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HEMove your finger slowly from left to right across the pattern while watching your finger with your eyes, taking about two seconds to travel across the whole width. When you reach the right speed you should start to perceive the stripes smoothly drifting to the right even though they are not actually moving. Then move your finger back the other way and follow it again with your eyes and the stripes should appear to drift smoothly to the left. After a little practice you can even switch the apparent direction of motion on your own without needing your finger as a guide.
This visual phenomenon is known as Sigma Motion.
It was named by the neuroscientist O. J. Grüsser, but the effect was first described by James Pomerantz.
What’s fascinating is that the motion you see is not actually happening.
Your brain creates the illusion because of the interaction between image reversal and eye movements.
Here’s what’s happening psychologically:
When your eyes move, the brain sends a motor command to the eye muscles.
At the same time, it sends a copy of that command — called an Efference Copy — to the visual system.
This internal signal helps the brain distinguish between:
• Movement happening in the world
• Movement caused by your own eyes
In illusions like this, the signals don’t perfectly align, so the brain interprets the visual change as external motion.
So even though the image is static, your brain constructs movement from the mismatch.
It’s a powerful reminder of something important in psychology and neuroscience:
We don’t see reality exactly as it is.
We see the brain’s interpretation of it.
Sometimes your brain invents motion where none exists.
💬 Comment “VISION” if the illusion fooled you.
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