Our perception of color is constrained by the way different types of cone cells in our retina respond to light of different wavelengths. Our retina consists of short (S), medium (M), and long (L) wavelength cones whose responses are tuned to colors near the blue, green, and red gamut, respectively. The color we experience whenever we look at an object depends on how much the distribution of wavelengths reflecting off the object stimulates the three types of cones. For example, an orange will stimulate both L and M cones, to specific degrees, resulting in our particular color experience of orange.

Because each type of cone responds to a fairly wide range of wavelengths (see diagram), some combinations of activity across the three types of cones are not possible. For example, since medium-wavelength light that activates M cones also partially activates L and S cones, it is not possible for a naturally occurring color to stimulate M cones without also stimulating L and S cones. That is, until now.

A team of researchers led by James Fong at UC Berkeley has devised a new way to stimulate specific cones in the retina and thus create a novel color experience. Previous research has demonstrated the possibility of targeting one or two individual cone cells at a time, but the new research published in Science extends this technique to allow researchers to stimulate thousands of retinal cone cells at a time, while still allowing for the targeting of a single cone type.

Five observers participated in the study that used the Oz technique to stimulate specific retinal cones. After stimulating observers’ M-cones, participants reported experiencing a never-before-seen color the researchers termed “Olo” (referring to the middle three letters of the word color). By conducting a series of color-matching experiments, Fong and colleagues were able to ascertain the hue, brightness, and saturation to render the most similar-looking target color that we can all see with our naked eyes. That target color was a highly saturated teal, similar to the one shown in the left box below:

Although it is impossible to experience the true Olo without participating in the Oz procedure, it may be possible to approach the experience by perceiving the teal color above after having adapted the L cones. To try it, you can stare at the red square on the right, keeping your eyes fixated on the central cross, for about 30 seconds. After 30 seconds, quickly move your eyes to the teal square on the left. For a couple of seconds, you should experience a particularly vibrant teal color that may be reminiscent of the Olo color the lab participants got to experience.

The ability to stimulate specific cones at the scale of thousands at a time represents an unprecedented advance in the science of color vision. This research may have implications beyond seeing novel colors, with potential applications for improving color vision in those who experience color blindness.