Headache-Inducing Spiral Illusion Explained

Four Circles Illusion

By Natalie Wolchover via LifesLittleMysteries.com

Warning: This optical illusion might give you a headache. At a glance, the swirls of tilted black-and-white squares create the perception of a spiral. Look more closely and you realize that the squares don’t form a coil at all; they trace out four perfectly round, concentric circles. The cognitive dissonance between your overall impression of spiraling and your recognition of individual circles … well, it hurts.

This “intertwining illusion” sends the brain conflicting cues.
CREDIT: Pinna & Gregory, 2002

The illusion — called the “intertwining illusion” — has been a hit on social media recently, and it also happens to be the subject of study by researchers around the world. Because optical illusions harness the shift between what the eyes see and what the brain perceives, teasing out how that shift happens enables scientists to understand the inner workings of the human visual system.

When confronted with an optical illusion, or any other scene, “the visual system is interested in inferring what regions of an image are part of the same object or were made by the same process,” explained Alvin Raj, a researcher in the Computer Science and Artificial Intelligence Lab at the Massachusetts Institute of Technology, who uses spiral illusions to study peripheral vision mechanisms.

But in this case, the visual system receives conflicting cues: Some say “circle,” and some say “spiral.” At the periphery of your vision, the spiral cues win.

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