Illusions: Colors Out of Space This is the 11th article in the Mind Matters series on the neuroscience behind visual illusions Yellow moon and blue moon Here we have two moons out of space. One yellow and one blue. Or are they? Actually both moons are exactly the same color in this illusion by psychologist Akiyoshi Kitaoka of Ritsumeikan University in Japan; only the surrounding colors are different. The appearance of colors is all about their context.
Eye shadow This Japanese manga girl by Kitaoka looks like she has one blue eye and one gray eye. In fact, both eyes are exactly the same shade of gray. The girl's right eye only looks the same as the turquoise hair clip because of the reddish context. Part of the process of seeing color is that three different kinds of photoreceptors in the eye are tuned to three overlapping families of color: red, green and blue (which are activated by visible light of long, medium and short wavelengths). These signals are then instantaneously compared with signals from nearby regions in the same scene. As the signals are passed along to higher and higher processing centers in the brain, they continue to be compared with larger and larger swaths of the surrounding scene. This "opponent process," as scientists call it, means that color and brightness are always relative.
Red rings This image by Kitaoka contains a number of bluegreen circular structures. The red rings are purely a creation of your brain. A process called color constancy makes an object look the same under different lighting conditions, even though the color of the light reflecting from the object is physically different. Color constancy is an incredibly important process that helps us to recognize objects, friends and family both in the firelight of the cave and in the bright sun of the savanna. Because the entire image here is drawn in shades of blue, the brain mistakenly assumes that the image is illuminated by blue light, and that the physically gray rings inside the blue structures must therefore be reddish. The visual system subtracts the blue "ambient lighting" from the gray rings, and gray minus blue results in a pastel red color.
White's effect In 1979 Michael White of the Tasmanian College of Advanced Education described an illusion that changed everything in visual science. The gray bars on the left look brighter than the gray bars on the right. In fact, all the gray bars are physically identical. Before White discovered this effect, all brightness illusions were thought to result from opponent processes—that is, a gray object should look dark when surrounded by light and light when surrounded by dark. But in this illusion the lighterlooking gray bars are surrounded by white stimuli, and the darkerlooking gray bars are surrounded by black. The brain mechanisms underlying White's effect remain unknown.
Sparkling color In Light of Sapphires, by Kitaoka, the blue dots appear to scintillate as you move your eyes around the image. But when you focus on one dot, the scintillation stops. The blue color appears more saturated for the dot in focus than for dots in the visual periphery. This effect is a colorful variant of the scintillating grid illusion discovered in 1994 by Elke Lingelbach of the Institute for Optometry Aalen in Germany and her colleagues Michael Schrauf, Bernd Lingelbach and Eugene Wist.
Neon color spreading The colors from the small crosses appear to spread onto the white expanse surrounding each intersection. The effect resembles the glare from a neon light. This illusion was reported in 1971 by Dario Varin of the University of Milan in Italy and a few years later by Harrie van Tuijl of the University of Nijmegen in the Netherlands. Its neural causes are currently unknown.
Colored ray In this neon color spreading illusion, the yellow spreads in a direction perpendicular to the black bars.
Matisse's multicolored face A group of 20thcentury European artists led by Henri Matisse and André Derain used such vivid, unusual colors in their paintings that one critic dubbed these works les Fauves ("wild beasts"). This style became known as Fauvism. Derain's 1905 portrait of Mattisse is characteristic of this style. Using a grayscale version of a similar painting, Livingstone showed that the weird colors work because they have the correct luminance.