Sandrine Ceurstemont, editor, New Scientist TV
Is that a snake slithering in the grass? Luckily, its movement is only an illusion.
In this animation by Michael Pickard from the University of Sunderland in the UK and colleagues, the impression of motion is created when a pattern changes shade. In the first example, a snake in the background appears to move horizontally. In the second version of the illusion, diamonds seem to dance around.
Although several different factors are responsible for the brain trick, changes in brightness play the biggest part. A careful look at the snake video reveals that the grid pattern is made up of alternating light and dark edges. Illusory movement is produced when the diamond-shaped spaces cycle between these shades, matching one edge then the other.
The effect is also amplified by the brightness of the background, which is deliberately set midway between the darkest and lightest shade. "As each diamond cycles from dark to light, its luminance momentarily equates to that of the background but is out of phase with the others," says Pickard. "This spatial imprecision may be picked up by our peripheral vision to add to the sense of movement."
The speed of the brightness change and the way we perceive the overall contrast of the pattern are also thought to affect the illusion.
The brain trick was recently presented at the European Conference on Visual Perception in Alghero, Italy.
If you enjoyed this post, check out our previous Friday Illusions, such as a still heart that seems to beat or tricky stripes that create a stepping motion.
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