Are you in the vanishing point? (Image: Markus Moellenberg/Corbis)



Video: Are these shapes lined up? Pause the video if you aren’t sure Video: Are these shapes lined up? Pause the video if you aren’t sure

Imagine a dog walking behind a picket fence. Only sections of moving dog are visible, but without having to think about it you know there’s a whole dog there. How the brain manages this trick is still unclear, but Peter Scarfe from University College London has come up with some interesting illusions while trying to understand the rules that the brain applies to moving objects.

In the video above, three circles seem to be lined up. When the video is played, and lines within the shapes begin to move, the row of objects appears to shift and become misaligned. Pause the video, however, and the objects are clearly in a row. “Even if we hold a ruler up to the screen and prove to ourselves that these objects are aligned with one another, we still perceive them as misaligned,” says Scarfe.

“Normally, in our everyday world, objects change position when they move. This illusion represents a special case where we have an object producing motion in a given direction, but staying stationary.”


One explanation is that the brain is “tuned” to the likelihood of the environment behaving in a certain way. For example, the visual system can make a good guess that moving objects will probably change position, says Scarfe, because a person will encounter this many times in everyday life. The illusion may therefore occur because the brain’s prior knowledge about the world influences perception.

In the second part of the video, a number of circles are grouped into two blocks. When each circle has moving stripes inside it, the two blocks appear to shift out of line with each other – effectively, the two groups start to act like separate objects.

This illusion could be likened to snow falling. Each snowflake is drifting in a different direction, but the overall effect is that the snow is falling vertically. “The brain has some understanding of which signals to group together,” says Sharfe, and these illusions should help to explain how the brain applies these rules, and at what stage of the visual system. “That is what we are hoping to find out by using these stimuli,” he says.

Read more: Moving illusions: Brain-tricking motion