How Do Optical Illusions Work?


The image you’re looking at isn’t moving – it’s an illusion! Images like this appear to move as the result of what is known as illusory motion. Through the use of curvature, colour and shape, images that are static give the appearance of motion. But why does this happen?

Cognitive interruption in the brain is one theory. This interruption is triggered by edges found between black and white (note the subtle use of black and white converging throughout this image) or perhaps the way our brains perceive the transition from light to dark as motion.

Luminance, our sense of light and dark, is pretty unreliable. Our brains are able to perceive lighter values much more quickly than dark values. This explains why the spiral seems to rotate in the direction of the lighter shades. There are also key points where your perception of motion is reset: blinking, shifting your eyes, and looking away and back fuels the illusion of motion.

A study into illusory motion conducted in 2012 showed that tiny eye movements and blinking can make the geometric drawing of “snakes” appear to dance.


“Visual illusions demonstrate the ways in which the brain creates a mental representation that differs from the physical world,” said Barrow Neurological Institute researcher Susana Martinez-Conde. “By studying illusions, we can learn the mechanisms by which the brain constructs our conscious experience of the world.”

Earlier studies of the “Rotating Snakes” indicated the perception of motion was triggered by the eyes moving slowly across the illusion. But by tracking eye movements in eight volunteers, the vision neuroscientists found a different explanation: fast eye movements called “saccades,” some of which are microscopic and undetectable by the viewer, drive the illusory motion. Your eye moves at such a speed, it creates the impression of motion, just like a movie reel.

The difference between what we see and what we THINK we see is one of the most fascinating things about vision. For more information about the weird and wonderful mysteries about your eyes, check out our blog post on Weird and Wonderful facts about eyes.