An invisible world is all around you—but only a few can see it, study claims

We know that the sky is blue and the sun is yellow. But what if that’s not entirely true? What if the way you see the sky is not the same as how someone else sees it? There could be an entire world hidden in something as ordinary as color.

Color is not a fixed property of an object. In fact, it’s an experience our brain builds from signals sent by the cone cells in our eyes. And some people are born with more than just red, blue, and green cones. Scientists are still trying to figure out if that gives them access to a version of the world the rest of us can’t perceive at all.

The condition is called tetrachromacy. It is caused by a gene on the X chromosome that produces an extra light-sensitive molecule, or photopigment, in the eye's cone cells. Scientists shared that about 12% of women have this gene, and among Caucasian women, that number reaches as high as 50%. 

If that fourth photopigment is put to use, it could open up an entire extra dimension of color. It would be a world that is currently invisible within the same world we all share.

Visual neuroscientist Jenny Bosten and her team performed a test with 24 women who were believed to carry this gene. The research team showed them color pairs that look identical to ordinary three-cone vision, but they appear different to those who have a true fourth dimension of color. Only one participant (known as CDA29) detected differences invisible to everyone else in the study. 

Explaining this, Bosten told Popular Mechanics, "If someone truly had four-dimensional color vision, we could never know what that looks like to them. All we can do is infer it from whether they discriminate colors in ways that their extra cone class would predict."

If this fourth color dimension really exists, then why did almost no one with the gene actually perceive it? Jenny Bosten explained the reasoning behind this. She shared that this is because of how closely packed the cones are. Typical cones have peak sensitivities that are about 30 nanometers apart. A fourth cone might sit just a few nanometers from what already exists.

And when the gap is this small, our brain perceives the new signals as a color it has already recognized. This is why the brain doesn't register it as something new.

At UC Berkeley, computer scientist Ren Ng is now trying to design a real test for tetrachromacy. This is similar to how doctors use the dot-pattern tests for color blindness. “There’s no wavelength in the world that can stimulate only the M cone,” Ren Ng noted. “I began wondering what it would look like if you could just stimulate all the M cone cells. Would it be like the greenest green you’ve ever seen?”

His team has modeled a four-dimensional color space based on the genetics of a fourth cone.  They have even predicted two colors invisible to trichromats, nicknamed "keef" and "litz." They also built a printer using four inks, cyan, violet, pink, and yellow, instead of the usual three, to finally reveal the colors hiding in plain sight.