Why is this optical illusion hardwired into our brain?

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The Motion Aftereffect is an optical illusion in which stationary objects appear to be in motion if a person has just been looking at a moving background. And the effect appears to work on the deepest levels of our subconscious.

We've known about this particular optical illusion for a long, long time - it was first described by the Greek philosopher Aristotle. But it's only now that we're starting to understand just how powerful this illusion really is, thanks to work by researchers at the University of Rochester. They've also created the video up top so that you can see the motion aftereffect for yourselves, featuring Rocky, the university's mascot .


Their new research has found that humans perceive the optical illusion even if the initial stimulus - the background motion - is so brief that they can barely perceive it. Test subjects still experienced the aftereffect when they were shown a moving pattern for just 1/40 of a second, which is far too short for motion to be consciously perceived. Even if we can't consciously see any movement, neurons in our brain - specifically in a part of the motion center of the brain known as cortical area MT - will automatically adjust so that they respond to a stationary object as though it's moving.

This means that the optical illusion is operating on a far deeper level than most, and it suggests that we have gleaned some evolutionary advantage from this faulty perception of motion. The researchers will next turn their attention to this subject, as they hope to demonstrate some link between the motion aftereffect and improved ability to estimate the speed and direction of objects moving around us.