This optical illusion was created by the German physiologist Ewald Hering in 1861. It is a very common perceptual illusion used by neuroscientists to study the mechanism of perception. It’s also an intriguing picture for the viewers because they don’t understand how it works.
We can find two similar illusions: The Orbison Illusion and The Wundt Illusion.
What to see?
In this image you can see two vertical lines. When the viewer is watching this illusion he sees a pair of bowed or curved lines placed in front of a bunch of other lines oriented like the spikes of a bicycle wheel. In reality the two curved lines are parallel and only appear to be curved because of the background.
If you want to see them straight try to squint your eyes.
How it works?
The fact that the illusion takes place only when the image is horizontal or vertical, challenged the scientist to find explanation for how the illusory effect is generated. If we rotated the parallel lines to another angle the illusion doesn’t happen.
The effect of the Hering Illusion is associated with the “angular placement”. The distortion we perceive is produced by the angular lines behind the parallel ones. In other words the straight parallel lines are skewed because of the effect produced by the lined pattern on the background that simulates a perspective design, and creates a false impression of depth.
Another trick in this illusion is made by the central point, from which all the lines start. The fact that all the lines are intersecting in a point, just like the spokes of a bike, give us the impression that we are moving forward that point. Since we are actually standing we perceive the straight lines as curved ones.