For the first time, humans see colors that our eyes cannot distinguish

For the first time, humans see colors that our eyes cannot distinguish

There are colors that we humans will never see. These colors are also called impossible or forbidden colors. Only six people have ever seen these colors, namely in a science experiment.

What are these mysterious colors? As strange as it sounds, it makes sense from a scientific point of view that humans cannot see all colors. Colors together make up part of our visible light, which moves in waves. The length of this wave is called the wavelength. We perceive different wavelengths as different colours. The wavelengths that we see with our eyes are called the visible spectrum.

The spectrum visible to humans is between a wavelength of approximately 400 nanometers and 750 nanometers. We envision 400 nm violet and 750 nm red. Among the wavelengths we see as blue, green, yellow and orange.

The light that we humans perceive is electromagnetic radiation. X-rays and gamma rays, for example, also belong to the electromagnetic spectrum, but they are not visible to humans and most likely not to animals either. However, there is scientific evidence that snakes, some butterflies, and land animals have sensory receptors that allow them to perceive magnetic radiation on the Earth’s surface during long-distance migrations.


Anyway, all animals and humans need certain radiation in specific wavelengths to see colours. But then we’re not there yet. We also need the right equipment, namely our eyes and our brain. So the human eye is equipped with color receptors, which we also call cones. Cones are located on the retina and are sensitive to wavelengths of radiation, which we associate with blue, red, and green.

However: there is radiation that can be observed by animals, but not by humans. These are colors that are not visible to humans. Biologist Casper van der Kooi is affiliated with the University of Groningen and is involved, among other things, in the optics of flowers and animals. “It has a magical charge, colors that are there and we can’t see them. But some animals have different receptors that allow them to perceive different wavelengths. Snakes, for example, can perceive infrared radiation. People can feel this radiation, in the form of heat, but they can’t See it. And the opposite is true for bees: they don’t have red receptors, but they do have receptors that allow them to see ultraviolet. This is ultraviolet light and it’s not visible to humans. It’s much brighter than purple.”

Butterflies can also see ultraviolet light with special receptors. Where humans have three types of cones, most butterflies have four. We are talking about most butterflies, because Japanese scientists have studied a butterfly with no fewer than fifteen different color receptors. The butterfly Graphium is called Sarpedon and thus the animal can see colors that we cannot imagine through the three cones. Perhaps these are not just UV receptors, but also different colors of blue and green. Where we can hardly see a difference between two nearly identical shades of blue, van der Kooy says, it is likely that the graphium will see two completely different colours.

Purple is an invention of our brains

Infrared and ultraviolet light may be a piece of cake for some animals, but for humans these are impossible colors. We lack receptors. But there are more colors that cause problems for people. So we have to look at that other part of our light reception: the brain. Color perception not only occurs with the cones in our eyes, but also with the interpretation we need our brains for. The purple colors that people see are not picked up by the cones in the retina. After all, we do not have UV receptors, but we do have blue and red receptors. Our brain turns it purple. It is not the purple color that butterflies and bees perceive through UV receptors. It is actually an invention, a good guess for our brains.

The so-called chromaticity diagram clearly shows the colors produced by the human brain. Along the curve are all the colors that are perceived purely by human receptors. All the colors there and along the straight line between violet and red are combined by our brain. And we’ve come a long way with that.

Chromaticity chart of the spectrum visible to humans.Photo by Ted Du Bois

color scheme

In a chromaticity diagram we can clearly see which colors fall into the spectrum visible to humans. The colors along the arc, starting with violet in the lower left corner and then progressing through blue, green, yellow, orange and finally red in the lower right corner, are exactly the colors our receptors see. The entire color field in between is the colors that our brain composes, following the pure color signals from the receptors located in the retina. This also applies to the straight line connecting violet and red, that is, the color purple. In the middle, white light turns on, which can vary from warm white light with red or yellow tones, to cool light with blue tones.

However, we are missing some color, say American scientists Brian Tso and Vince Belloc. Tsou and Billock research colors for the US Air Force and discover colors that not only our brain can combine. Those are the colors they call impossible or forbidden. Scientists claim to have successfully made test subjects see these colors during experiments. It comes to blue, yellow, red, and green, which are definitely no match for green and brown, respectively. Green is the color you get when you mix blue and yellow. Some greens lean more towards blue while other greens are more yellow, but there is no green for blues and yellows. The same principle applies to red and green. Red-green is different from brown, because brown never looks the same as green and red. Green and red disappear into brown, just as yellow and blue disappear into green.

The nerve pathway is already full

What makes yellow and blue or red and green a forbidden color? This has to do with the brain. Cells in the cones transmit blue, green, and red data to neurons, which send color signals to the brain. It consists of four colors: blue, yellow, green and red. How exactly the nerves transmit the signal to the brain should explain the color opposition theory. According to this theory, there are two neural pathways: one for blue and yellow and the other for red and green, so that these colors are opposite to each other and cannot be perceived at the same time. One nerve pathway passes yellow or blue, but not both at the same time. The same goes for the other nerve pathway: it turns red or green.

This is the theory, but can it be circumvented? Tsou and Billock have always been shy about the details of the experiment. Their investigative reporting is covered in US defense secrets, and they could not be reached for comment. But neuroscientist Jeroen Goossens of the Donders Institute at Radboud University doesn’t think the concept of being able to see impossible colors is all that crazy. He himself has done a lot of brain research to see the light. “The classic idea is that blue and yellow compete with each other and one of the two colors wins in signal transmission. If you make sure that yellow and blue are equal in brightness, you guarantee a tie for the nerves.”

Goossens also stresses that it is important to maintain a steady focus. The eyeball is always making small movements, and so is the retina. Therefore it is difficult to consistently focus on areas of color. Mirrors moving along with these small movements fix the gaze on the surface of the color, so that it is, as it were, glued to the retina.

Third, they did not choose random participants in the experiment, but participants who had receptors with the most similar sensitivity. “If your blue receptor is more sensitive than your green receptor, then, of course, the experiment will not work. Then the yellow color will still be suppressed. Under these conditions, it is already possible to notice new colors.

Just describe it

According to American researchers, the experiment was successful in six of the seven participants. The seventh participant was shown to see only gray instead of yellow and blue or red and green. The other six could see impossible colors, but had great difficulty describing them. They can visualize colors clearly for hours after the experience.

The experience can also be temporarily simulated at home. You need an area of ​​equally bright blue and yellow (or red and green, of course). A cross in the middle helps your eyes focus. With a little practice it should be possible to visually blend areas of yellow and blue together and see yellow and blue.

The question now is what happens to yellow, blue, infrared or ultraviolet light if we can’t see it. Do these colors remain invisible or does our brain interpret this as a different colour? This depends on its color. A dandelion is a uniform yellow to the human eye, but to a butterfly’s eyes, the outer edges of the petals have an ultraviolet hue. Ultraviolet rays are not visible to us, but we see them as yellow. Blue and yellow will pass to green, and red and green to brown.

Whether it’s our cones or our brains, some colors will never be seen by most humans. But given the right circumstances, it seems to be taken for granted for a few people.

Seeing Yellow and Blue, tip Jeroen Goossens

Place the image below on the entire screen, and sit with your nose and forehead on top of the screen. Place the tip of your nose on the line between yellow and blue Only yellow in your left eyeand the Blue only in your right eye He falls.

Serving Stare straight ahead, and notice what happens to the colors on the centerline. You may then be able to see the colors vary from yellow to yellow, blue to blue, blue, yellow, and so on.

Blue, yellow and everything in between.  Sculpture by Jeroen Goossens

Blue, yellow and everything in between.Sculpture by Jeroen Goossens

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