Because I once wrote a book about colour (Bright Earth, since you ask), I have often been asked if there are any colours still to be discovered. My answer is that we have long known the full gamut of colours that the human eye can register. We know the range of wavelengths encompassed by visible light, and what sensations are triggered in our brain’s visual system when the light-sensitive cone cells in our retinas are hit with all combinations of these wavelengths.
Sure, there’s some individual variation – some people are more sensitive than others to short-wavelength light reaching into the ultraviolet, say – but no one is ever going to mix up a paint of a hue so astonishingly different from anything anyone has set eyes on before that we won’t even know what to call it.
That answer is both confirmed and challenged by a new discovery from a team of scientists at the University of California at Berkeley. They claim to have produced a genuinely “novel colour” lying outside the natural human gamut. On the other hand, those who have seen it say that it looks sort of cyan.
The team calls this new colour olo, and seeing it sounds like a real blast that’s not done justice by saying it’s merely blue-green. Olo apparently has “unprecedented saturation”, referring to the intensity of hue: think of the garishly high saturation of Technicolor movies, pioneered in The Wizard of Oz.
Sadly you can’t see olo for yourself, although you can Google the closest approximation that those who have laid eyes on it can come up with. To do so, they needed to dilute the intense saturation of olo with white light. To see the real thing requires that laser light be fired directly on to M cone cells, one of the three types of cell in your retina that produce colour vision.
That, needless to say, is not something that happens in everyday life. When light enters your eye, it falls on to all of your retinal cells: the three types of cone cell (sensitive, roughly speaking, to red, green, and purple light) and the rod cells that lack wavelength (and thus colour) discrimination but which are better at registering very dim light (this is why objects tend to lose their colour at night). Exclusively activating M cone cells demands high-tech apparatus: specifically, the system that the Berkeley team has constructed and which they call – naturally – Oz.
This set-up is able to identify which of the three types each of 1,000-2,000 cone cells are in a patch of retina for each individual being tested, and then to deliver “microdoses” of laser light (with a specific wavelength) to cells of a given type, such as M cells. The task is made trickier because our eyes are constantly moving, so Oz has to track eye motion at high speed to make sure the laser is hitting the right targets. If the laser light arrives off-target and activates the wrong cells, for example when the researchers intentionally jitter the laser, the subject merely sees the laser’s natural colour.
The subjects are asked to match the colour of what they see with colours in the normal gamut produced by display technologies – something they could do only after desaturating olo with white light. The subjects all rated olo as way more saturated than even the most intense of ordinary colours.
It’s because of this wholly non-natural way of selectively stimulating the cone cells that the Berkeley team can fairly lay claim to a “novel colour”. All the same, evidently the brain interprets this unusual signal within its usual reference frame, producing a sensation of a kind of green – as one would expect from M cone cells, which handle medium-wavelength (green) light.
Why do this research? The degree of control that Oz offers over how our photosensitive cells are activated could help to answer challenging questions in visual perception, such as how many cone cells need to be stimulated to elicit an unambiguous perception of colour at all.
We could also investigate how flexible the human visual system is for dealing with the unfamiliar. Oz could be used, for example, to “trick” some cone cells of one type into responding like another, so that people with colour blindness, because they lack a particular type of cone cell, might see normal full colour.
Or, more mind-bogglingly, we might be able to experience with our three types of cone cell what the world looks like to birds, reptiles and fish that have four.
