A team of five scientists successfully perceived a colour never before seen by human eyes, after use of lasers and tracking technology to selectively activate certain cells in their retinas.

The blue-greenish shade has an intensity , outside the natural range of colours seen by humans. According to Kimberly Jameson, a colour-vision scientist at the University of California, this is an “extraordinary achievement”,

Even before this researchers have stimulated individual cone cells, which are the photoreceptors in the eye whose signals the brain interprets as colour. But this time it was done across an area large enough to alter a person’s vision substantially.

“What is novel in this study is the evidence that such new colours can, in fact, be perceived,” says Sérgio Nascimento,  a physicist specializing in human vision at the University of Minho in Braga, Portugal.

OFF THE CHARTS INTENSE

The researchers, who published details of the technique in Science Advances , call the imperceptible colour ‘olo’. A computer scientist and vision researcher at the University of California, Berkeley Ren Ng said that, It is something like a peacock blue or teal, “but the level of saturation is off-the-charts.  Ren was both a co-author of the study and one of the test participants. 

The participants looked into a device called Oz which consists of mirrors, lasers and optical devices and run by software called Wizard — works by controlling the precise doses of light delivered to each cell in the retina, to generate the signals the brain uses to interpret colour or to create signals it has never experienced before.

Ng says the technique has the potential to perceive more new colours, even for colorblindness, for which there are no effective treatments, to perceive differences in shades.

It may need more efforts and inputs . So far, the research workers can carefully control colour in just a small area of vision and the method requires high technology available to very few labs. 

Jenny Bosten, a visual neuroscientist at the University of Sussex in Brighton, said that, despite its narrow scope of applicability, the research is “an impressive technical achievement. There is a wide scope & potential for future research using the technique.

IMPRINTS OF LIGHT

The perception of colour comes from the brain comparing the signals it receives from three types of light-detecting cone cell. Each one is sensitive to a different but overlapping range of wavelengths. At shorter, bluer wavelengths, the S cone is most responsive, whereas the M, or medium, cone is activated most by greenish light.

The L cone is more sensitive than the others to longer-wavelength red light. Every colour that humans can see comes to the brain as a characteristic level of activation of these three cell types, analogous to a fingerprint or set of coordinates. 

Because the M cone is in the middle of the spectrum, light activating it always activates neighbouring S or L cones, as well. Ng and his colleagues wondered whether, if the M cone was stimulated by itself, it would create a new colour. 

The team first mapped the retinas of each trial participant, marking the position and type of each cell, using a technique developed by co-authors based at the University of Washington in Seattle. This allowed them to track each person’s eye movements and train laser light on individual cone cells.

They then stimulated just the M cones with microdoses of laser light. To test what the participants were seeing, the team asked them to match the colour they perceived with examples of light of a single wavelength.

There was no match — the olo colour seemed to be more intense than even the most vibrant blue-green colour possible in normal vision. Participants had to ‘wash out’ the olo colour by adding white light to make it match the closest natural colour, says Ng.

Prof John Barbur, a vision scientist at City St George’s, University of London, who was not involved in the study, said that while the research is a “technological feat” in stimulating selective cone cells, the discovery of a new colour is “open to argument”. 

He explained that if, for example, the red cone cells (L) were stimulated in large numbers, people would “perceive a deep red”, but the perceived brightness may change depending on changes to red cone sensitivity, which is not unlike what happened in this study.

But the study’s co-author Prof Ng admitted that although olo is “certainly very technically difficult” to see, the team is studying the findings to see what it could potentially mean for colour blind people, who find it difficult to distinguish between certain colours.