New colour seen for the first time by tricking the eyes

Five people have witnessed an intense green-blue colour that has never been seen by humans before, thanks to a device that might one day enable those with a type of colour blindness to experience typical vision.

We perceive colour via the retina at the back of the eye, which typically contains three types of light-detecting cone cells – called S, M and L – that absorb a range of blue, green or red light, respectively, and then send signals to the brain. When we see anything at the blue-green end of the visible spectrum, at least two types of cone cells are activated at the same time because there is some overlap in the wavelengths they detect.

Ren Ng at the University of California, Berkeley, wondered what colour people would perceive if only one type of cone was activated in this part of the spectrum. He was inspired by a device called Oz, developed by other researchers studying how the eye works, that uses a laser capable of stimulating single cone cells.

Ng and his colleagues, including the scientists who built Oz, upgraded the device so that it could deliver light to a small square patch of about 1000 cone cells in the retina. Stimulating a single cone cell doesn’t generate enough of a signal to induce colour perception, says Ng.

The researchers tested the upgraded version on five people, stimulating only the M cones in this small area of one eye, while the other was closed. The participants said they saw a blue-green colour, which the researchers have called olo, that was more intense than any they had seen before. “It’s hard to describe; it’s very brilliant,” says Ng, who has also seen olo.

To verify these results, the participants took a colour-matching test. Each viewed olo and a second colour that they could tune via a dial to any shade on the standard visible spectrum, until it matched olo as closely as possible. They all dialled until it was an intense teal colour, which supports them seeing olo as they described.

In another part of the experiment, the participants used a dial to add white light to either olo or a vivid teal until they matched even closer. All the participants diluted olo, which supports it being the more intense of the two shades.

Andrew Stockman at University College London describes the research as “kind of fun”, but with potential medical implications. For instance, the device could one day enable people with red-green colour blindness, who find it hard to distinguish between these colours, to experience typical vision, he says. That is because the condition is sometimes caused by M and L cones both being activated by wavelengths of light that are very similar. Stimulating one over the other could enable people to see a wider range of shades, though this needs to be tested in trials, says Stockman.