Saturday, January 14, 2012

New Scientist TV

Friday Illusion: Tricky stripes create stepping motion

Don't believe your eyes as you watch this video: although the rectangles moving in sync suddenly seem to shuffle, their motion hasn't actually changed. Keep watching when a backdrop of morphing stripes appears and a caterpillar-like motion can be seen.
Created by graduate student Sebastiaan Mathôt from VU University of Amsterdam, the brain trick occurs when the background is striped rather than solid, an illusion originally developed by researcher Stuart Anstis from University of California, San Diego. The effect is caused by the influence of contrast on motion. When there is a big difference in contrast between a moving object and its background, it appears to move faster than when brightness levels are similar.

In this case, the top rectangle is brighter than the maroon one below it. So when their front edges are superimposed on a dark stripe, the top block seems to move faster than the bottom one. Similarly, the speed effect is reversed when they cover the lighter stripe.

In the second version of the illusion with morphing stripes, the front and back edges of the blocks sit on different colours. Since the motion of each edge is perceived independently, the front and back of the blocks appear to move at different speeds, creating a caterpillar-like sashay.
According to Anstis, the contrast effect is also experienced by drivers. On a foggy day, the difference in brightness between an object and its surroundings is generally less than on a sunny day. This can cause drivers to underestimate how fast they're travelling and speed up, sometimes with disastrous consequences. 
Were you able to see the illusion? Let us know in the comments section below.
If you enjoyed this illusion, see how a change in contrast can fool your brain or watch how a shadow can alter an object's path.

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