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.
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.