Four Circles: A Most Amazing Optical Illusion
AT first glance, this image appears to have a small round circle surrounded by a line spiraling outward. But look more carefully. Follow the outer line around and see what happens. Follow any of the "spiraling" lines and see what happens. You may be surprised.
This optical illusion – called the "intertwining illusion" is 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.
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. Although the squares actually form rings, the tilt of the squares is consistent with a spiral. The offset between the black squares in one ring with the black squares in neighboring rings also creates the perception of a spiral, as does the offset between the white squares in adjacent rings. The spiral cues beat out the circle cues.
When the brain tries to make sense of complex stimuli, it perceives 3D scenes it might actually be standing in front of, rather than 2D images that it didn't evolve to understand. So, when your brain decides that those rings are actually spirals, it's probably not deciding they're spirals lying within a flat surface in front of you. Instead, spirals in real life are more often due to something going in a circle while simultaneously changing in distance from you.
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