Stare at the image below and really fixate on a point in the center.
You may notice that in about 20-30 seconds, the colors start to fade away until the entire picture appears uniform, although as soon as your focus moves even slightly, the original pattern will return.
Stare at the image for about half a minute without moving your eyes and watch as it gradually disappears. This is a variation of #Troxler’s effect which essentially says that if you fixate your eyes on a certain point, stimuli near that point will gradually fade. pic.twitter.com/qjIrhKpO2V
— David McPhillips (@primaryeyecare1) March 12, 2018
The image, shared on Twitter by David McPhillips from Primary Eye Care, is an illustration of the Troxler effect which describes how rigidly fixating on a particular element in the visual field for even a short period of time can cause surrounding stationary images away from this point to gradually fade away and disappear to be replaced by the background. It was discovered by Swiss physician, philosopher and politician Ignaz Paul Vital Troxler (1780-1866) in 1804.
The illusion demonstrates one of the general principles in sensory systems—that our brain automatically filters out unchanging stimuli. For example, if you drop a small piece of paper on the inside of your forearm, you will only be able to feel it for a short period of time before the sensation fades away, because the tactile neurons have adapted and begin to ignore the unimportant stimulus. This is why you don't tend to notice the clothes that you're wearing.
These principles are helpful, in evolutionary terms, because they have enabled us to focus our attention on more important things, such as dangerous predators.
The Troxler effect, also known as Troxler's fading, occurs with any stationary stimulus, although it is particularly fast-acting and noticeable with images that have low contrast between the different parts, according to the Illusions Index.
Optical illusions can be categorized into three main categories, according to the late British neuropsychologist Richard Gregory: physical, physiological and cognitive.
Physical illusions are caused by the physical environment, for example, the apparent bending of a stick which is half submerged in water. Physiological illusions arise in the eye or visual pathway and include things like afterimages—such as the glow that temporarily remains in one's vision after looking at a phone screen in the dark. Cognitive illusions meanwhile, like the Troxler effect, are caused by unconscious inferences in the brain.
