Researchers at the University of California, Berkeley have enabled participants in a recent study to perceive a previously unseen color, named “olo,” using a technique called Oz. The process involved directing a laser into the eye to stimulate only one type of color-sensitive photoreceptor cell—specifically, the M cones that are sensitive to green light.

“There’s no light in nature that can only stimulate the M-cones,” said Austin Roorda, professor of optometry and vision science at UC Berkeley’s School of Optometry. He explained that human eyes contain three types of cones: L (long), M (middle), and S (short) wavelength-sensitive cones. Typically, natural light stimulates more than one cone type at once, making it impossible to see olo outside controlled laboratory conditions.

The study was published in April and included collaboration with Ren Ng, professor of electrical engineering and computer sciences at UC Berkeley. Their work began when Ng asked Roorda what would happen if they could target thousands of M cones alone—a scenario not found in nature. The result was a highly saturated teal described as “the greenest of all greens.”

Ng and Roorda were among five initial subjects who saw olo during the experiment. Since then, additional participants have experienced it as well. Describing his experience, Roorda said he had to remain completely still while looking into the Oz machine; the patch of color appeared much more vivid than any naturally occurring green.

“It’s really about the capacity of the human brain to develop new perceptions to attribute to new sensory inputs,” said Roorda. “This could apply to any sensory inputs. It just turns out that we have a platform where we can directly manipulate the sensory inputs into the brain through the visual system in an unprecedented way.”

Roorda noted that most people are trichromats—they see colors using three types of photoreceptors—and can distinguish up to 10 million hues. He believes this research could expand understanding both of how humans process color and how perception itself can adapt or change.

“The human color vision system is really quite incredible,” said Roorda. “This Oz platform not only allows us to elicit color sensations that natural light would not, but we can use this as a tool to try to understand the basic processing of colors that humans perform when we’re looking at the world.”

He added that perception can be shaped by exposure: children’s brains are especially open to new stimuli, but even adults may retain some flexibility for novel experiences like seeing olo.

Roorda outlined two theories regarding how people perceive colors: one suggests individuals are born with fixed perceptual categories (“a box of crayons”), while another proposes there is no limit on possible percepts or colors humans might recognize.

Research from other institutions supports these ideas. In 2009, scientists at the University of Washington used gene therapy on squirrel monkeys—naturally dichromatic—to give them a third cone type; after treatment, these monkeys could distinguish red from green tones for the first time (https://www.nature.com/articles/nature08401).

Atsu Kotani, Ph.D. student and collaborator on Oz, has run simulations showing computers can interpret input from four cone types and generate an extra dimension in vision; Roorda suggested this indicates similar potential for human brains.

The technology also has implications for understanding and treating eye diseases where cone cells are damaged or lost over time—a cause of vision loss for many people worldwide (https://nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinitis-pigmentosa). Hannah Doyle, fourth-year Ph.D. student in electrical engineering at UC Berkeley who ran experiments with Oz, explained researchers can now simulate specific patterns of cone loss using their imaging platform.

“You could wonder, how would you do looking at an eye chart if you’ve lost 70% of your cones?” said Doyle. “Can you still read the letters? What’s the smallest letter you could read? It turns out that you can lose a lot of cones and still perform almost completely normally on an eye chart.”

Roorda added this information helps guide development targets for therapies such as gene or stem cell treatments aimed at restoring partial cone function—if patients retain even 30% normal density they may maintain high quality vision.

“I might be a little biased, but it’s our most precious sense,” he said. “It’s the last one I would want to give up, for sure. If you can give someone a better quality of life by maintaining their vision … this is really important for the individual, but also for the health benefit of the world.”