In L. Frank Baum’s novel The Wonderful Wizard of Oz, the Emerald City is said to be such a brilliant shade of green that visitors must wear green-tinted glasses to protect their eyes from the “brightness and glory” of the city. The glasses, however, are one of the wizard’s many deceits, as green-tinted glasses would only make the city appear greener. Using a technique called “Oz,” scientists at the University of California, Berkeley (UC Berkeley), have found a way to manipulate the human eye into seeing a new color — a saturated blue-green color the team has named “olo.” “It was like a profoundly saturated teal … the most saturated natural color was just pale by comparison,” said Austin Roorda, a professor of optometry and vision science at UC Berkeley’s Herbert Wertheim School of Optometry & Vision Science, and one of the creators of Oz. Oz works by using tiny doses of laser light to individually control up to 1000 photoreceptors in the eye at one time. Using Oz, the team is able to show people not only a green more stunning than anything visible with ordinary vision, but also other colors, lines, moving dots, and images of babies and fish. The team believes the platform could also be used to answer basic questions about vision and vision loss. “We’ve created a system that can track, target, and stimulate photoreceptor cells with such high precision that we can now answer very basic, but also very thought-provoking, questions about the nature of human color vision,” said James Carl Fong, a doctoral student at UC Berkeley. “It gives us a way to study the human retina at a new scale that has never been possible in practice.” Participants in the study described the new color as a blue-green or peacock green far more saturated than the nearest monochromatic color. Courtesy of Marissa Gutierrez/UC Berkeley. Humans are able to see in color due to three different types of photoreceptor “cone” cells embedded in the retina. Each type of cone is sensitive to different wavelengths of light: S cones detect shorter, bluer wavelengths; M cones detect medium, greenish wavelengths; and L cones detect longer, reddish wavelengths. Because of an evolutionary quirk, the light wavelengths that activate the M and L cones are almost entirely overlapping, meaning that 85% of the light that activates M cones also activates L cones. “There’s no wavelength in the world that can stimulate only the M cone,” said senior author Ren Ng, a professor at UC Berkeley, “I began wondering what it would look like if you could just stimulate all the M cone cells. Would it be like the greenest green you’ve ever seen?” For Oz to work, the unique arrangements of the S, M, and L cone cells on an individual’s retina must be mapped. The researchers collaborated with Ramkumar Sabesan and Vimal Prahbhu Pandiyan at the University of Washington, who have developed an optical system that can image the human retina and identify each cone cell. With an individual’s cone map in hand, the Oz system can be programmed to rapidly scan a laser beam over a small patch of the retina, delivering tiny pulses of energy when the beam reaches a cone that it wants to activate, and otherwise staying off. The Oz software takes a color image (left column) and calculates which cone cells in the retina need to be activated for a person to see that image (center). It then calculates the pattern of laser microdoses that need to be delivered to the retina to activate those cones (right). Courtesy of James Carl Fong/UC Berkeley. The laser beam is just one color — the same hue as a green laser pointer — but by activating a combination of S, M and L cone cells, it can trick the eye into seeing images in full technicolor. Or, by primarily activating the M cone cells, Oz can show people the color olo. Beyond projecting movies and showing new colors, the research team is finding ways to use the technique to study eye disease and vision loss. “Many diseases that cause visual impairment involve lost cone cells,” said Hannah Doyle, a doctoral student at UC Berkeley. “One application that I’m exploring now is to use this cone by cone activation to simulate cone loss in healthy subjects.” They are also exploring whether Oz could help people with color blindness to see all the colors of the rainbow or if the technique could be used to allow humans to see in tetrachromatic color, as if they had four sets of cone cells. It may also help answer more fundamental questions about how the brain makes sense of the complex world around us. “We found that we can recreate a normal visual experience just by manipulating the cells — not by casting an image, but just by stimulating the photoreceptors. And we found that we can also expand that visual experience, which we did with olo,” Roorda said. “It’s still a mystery whether, if you expand the signals or generate new sensory inputs, will the brain be able to make sense of them and appreciate them?” The research was published in Science Advances (www.doi.org/10.1126/sciadv.adu1052).
