Optical illusions fascinate humans. Greek builders used one to ensure  that their columns appeared straight (building them with a bulge). Best known is the mental flip involved in the case of the young girl/old woman faces or (above) the right turn, left turn dancer  suggesting dominant right or left brain, (foot shadows have an impact too!)

Published in BMC Neuroscience,  University College London researchers looked at two well known illusions: the Ebbinghaus  illusion, where an object surrounded by small circles appears bigger than the same object surrounded by bigger circles (left), and the Ponzo illusion, where an object within converging lines (like train tracks or a corridor) is perceived to be larger than a same sized object nearer to the observer.

Their results show the Ponzo illusion holds true, regardless of which eye is used or whether the environmental clues are presented to a different eye than the objects.

This suggests that our clues about relative size at a distance are determined after the two-dimensional images seen by the eyes have been processed into a single, three-dimensional image.

In contrast the Ebbinghaus illusion does not work as well if the central object is presented to a different eye than the surrounding circles and shows that determination of an object’s size relative to others in the same plane occurs before three-dimension processing.

Researcher, Chen Song said, “Although our perception of size is distorted by environmental clues, this study shows  the extent of distortion and the brain mechanisms involved, are dependent on the type of environmental contexts.”

So while celebrity illusionists retain the skill to fool the eye, scientists can use the visual tricks to further our understanding of how we relate to the world around us.