Troxler fading and neural adaptation may partly explain Caputo's strange-face illusion. As we gaze long and steady into our reflected face, the unchanging nature of the visual stimulus causes facial features to disappear and then reappear, as we blink or make involuntary eye movements, thereby “refreshing” our neuronal responses. In the absence of visual information, our brain will “fill in” the gaps according to our experiences, expectations, best guesses, and even hardwired neural mechanisms involved in shape and face perception. The result can be amusing or disquieting.
A few years ago I had the opportunity to collaborate with cognitive scientist Daniel Simons (author, with Christopher Chabris, of The Invisible Gorilla, Random House, 2010) and his colleagues at the University of Illinois at Urbana-Champaign on an experiment to investigate visual fading in entire scenes. You can experience the effect by focusing your gaze precisely at the center of the blurry image, while paying attention to the entire scene. Careful staring for just a few seconds will minimize your eye movements, causing the scene to fade to gray. Now stop focusing your gaze, and the scene will come right back. Scientists can make nonblurred scenes fade in the laboratory by completely removing or by compensating for the observer's eye movements.
A Fading Impression
Impression, Sunrise, by French painter Claude Monet in 1872, gave its name to the Impressionist movement. The subject of the painting is the harbor of Le Havre in France, as seen from Monet's window. It was not, however, Monet's actual view of the scene, as he later explained, but his “impression”—hence the title. Indeed, Sunrise does not accurately represent reality: the rising sun appears much brighter than the surrounding sky, as it should in real life, but that perception is an illusion. Monet used pigments of matching luminance, or brightness, but different chromatic content, or hues, to represent the sun and the sky. Harvard University neurobiologist Margaret Livingstone has proposed that this equiluminant quality, where objects in the image have the same luminance as the background, is what gives the sun its eerie, almost pulsating, lifelike appearance. A black-and-white reproduction reveals that the sun has the same physical luminance as the background clouds.
Equiluminant objects are somewhat difficult to see, which makes them more susceptible to Troxler fading. Neuro-ophthalmologists Avinoam B. Safran and Theodor Landis of Geneva University Hospital in Switzerland noticed that if you fix your gaze on the image of the sailor in Monet's painting for several seconds, while paying attention to the sun, the solar disk will disappear progressively, being replaced by the surrounding sky.
How Many Dots?
This pattern, generated by vision scientist Akiyoshi Kitaoka of Ritsumeikan University in Japan, contains a yellow dot at the center of each set of four blue “petals.” That makes a total of 61 yellow dots! To prove it to yourself, examine each “flower” in turn and see that all of them contain a centralyellow spot. The problem is that you can see only one spot at a time. The othersdisappear when you are not looking at them directly.
In the left image of the pair that makes up this illusion, also by Kitaoka, each of the four squares in the corners has three ladybugs. The little insects are easy to see as you look around the image, but when you stare at the center of the pattern they vanish immediately. The right-hand image shows they do not become invisible by virtue of the small size: in the absence of the squares, all 12 ladybugs are perceptually prominent, no matter where in the image you look.
The Healing Grid