world in grayscale? Certainly–they are just much more rare. This condition is known as achromatopsia, and is more accurately described as “total color blindness.” They may not have any cones at all. However, this is rarer than the other types of colorblindness and is generally not what we mean when we say “color blind.”

Did I miss anything? Do you have follow-up questions? I have plenty of vision scientists to consult if you come up with a real stumper!


Image credits:

Color pencils, Colour Blind Awareness

Simplified cone response curves, based on Stockman et al.

Ishihara plate, featured image, source unknown

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Vy is a recent graduate working in a neuroscience lab with children and monkeys. She likes sewing, knitting, lifting weights, and reading in her free time. Especially reading about science!


  1. February 19, 2012 at 10:23 pm —

    The M and L response curves are relatively close to each other, but I still feel compelled to comment and ask — if I am not colorblind, am I supposed to see the “defect in M cell” picture and “defect in L cell” picture as almost exactly the same? (Or have I just discovered that I have some degree of colorblindness?)

    • February 19, 2012 at 10:50 pm —

      i see them almost identically with L being slightly darker (especially noticeable in the right side of the spectrum)

    • February 19, 2012 at 10:53 pm —

      Yes, the M and L (red and green) curves are quite close to each other. But that’s not the only reason those two pictures would appear so similar. Vision scientists believe that red and green are opponent colors, and that when the visual system codes “red,” it is at the same time actively suppressing “green” (and vice versa). Another way to think about this is that green and red never mix to become one perceptible new color–it’s just a muddy brown. So when red is perceived, you cannot simultaneously perceive green…

      (The neuroanatomical data are slightly more complicated, but that’s the main idea…)

      You should notice slight differences though–in the L defect picture, the purple color pencil appears more blue than in the M defect picture. That’s because in the L picture, more red is being subtracted from the purple.

  2. February 20, 2012 at 2:40 am —

    I’ve never understood why “purple is created from mixing blue and red together”. Purple is on the end of the spectrum, not between red and blue in terms of wavelength/frequency.

    • February 20, 2012 at 8:35 am —

      You’ve caught onto something there. I thought this topic fell out of the scope of this post, but here’s a brief summary: there are several ways to model the visible color space. For example, the popular RGB model is used to describe colors on computer displays. However, the human eye and brain don’t use this color model…

      There is also a difference between subtractive and additive color mixing. In school and in the arts subtractive models are the most intuitive, because this is what happens when you combine pigments and *take away light*: red and blue then make purple. In additive color mixing (which is how the brain would probably see things), visible light from different parts of the color spectrum are added together. In this case red and blue really make magenta, which is close enough to purple that I could fudge it…

      See here for examples. You could also probably play with the color mixer in Photoshop or something like it to get a more intuitive grasp on additive colors:

  3. February 20, 2012 at 12:13 pm —

    I know about additive colors. In the RGB model, approximately 2 parts blue to one part red gives purple. Which is what doesn’t make sense to me.

    • February 20, 2012 at 10:16 pm —

      The RGB model does not describe human color perception, so you can’t think about RGB color (e.g., 2 parts blue + 1 part red = purple, for example) in terms of cone cells responding to wavelengths of light.

  4. February 24, 2012 at 10:25 pm —

    I LOVE this topic!

    It’s an example of something WAAAAAY more complicated than it looks (see what I did there), and something that some very clever people have devoted some very clever time to getting very clever results: Newton, Maxwell, and many others.

    It is complicated (in areas I work in) because there is a difference between definitions of “brightness.” To a physicist, optical power is the number of photons times the energy of each one. To a Perceptual Psychologist, “brightness” is the number of photons times the eye’s sensitivity to each one. These are differentiated by referring to the physics “brightness” as “radiometric” and the perceptual “brightness” as “photometric.” By definition, IR or UV have zero photometric intensity. But what about creatures that have a different range of perceptible wavelengths than us?

    To a cat, the short, violet, wavelengths are brighter than to us, like many insects. At the red end, their vision “cuts off” at a shorter wavelength than ours. That’s why people sometimes think their cats are DIFFERENT because they don’t chase a laser-pointer: they just have an older model with a longer wavelength that cats don’t see. To us, a whole range of long wavelengths look “red,” though with varying sensitivity: the questions of sensitivity and hue of a pure wavelength are separate.

    Incidentally, with few exceptions, insects can’t see red, but birds can. Plants that “intend” to attract hummingbirds or sunbirds to their flowers are usually red, and, since those birds can’t smell, they are usu scentless. That is called the “bird-pollination syndrome,” which is subdivided. There are also “Moth-,” “Bee-,” “Bat-,” “Fly-,” “Wasp-” syndromes.

    And yes, there are many hues that we can perceive that are NOT part of the rainbow: all the magentas fall into this category. There are also many hues that cannot be reproduced on a computer screen or photograph: Look-up the “CIE Chromaticity Chart”:

    Since the “colour space” that we can perceive only APPROXIMATES a triangle that can be drawn for three CRT primary colours, any colour outside that triangle cannot be accurately reproduced by a CRT: GO OUTSIDE AND LOOK AT FLOWERS, BUGS, OIL-SHEEN, ROCKS/JEWELS, THE SKY AND EACH OTHER. (Ahem…)

    Worse, for us humans, there are further complications…. Our night-vision does not convey colours. In dim light, our overall sensitivity curve is curtailed in the red too: red looks black at dusk. Coincidentally, this made photography possible: early photographic emulsions were not sensitive to red: in Black-and-White photography, red looks black, just as we are accustomed-to in dim light. Were this not true, B&W photographs would not be acceptable.

    Bright-light sensitivity is summarized in the “Photopic Response Curve.” Dim-light in the “Scotopic Response Curve.” Neither says anything about perceived hue, and the photopic curve may, for all I know, not apply to those who are “color challenged.”

    Weirdly, our closest relatives, the chimps, have an even more complicated system: males are (by our definition) colourblind, but females not necessarily so: the extra cone-type necessary for tristimulus perception is sex-linked! In birds and certain other creatures, there can be more than three optical primary colours, and, here’s the kicker, some female humans may have FOUR. But how do we trick them into our labs and strap them to the dissection-table?

    Some clever experiments have been done on honeybees, training them to visit a SHAPE regardless of the colours used to draw it. They have an entirely alien sensorium to ours.

    But that’s enough… I could post a zillion linques, but you’re smart enough to find your own. If you find something GOOD though, I hope you’ll be generous enough to post it here.

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