Oz Prototype May Reveal a New Colour Beyond Human Vision

For the first time, people may have caught sight of a spectrum of colour lying just outside normal human vision – including a “blue-green of unprecedented saturation”.

You have never encountered it because you cannot: it belongs to a colour space that human eyes cannot ordinarily reach.

The Oz prototype and human colour vision

That is, it cannot be seen naturally. Scientists at the University of California, Berkeley, and the University of Washington say they have developed a method to take over the retina and artificially broaden the human colour gamut.

Much like Dorothy in The Wizard of Oz, the researchers believe they may be able to reveal an entirely unfamiliar world of shades.

Their evidence is an ‘Oz’ prototype that changes how colour information moves between cells in the eye before reaching the brain. According to the team, the resulting activation patterns cannot occur in ordinary viewing conditions.

The device sends laser flashes in one monochromatic colour – generally seen as green – to individual cone cells that capture colour.

Under normal circumstances, each colour we perceive activates several of the more than six million cone cells in the retina.

People are generally trichromats, with three cone-cell types that respond to long, medium and short (L, M, S) wavelengths within the visible spectrum.

L cones are most responsive to red, M cones to green and S cones to blue. As their signals meet and combine en route to the brain, they create the familiar colour spectrum.

As illustrated below, the sensitivity function of the M cone (green) overlaps fully with those of the red and blue cones. Consequently, no naturally occurring light wavelength activates M cones alone.

How the Oz prototype produced olo

Oz bypasses this limitation by directing a laser solely at M cones. In theory, this sends the brain a colour signal it has not previously encountered.

To examine the proposal, three participants stared at a neutral grey background as green laser light was flashed onto their retinas. As predicted, the brain did not interpret the signal from the small set of targeted M cells as any recognised colour.

When offered red, green and blue light to combine, participants were unable to recreate the colour they observed. Instead, they needed to introduce large amounts of white light to reduce its saturation sufficiently.

The research group, headed by Berkeley electrical engineer James Fong, called the newly perceived colour “olo”. Its nearest visible equivalent is shown in the “match” box in the image below.

Fong and his colleagues subsequently asked participants to watch a moving dot while Oz microdoses targeted only selected cone cells.

They say this enabled participants to perceive “different colors of the rainbow, unprecedented colors beyond the natural human gamut, and imagery like brilliant red lines or rotating dots on an olo background.”

Put another way, if this new rainbow of colours truly exists, it could in principle be viewed in video as well as still images.

Debate over a new colour beyond the natural gamut

Although Fong and colleagues maintain that their findings offer “unequivocal proof” of a new colour, University of London vision scientist John Barbur, who did not take part in the research, told the BBC’s Hafsa Kalil that the assertion is “open to argument”.

Barbur describes the ability to target a small number of cones as a “technological feat”. However, he notes that it may alter a shade’s apparent brightness, perhaps making an existing colour more intense rather than generating an entirely new one.

As with any prototype, the system has constraints. Colours seen by participants through the Oz method appeared at the edge of their vision, slightly away from their fixed focal point. Peripheral cone cells are more widely spaced and are therefore easier to target, but they generally provide lower acuity and thus a less sharp image.

The team intends to develop the Oz prototype further, hoping it can investigate the visual system at the level of individual cells and perhaps even help treat people with colour blindness.

“Oz represents a new class of experimental platform for vision science and neuroscience, which strives for complete control of the first neural layer to the brain, programmability of every photoreceptor's activation at every point in time,” write Fong and colleagues.

“Our prototype is an advance toward this class of neural control, and we demonstrate its ability to accurately deliver microdoses to target cones.”

The study appeared in Science Advances.