How Impossible Colors, like Stygian Blue, Function

Here’s a brain-bending idea — our eyes don’t offer us a complete visual view of the world. In fact, there’s much we can’t perceive, such as ultraviolet wavelengths or impossible colors like stygian blue.

There’s actually no such thing as blue. Or red, green, fuchsia, or lavender. In truth, color doesn’t exist as an absolute entity. It’s purely a creation of our minds. (Dude!)

Take a banana, for instance. It isn’t inherently yellow. To demonstrate this, walk to your kitchen in the middle of the night and hold a banana up to your face. What color does it appear to be? More like a dull, grayish-black, but certainly not the bright yellow you’re used to. That’s because colors aren’t emitted by objects; they’re reflected. A banana is yellow because visible light bounces off it and returns as yellow.

White light, like sunlight or the glow from a bright light bulb, contains a mixture of wavelengths across the visible spectrum. When this white light passes through a prism, you can observe the individual spectral colors: violet, indigo, blue, green, yellow, orange, and red.

When white light strikes a banana peel, something fascinating happens. The peel contains a pigment called xanthophyll, which absorbs specific wavelengths while reflecting others. The yellow hue we perceive is the result of xanthophyll reflecting that particular wavelength.

However, the yellow of the banana doesn't truly exist until it is detected by the millions of cone cells in our retinas, which are sensitive to different wavelengths of light. These cones, categorized into blue, red, and green types, send signals to the brain, where the information is processed and perceived as the color yellow [source: Pappas].

The takeaway from this is clear: colors do not exist without our visual system and brain. Even when they do, they exist only in our minds. This raises an intriguing question: Are there colors in the visible spectrum that our cones and brains are incapable of seeing? In fact, there are—these are known as impossible or forbidden colors, which defy the rules of perception. Some scientists believe they have found a way to perceive these elusive colors.

Now, let's delve deeper into the science behind how we perceive color.

Color Opponency

As we’ve previously covered, the colors we identify as red, green, yellow, dark blue, and others emerge from the light that is reflected and detected by our eye's cones, which are then interpreted by our brains. To comprehend why impossible colors break the visual perception rules, we must first explore how our cones and brains work together.

Each eye is equipped with about 6 million cones, which are densely packed in the center of the retina [source: Pantone]. These cones fall into three categories: short, medium, and long wavelengths. When a cone detects a strong signal within its wavelength range, it sends an electrical signal to the brain, which then combines signals from all cones to reconstruct an image of the true color.

The brain isn’t like a computer, but it has its own complex array of highly specialized cells. The ones responsible for processing the electrical signals from the cones are called opponent neurons [source: Wolchover]. These neurons come in two types: red-green opponent neurons and blue-yellow opponent neurons, both of which are found in the brain’s visual cortex.

These brain cells are referred to as opponent neurons because they operate in a binary fashion: the red-green opponent neuron can signal either red or green, but never both at the same time. Similarly, the blue-yellow opponent neuron can signal either blue or yellow, but not both simultaneously.

When you view a pure yellow image, the yellow component of the blue-yellow opponent neuron is activated, while the blue part is suppressed. Switch to a pure blue image, and now the blue part of the neuron becomes active while the yellow is suppressed. Now, try to imagine an image that is both equally blue and yellow at the same time—this is impossible, as the opponent neurons cannot be excited and inhibited at the same time.

This is why bluish yellow is an impossible color. The same applies to reddish green. You might be thinking, 'Hold on, I know exactly what yellow and blue look like together—it's green! And red and green together make a kind of muddy brown, right?' Nice try, but that's the result of mixing two colors together, not from a single pigment that's equally blue-yellow or equally red-green.

Experiments With Impossible Colors

Way back in 1801, long before the discovery of cones and neurons, the English physician Thomas Young hypothesized that the human eye has three types of color receptors: blue, green, and red. This idea, known as Young's trichromatic color theory, was proven to be correct in the 1960s when scientists discovered that cones (so named for their shape) are indeed sensitive to blue, green, and red light [source: Nassau].

The opponent color theory of perception dates back to the 1870s when German physiologist Ewald Hering proposed that our vision is governed by pairs of opposing colors: red versus green and blue versus yellow. Hering's theory holds up because there are no colors that could be described as reddish-green or yellowish-blue, while all other colors in the visible spectrum are combinations of red or green light with yellow or blue.

For more than a century, both the trichromatic color theory and the opponent theory were considered undeniable truths in color perception. Together, these theories assert that the human eye or brain cannot perceive colors such as red-green or blue-yellow.

Fortunately, a few adventurous scientists like to challenge the boundaries of what’s possible. In the early 1980s, visual scientists Hewitt Crane and Thomas Piantanida conducted an experiment with the aim of tricking the brain into perceiving impossible colors.

In Crane and Piantanida's experiment, participants were asked to focus on an image with a vertical red stripe next to a vertical green stripe. Their heads were held in place with a chin rest, and their eye movements were monitored by a camera. With every tiny eye movement, the red and green stripes were adjusted to keep the participant’s gaze fixed on the opposing colors.

The results, published in the journal *Science* in 1983, were extraordinary. If participants gazed at the opposing colors for long enough, the boundary between them would fade, and new ‘forbidden’ or impossible colors would emerge. These colors were so unprecedented that the subjects found it difficult to even describe them [source: Wolchover].

By stabilizing the image to track eye movements, Crane and Piantanida hypothesized that different parts of the eye were exposed to varying wavelengths of light, leading to the simultaneous excitation of some opponent neurons and the inhibition of others.

Strangely, Crane and Piantanida's groundbreaking experiment was dismissed as nothing more than a parlor trick, and several other vision scientists failed to replicate the dramatic results. It wasn’t until the 21st century that the concept of impossible colors began to gain new attention.

How to See Impossible Colors

When research teams attempted to replicate Crane and Piantanida’s revolutionary experiments with impossible colors, they often faced disappointing outcomes. Instead of perceiving entirely new shades like greenish-red or blueish-yellow, participants most often described the resulting color as a muddy brown [source: Wolchover]. Some saw green fields with pixelated red dots scattered across them. The idea of impossible colors became somewhat of a scientific joke.

However, in 2010, the concept of impossible colors made a comeback in the headlines. This time, two visual researchers from Wright-Patterson Air Force Base in Ohio believed they had uncovered why Crane and Piantanida succeeded where others had failed.

In an article for *Scientific American*, biophysicists Vincent Billock and Brian Tsou highlighted the combination of eye tracking and luminance (brightness) as crucial factors in tricking the brain into perceiving impossible colors.

Billock and Tsou conducted their own experiments where participants were again positioned with a chinrest, and their eye movements were tracked using the latest retinal technology. With images stabilized to the subjects' gaze, they manipulated the brightness or luminance of the opposing color stripes.

When there was a noticeable difference in brightness, the participants saw the pixelated colors reported in previous experiments. However, when the two self-luminous colors were equaled in brightness — perfectly equiluminant — six out of seven observers reported seeing impossible colors. What's more, two of them could still perceive the new colors in their minds for hours after the experiment had ended.