Jastrow illusion

[3] The oldest reference to this illusion can be found in The World of Wonders, an 1873 book about curiosities of nature, science and art.

On the other side of the Atlantic, German scientist Wilhelm Wundt was also pioneering in psychology research.

He wrote one of the first books about geometric optical illusions in which he copied the design previously published by Müller-Lyer.

[6] There are several competing explanations of why the brain perceives the difference in size between the ring segments, none of which has been accepted as definitive.

One explanation relates to how the mind interprets the two-dimensional images on the retina as a three-dimensional world.

Another explanation relates to the fact that the mind can only attend to a small field of vision, which is reconstructed by our consciousness.

The most commonly used explanation is that the brain is confused by the difference in size between the large and the small radius.

The cut angle is most effective at zero degrees, which occurs when the line extends through the centre of the circle segments.

[12] Manfredo Massironi and his colleagues from the universities of Rome and Verona modified the Jastrow illusion to develop a diagnostic test for unilateral spatial neglect.

People that suffer from neglect do not experience the illusion when the overlapping part of the segments is on the side where their perception is missing.

The children were asked to play a game called "Big and Little" and point out which segment was really bigger than the other.