Optical Illusion Combines Phi Phenomenon and Oscillating High Contrast Patterns

Article: The Circles in This Illusion Actually Aren't Changing Shape or Moving at All
Source: ScienceAlert 
Published: November 5, 2021

A new optical illusion is making its rounds for the enjoyment of netizens. The original version, which first appeared in 2020 and attributed to Japanese digital artist Jagarikin, displays a pair of rotating blue-and-yellow circles each encompassing one or four arrows that change directions, with the direction that the arrows point to influencing the perception of the forward motion of the circles. Other variations of the illusion have been created since then, including a black and white version and a version in rainbow colors. (A variation using Necker cubes also seems to be related.) The latest version in rainbow colors has been dissected by viewers to demonstrate, for example, that the illusion persists even when the arrows are removed and that the circles are indeed stationary. For cognitive—in addition to visual—entertainment, curious observers have also investigated the underlying properties that give the illusion its effect. The first is the phi phenomenon, which most of us are familiar with in the form of animated films. In its simplest instantiation, spots moving in succession in the form of a circle create the illusion of forward motion. (In a related phenomenon, called the reverse phi phenomenon, if the second point becomes light rather than dark, then we perceive the motion as moving in the opposite or reverse direction.) Other elements of the optical illusion perhaps include the Müller-Lyer illusion (as seen in a star formation here), wherein varying the direction of arrowheads influences the perception of length. (Additionally, the version in black and white seems to make use of the barberpole illusion.) And finally, it has been noted (in still frame) that each circle is flanked by inner and outer edges with colors that contrast with the body of the circle. The high contrast suggests that the subtleties of the illusion also rely on oscillating positive-negative patterns, for example as seen in two-stroke or four-stroke apparent motion. For both visual and cognitive reasons, optical illusions provide a perplexing but fun reminder of the complex, and sometimes inaccurate, ways in which our visual systems represent the world we see.

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