Imagine seeing a colour so vivid, so unlike anything in nature, that your brain has no reference for it. That’s exactly what happened to five people who took part in an experiment published in the journal Science Advances this week. 

The newly perceived colour, which is described as a hyper-saturated blue-green shade, has been named olo, and it unlocked a visual experience that was previously impossible with natural light alone. 

To understand the experiment, we first need to understand how we see colour. 

At the back of your eye lies the retina, which contains three types of photoreceptor cone cells - S, M, and L cones -each tuned to different wavelengths of light: 

• S cones respond to short, bluish wavelengths,
• M cones to medium, greenish wavelengths,
• L cones to long, reddish wavelengths. 

Together, they allow your brain to mix and interpret the colours of the world around you. But here’s the catch: in nature, the M and L cones are almost always activated together due to how their sensitivities overlap. This means it's essentially impossible to isolate just the M cones with regular light. As a result, a truly pure green, seen by stimulating only M cones, is something no one had ever seen - until now. 

The researchers used pinpoint lasers and advanced optics to stimulate individual photoreceptors in the human eye. 

Using a laser no more powerful than a standard green laser pointer, the team was able to target cone cells with surgical precision. 

First, they needed to create a cone map, a detailed image showing the exact arrangement of cone types on each participant’s retina. This map allowed the system to fire microscopic pulses of light solely at the M cones, leaving the others in darkness. 

When volunteers looked into the laser system, they described a colour that doesn’t exist in nature. Some said it was like a supercharged peacock green, others just stared in amazement. As soon as the laser was adjusted to stimulate even a few non-M cones, the olo colour vanished and was replaced instantly by the familiar green of the laser. 

The implications of this go far beyond a cool visual trick and open up new possibilities in both science and medicine. By allowing researchers to control cone cells at this level of precision, it may help us: 

• Understand how colour vision truly works on a cellular level,
• Simulate what it's like to lose certain types of vision, helping us study degenerative eye conditions like macular degeneration,
• Explore how to expand human perception, possibly giving people with colour blindness access to colours they've never seen before or even enable humans to experience tetrachromacy. 

Sadly, most of us will never get to see olo ourselves, but its discovery is a powerful reminder of how much there is still to learn about the human body, even something as seemingly simple as colour. 

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