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Home  Illusions Sigma Motion
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| Sigma Motion |
| Written by Justin Whelan | |
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Warning: this may take practice to see, but I feel it’s worth it. The image above should (hopefully) flicker wildly. Now move your finger along it from left to right, with a speed such that you travel the distance in about 3 seconds, while following the finger with your gaze. When you hit the right speed, you should perceive a smooth rightwards motion of the stripes. Now move your finger back. If you follow it again with your eyes, you will see smooth leftwards motion of the stripes! After a little practice, you can voluntarily switch movement direction without your finger as a guiding target. While you perceive the illusory motion, your eyes do a smooth pursuit with occasional backwards saccades, also known as oculokinetic nystagmus. Comment. The stimulus is just a rapid phase reversal (at 30 rev/s) of a bar pattern. The occasional jerks are due to aliasing of the reversal frequency by the raster frequency of your display and would not occur if I could synchronize to your screen build-up. This phenomenon was named “Sigma Motion” by O. J. Grüsser, but first reported by James Pomerantz. It demonstrates the interaction of image reversal and eye movements, with efference copy at work. Sources Behrens F, Grüsser OJ (1979) Smooth pursuit eye movements and optokinetic nystagmus elicited by intermittently illuminated stationary patterns. Exp Brain Res 37:317–336
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