More than the Eye Can See

Visual artists engage in a lifelong battle/love affair with color. In yet another neurobiological plot-twist, there are physical limitations to human vision which inhibits our ability to see some colors! No, I’m not talking about infrared and ultraviolet wavelengths and the like (come now, that’s too easy!). These colors are actually inside the spectrum of visual light waves, so we should technically be able to see them.


visible spectrumSorry I’m not sorry about this, dear readers. But it will be short and sweet and you’ll understand everything much better. Promise.

First thing’s first: how do we detect color in the first place?

Light consists of short, rapid electromagnetic vibrations or waves.

red apple color sketchWhen light hits an object, the object absorbs some of those light waves and the rest reflect away. It is the chemical make-up of the object’s surface that determines how much of the light is absorbed and reflected.
Each element has a unique electron formation and so can only absorb a certain amount of energy.

Stay with me now…

So when light hits a banana, the atoms composing the fruit’s skin are capable of absorbing all kinds of
wavelengths… except those with wavelengths of about 570-580 nanometers (shades of yellow).

Some of these rejected waves eventually come into contact with the eye, more specifically in the BACK of the eye. The retina is a layer of light-sensitive tissue that lines the inner surface of the eye. It is here where light sensing cells called rods and cones come into play.

rod: sensitive to light intensity
cone: sensitive to wavelength


eye anatomyFunnily enough, our knowledge of color vision has had a complicated road.

One of the first major theories that developed was called the Trichromatic Theory (a.k.a. the Young-Helmholtz theory of color vision). It claimed that the eye contains three different kinds of cones:

S-cones: (“Short”) particularly sensitive to shorter waves, 400–500 nm, peaking around 420–440 nm wavelengths (blue)
M-cones: (“Medium”) particularly sensitive to medium waves, 450–630 nm, peaking around 534–545 nm wavelengths (green)
L-cones. (“Long”) particularly sensitive to longer waves, 500–700 nm, peaking around 564–580 nm wavelengths (red)

Notice I said their “peak”; the cones’ sensitivities overlap, so they work in combination to convey information about all visible colors. When our banana wavelengths of 570-580 nanometers hit the cones in the retina, a chemical reaction activates the M- and L- cones (red and green), and that in turn creates an electrical impulse. These impulses are picked up by the optic nerves and run along to the brain. The brain combines this information from the various cones to give rise to different perceptions of different wavelengths of light.

Are the banana light waves really yellow? Nope. It’s just human psychology.


The Trichromatic Theory does not quite explain all aspects of our color perception, however. For example, it cannot account for “complementary afterimages” (take a look at Figure 1). The trichromatic theory also does not explain why yellow appears to be a pure color.

Here enters Ewald Hering (1878), a German physiologist, who asserted that there are only two types of color receptors:
– one for red and green
– one for blue and yellow

This is called the Opponent Process Theory, as he believed that the activation of one color in the pair would inhibit the opposing color.

“the photochemical in the red-green receptor is broken down by red light and regenerates in the presence of green light. The chemical in the second type of receptor is broken down in the presence of yellow light and regenerates in the presence of blue light.“2

Figure 1

Figure 1: Stare at the white dot in the middle of the flag for about a minute, then move your eyes to a white surface. You should see an “afterimage” of the flag in the colors of red, white, and blue.

Hooray! The color yellow is now present as a primary color! This theory also explains red/green color blindness. However, scientists had a hard time believing that light could break down and then regenerate chemicals in the eye. So, Hering’s theory lived in the rubbish bin for about 100 years.


The Trichromatic Theory and the Opponent Process Theory appear at first to be at odds with one another, yes? But in the 1950s, scientists Hurvich and Jameson combined these two theories into one, massive, whopper of a color vision theory: the Dual Process Theory.

The Trichromatic Theory gives us our different types of cones (red, green, and blue).

The Opponent Process Theory comes into play as visual information from the cones is psychologically translated:

“…at various stages within the visual system the colors we describe as ‘red’ and ‘green’ are encoded by the same opponent-processing channel. The encoding process is such that if redness is signaled by an increase in the electrical activity in this channel, then greenness will be signaled by a decrease in activity.”3

Humans can see “red-yellow” and “blue-green” hues but not really “red-green” and “yellow-blue.” These mysterious colors are called forbidden colors.


So there are colors humans can’t quite see. However, they are not as unattainable as the name suggests.


Figure 2

A visual scientist named Hewitt Crane and his colleague Thomas Piantanida wrote a paper in 1983 claiming that these colors CAN be perceived. They used eye tracker technology to hold an image of juxtaposed stripes, either red/green or blue/yellow (see Figure 2), relative to the viewers’ eyes. Thus, the light “from each color stripe always entered the same retinal cells; for example, some cells always received yellow light, while other cells simultaneously received only blue light.”1

“After a few seconds of viewing, the stabilized boundary between the red and green stripes disappears. Under these conditions, observers reports that the field may have one or more of these distinctly different appearances: (i) the entire field appears to be a single unitary color composed of both red and green; (ii) the field appears to be composed entirely of a regular array of just resolvable red and green dots; or (iii) the field may appear as a series of island of one color on a background of the other color.”3

They also noted:

“Some observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with a large color vocabulary.”3


Primary color combinations of light (top) and pigment (bottom)

Primary color combinations of light (top) and pigment (bottom)

In 2006, Po-Jang Hsieh and his colleagues at Dartmouth College conducted a similar forbidden colors experiment, though subjects were asked to match the color they perceived instead of describing the experience, as in the 1983 experiment.

The subjects’ visual field (the computer screen) was split, the upper half showing alternating colored stripes and the lower half was a color-matching area. The viewer would press buttons to adjust the color on the lower half of the screen to match the “perceptually fill-in color” on the upper half.4 When all was said and done, Hsieh concluded that the “perceived colors are not ‘forbidden colors’ at all, but rather intermediate colors.”4

So the observers witnessed various shades of brown. Awesome.

Hsieh claimed that subjects from the 1983 experiment didn’t actually see forbidden colors, they simply “lacked the appropriate vocabulary to describe the color that they perceived.”4


…is what vision scientist Vince Billock said about Hsieh’s results. In fact, he and his colleagues have conducted various experiments over the past decade that might indeed back the forbidden colors theory.

According to Billock, Hsieh’s downfall was the lack of eye trackers in the 2006 experiment. A key ingredient to these experiments is the steady stream of lightwaves onto the cells in the retina. And, quite frankly, asking a subject to “hold still and fixate on a small area” is less precise than the retinal stabilization eye tracking offers.

Another factor is luminosity: the juxtaposed stripes of opponent colors must be equally bright. If they are not, subjects see a “pattern formation and other effects, including muddy and olive-like mixture colors that are probably closer to what Hseih saw.”1

Therefore, it seems as though Hsieh’s subject were not experiencing the same phenomena as subjects in Crane and Piantanid’s experiment.

So perhaps the forbidden colors are not just shades of brown. Billock himself claims that the perceived forbidden colors are not muddy at all, but surprisingly vivid: “It was like seeing purple for the first time and calling it bluish red.”1

Ultimately, I don’t think artists will be walking around one day seeing these exotic colors (it’s just not how we are built), but it’s always useful to know more about the variety of possibilities in Nature and to understand the physical limitations around which we humans must navigate.

color wheel


1. Wolchover, Natalie. “Red-Green & Blue-Yellow: The Stunning Colors You Can’t See.” LiveScience. January 17, 2012. Accessed March 24, 2014.
2.Garrett, Bob. “Vision and Visual Perception.” Brain and behavior. Chapter 10. Recording for the Blind & Dyslexic, 2003. Web.
3. Crane, Hewitt D.; Piantanida, Thomas P. (1983). “On Seeing Reddish Green and Yellowish Blue”. Science 221 (4615): 1078–80.
4.Hsieh, Po-Jang, and U. Tse Peter. “Illusory color mixing upon perceptual filling-in does not result in ‘forbidden colors’ and reveals cortical processing.” Journal of Vision 6, no. 6 (2006): 239-239.


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