Once again, the web has been invaded by an optical illusion that went viral in a matter of hours, leaving users baffled and scrambling to figure out how it works. Essentially, if you stare at the image at any given point without blinking or moving, its content disappears. The effect takes only about fifteen or twenty seconds to manifest, but its origin forces us to travel more than 200 years into the past.
It all started with a Reddit post by user NightBreeze13. A simple image with several colors, a white background, and blurred edges. If you look at the image for twenty seconds (in some cases it can take up to a minute), all the colors disappear. Sometimes one of those colors "resists" and stays longer, but the illusion works great as long as there are no blinks or movement. First, the answer is no: we're not sick, and there's nothing wrong with our eyes. And second... what exactly is this?
The magic of the web allowed us to confirm that the image has been in circulation since at least 2011, and that the illusion is the product of the so-called Troxler effect, originally identified by Swiss physician Ignaz Paul Vital Troxler in 1804. In relaxed terms, our vision adapts to all external stimuli, and when those stimuli don't change, they are ignored. Any basic movement in the environment (and that of our own eyes) is enough to break the effect, but when we are still and staring fixedly at the image, the colors fade away. Another example of neural adaptation that doesn't involve the eyes is putting on a pair of socks. At first we can feel their texture and temperature difference, but as the hours pass, those sensations disappear, just like the colors in the image.
Another interesting aspect of the Troxler effect is that the image doesn't need to be completely static. The second image is an animated GIF with a black cross in the center of a circle formed by lilac dots. If we focus on the cross, the dot that "disappears" will turn green, and after a few seconds it will start to devour the lilac dots like a Pac-man, leaving only the gray background.
