How Impossible Colors Work

As we've already discussed, the colors we perceive as red, green, yellow, burnt sienna and so forth are the result of reflected light being detected by cones in our eyes and then processed by our brains. To understand why so-called impossible colors break the rules of visual perception, we need to understand more about how our cones and our brains interact.

Each of your eyes contains roughly 6 million cones concentrated in the center of the retina [source: Pantone]. These cones come in three different wavelengths: short, medium and long. When a cone receives a strong signal in its wavelength zone, it sends electrical impulses to the brain. The brain's job is to combine the millions of electrical signals from each cone to recreate a composite "image" of the true color.

The brain, of course, is not a computer, but has its own complex lump of highly specialized cells. The cells responsible for processing the electrical signals from the cones are called opponent neurons [source: Wolchover]. There are two types of opponent neurons that reside in the brain's visual cortex: red-green opponent neurons and blue-yellow opponent neurons.

These brain cells are called opponent neurons because they function in a binary way: the red-green opponent neuron can either signal red or green, but not both. And the blue-yellow opponent neuron can signal either blue or yellow, but not both.

When you look at a pure yellow image, the yellow portion of the blue-yellow opponent neuron is excited and the blue portion is inhibited. Switch to a pure blue image and the blue portion of the opponent neuron is excited and the yellow is inhibited. Now imagine trying to see an image that is equally blue and yellow at the exact same time. The opponent neurons can't be both excited and inhibited simultaneously.

That, my friend, is why blue-yellow is an impossible color. The same is true for red-green. You might be saying, "Wait a second, I know exactly what yellow and blue look like together — it's green! And red and green make a kind of muddy brown, right?" Nice try, but that's the result of mixing together two colors, not a single pigment that's equally blue-yellow or equally red-green.

All the way back in 1801, long before scientists knew about cones and neurons, English physician Thomas Young theorized that the human eye has three types of color receptors: blue, green and red. Young's trichromatic color theory was proven correct in the 1960s, when cones (named for their shape) were discovered to have special sensitivity to blue, green and red light [source: Nassau].

The opponent theory of color perception has been around since the 1870s, when German physiologist Ewald Hering first postulated that our vision was ruled by opponent colors: red versus green and blue versus yellow. Hering's opponent theory is supported by the fact that there are no colors that could be described as reddish-green or yellowish-blue, but every other color in the visible spectrum can be created by combining red or green reflected light with yellow or blue [source: Billock and Tsou].

Both trichromatic color theory and opponent theory were treated as immutable truths of color perception for more than a century. Taken together, the two theories argue that it's impossible for the human eye or mind to perceive certain colors described as red-green or blue-yellow.

Thankfully, there are always a few rogue scientists who like to push the realms of possibility. In the early 1980s, visual scientists Hewitt Crane and Thomas Piantanida designed an experiment with the goal of tricking the brain into seeing impossible colors.

In Crane and Piantanida's experiment, subjects were instructed to stare at an image of two adjacent strips of red and green. The subjects' heads were stabilized with a chin rest and their eye movements were tracked by a camera. With every small twitch of a subject's eyes, the red and green image was automatically adjusted so that the subject's gaze remained fixed on the opposing colors [source: Billock and Tsou].

The results, published in the journal Science in 1983, were mind-blowing. If people stared at adjacent opposing colors long enough, the border between them would dissolve and a new "forbidden" color would emerge. The resulting color was so new that subjects had great difficulty even describing it [source: Wolchover].

By stabilizing the image to track eye movements, Crane and Piantanida theorized that different areas of the eye were being continuously bathed in different wavelengths of light, causing some opponent neurons to get excited and others to be inhibited at the same time.

Oddly, Crane and Piantanida's experiment was dismissed as a parlor trick, and several other vision scientists failed to achieve the same dramatic results. It wasn't until the 21st century that impossible colors were given a second life.

When teams of researchers tried to re-create Crane and Piantanida's revolutionary experiments with impossible colors, they often came up with disappointing results. Instead of seeing brand-new hues of greenish-red or blueish-yellow, subjects most frequently described the blended color as mud-brown [source: Wolchover]. Others would see fields of green with pixelated red dots scattered across it. Impossible colors became a scientific joke.

But in 2010, impossible colors were back in the headlines. This time, a pair of visual researchers from the Wright-Patterson Air Force Base in Ohio, believed they had determined why Crane and Piantanida had succeeded where others had failed.

In a Scientific American article, biophysicists Vincent Billock and Brian Tsou identified the combination of eye tracking and luminance (brightness) as key to tricking the brain into seeing impossible colors [source: Billock and Tsou].

Billock and Tsou ran their own experiments in which subjects were again strapped to a chinrest and monitored by the latest retinal tracking technology. With the images stabilized to the subjects' eye movements, Billock and Tsou played with the brightness or luminance of the two opposing color stripes.

If there was a difference in brightness, the subjects experienced the pixelated colors reported in earlier experiments. But if the two colors were equiluminant — exactly the same brightness — then six out of seven observers saw impossible colors [source: Billock and Tsou]. Even better, two of them could see the new colors in their minds for hours after the experiment was over.