This article explores whether an optical illusion can replicate the experience of "impossible" new colors, a phenomenon previously only achieved by using lasers to selectively stimulate individual cone cells in the human retina. The author explains that because the sensitivity spectra of our S, M, and L cone cells overlap, we never experience a "pure" signal from just one type. A recent study, however, used lasers to activate only M-cones, causing subjects to see a "blue-green of unprecedented saturation." The article presents an optical illusion—a shrinking red circle on a green background—that attempts to mimic this by desensitizing red-sensitive L-cones, thereby enhancing the perception of the green-sensitive M-cones. However, the author concludes that the limitations of the sRGB color gamut on standard displays and the unknown degree of cone saturation make it uncertain if the illusion truly takes a viewer outside their normal color gamut.
The accompanying community discussion delves into the evolutionary biology behind overlapping cone sensitivities, the debate over why plants are green despite it being a peak wavelength in the solar spectrum, and numerous user accounts of the illusion, with many describing a "shimmering" or "hypersaturated" effect. Notably, several users with colorblindness also reported experiencing the phenomenon.
Exploring "Impossible" Colors and the Biology of Vision
The Biological Limit of Vision: Human color perception is based on three types of retinal cone cells (S, M, and L) sensitive to different, but overlapping, wavelengths of light. This overlap means it is not normally possible to experience a signal from just one type of cone cell in isolation.
Hacking Vision with Lasers: A 2025 scientific paper by Fong et al. describes an experiment where lasers were used to selectively stimulate only the M-cones (green-sensitive) on subjects' retinas. This produced the perception of a new, hyper-saturated blue-green color that cannot be seen under normal conditions.
The Optical Illusion Alternative: The article presents an optical illusion designed to replicate the laser experiment's effect. By having the viewer stare at a shrinking red circle on a bluish-green background, the red-sensitive L-cones become desensitized. When the red circle disappears, the background color stimulates the M-cones more purely, causing the viewer to hallucinate an intensely saturated blue-green color.
The sRGB Screen Limitation: The author questions whether the illusion can truly show a new color, pointing out that computer and phone screens are limited to the sRGB color space. This gamut is a small triangle within the much larger space of all human-perceivable colors, and it cannot produce the pure, single-wavelength light of a laser.
Evolutionary Reason for Overlap: A community member explains that the M and L cones have similar sensitivities because they evolved from a gene duplication event on the X chromosome in primates, which was advantageous for distinguishing fruits from foliage.
The "Why Are Plants Green?" Debate: A major discussion point is the paradox of why plants reflect green light, a high-energy part of the solar spectrum, instead of absorbing it. Theories proposed by the community include preventing cellular damage from too much energy, reducing "noise" in the energy harvesting process, and it simply being a historical accident of evolution.
User Experiences and Colorblindness: Many users reported successfully experiencing the illusion, describing the hallucinated color as a "shimmering aura" or "magical" and "hypersaturated." Several users with common forms of red-green colorblindness (deuteranomaly) also confirmed they could see the effect, though their experience might differ from those with typical vision.
Interactive Tool for Exploration: The author created and shared a webpage where users can generate their own versions of the optical illusion, customizing the colors, size, and animation speed to explore different effects.