Video: Most of your eye’s color sensors don’t actually see color

We see color because of specialized light-sensing cells in our eyes called cones. One type, L-cones, sees the reds of strawberries and fire trucks; M-cones detect green leaves, and S-cones let us know the sky is blue. But vision scientists have now discovered that not all cones sense color (see video). The finding was made possible because, for the first time, scientists were able to look at individual photo-sensing cells. They did this by making use of an eye-tracking device and the same optical principles that enable telescopes to observe distant galaxies. The “adaptive optics” compensate for the many small irregularities in our eye’s lens so that single cones could be targeted by a flash of light. The eye-tracking device had to detect the eye’s rapid, irregular movement and predict its path to ensure the light hit the right target. With this technology, the researchers reliably and repeatedly stimulated 273 cone cells total in two lab-member volunteers, and most cells didn’t register any color at all. Instead, two-thirds send just a “white light” signal to the brain, the team reports today in Science Advances. In the past, researchers had thought that eyes sense a color by comparing what neighboring cones were detecting. The difference between what one type of cone experienced compared with what other types of cones nearby experienced led to a “red,” “green,” or “blue” signal. Thus, if all the neighbors were the same type, then the lack of contrast would lead to a “white” signal. But actually, the white-signaling cones were more likely to be surrounded by other cone types, the team reports. In retrospect, it makes sense that more cones see white, as that helps the brain discern the edges, lines, and other critical details that clearly define what is in our view. In contrast, the brain only needs a vague sense of the color to know, for example, a light is red or a strawberry is ripe. The work may lead to new ways to cure color blindness and other diseases.