The wavy colored fields in the image above do not exist. Well, except in your mind. There they do exist—in the Cartesian sense at least—following from the famous axiom "Cogito, ergo sum." Existence, it seems, depends on how you think about it. And illusions are all about the difference between reality and perception. In reality, the red and blue fields in the image below are identically-sized striped rectangles. The reason you see colored waves on the left is not well-understood, but clearly it has something to do with the direction of the arrowheads.

Distorted stripes, modified from @gsarcone. Right: black stripes reduce the illusory effect of waviness of colored for reasons not well understood. Credit: Chris Said

Here is the same illusion in motion:

Credit: Chris Said

The illusion, developed by Chris Said, a San Francisco based data analyst, went viral on January 3, 2020, after Said shared it on Twitter. We ourselves featured it in our latest  Illusions column in Scientific American MIND (March-April 2020).

Said’s recent creation belies its classic origins. The mechanisms of this puzzling effect date back to the Müller-Lyer illusion, named after its discoverer, and first reported in 1889.

The Müller-Lyer Illusion, in which line AB appears to be shorter than line BC, despite that they are equal in length. Credit: PolBr Wikimedia (CC BY-SA 4.0)

In a sense, Said's illusion is two illusions in one. First, the arrowheads make the lines appear to be shorter (or longer)—just like the original Müller-Lyer effect. This is the primary source of the waviness. The perceptual result may be related to differential neuronal sensitivities to objects of different sizes, which vision researchers refer to as the "spatial-frequency content" of objects in a scene. The size of a surface refers to its frequency in visual space (i.e. big objects have lower frequencies and small objects higher frequencies). Neuroscientists have established that different neurons are tuned to specific spatial frequencies, so different neurons are more or less sensitive to objects of different sizes. Thus, a prevalent scientific hypothesis is that the classic Müller-Lyer effect occurs because the inner vs outer arrowheads differentially activate different populations of spatial-frequency-tuned neurons. The second, and more mysterious aspect of the illusion, is that the colored lines appear to wave—when in reality they do not—even though the arrowheads themselves are not colored. It is as if the Müller-Lyer pattern manages to capture the colors of the lines that the arrowheads stem from, and then pull those color through the illusory undulations to create a color wave through space. Vision scientists call this type of illusory color spreading "color assimilation," but its neural bases remain unknown. Said's illusion very powerfully highlights this effect.

The artist Gianni Sarcone won the Best Illusion of the Year Contest’s Third Prize in 2017 for creating a dynamic version of Müller-Lyer Illusion, which in turn inspired Said’s brilliant variation. Sarcone also illustrated how motion can further enhance the illusory effect.

See Sarcone's Best Illusion of the Year Contest winning entry: