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  • Paul Anthony Jones

The Ponzo Illusion

(n.) an optical illusion in which two identical figures are made to appear of different sizes because of the effect of perspective

parallel train lines meeting in the distance, the Ponzo illusion

There is, as we’re steadily discovering here at HH, a name for everything. And that includes those optical illusions in which two identical lines appear to be different sizes, because they’re overlying a separate diminishing set of parallel lines:

As we explained over on Twitter, this kind of illusion is called the Ponzo Illusion, in honour of the acclaimed Italian psychologist Mario Ponzo (1882-1960) who discovered it.

Ponzo introduced this phenomenon in a psychological paper snappily titled, Regarding Some Illusions in the Field of Tactile Sensations After the Illusion of Aristotle and Analogous Phenomena in 1911. The Illusion of Aristotle? That’s the name of one of the very first sensory illusions ever described,: cross your index and middle fingers and then touch a small round object to the insides of your crossed fingertips, and your brain will tell you that you’re actually touching two objects, not one, because it’s so rarely called upon to decisions based on tactility with crossed fingers. Aristotle wrote about that phenomenon more than two millennia ago, and we’ve been dealing with challenges to our perception of things ever since. Including, of course, Ponzo’s Illusion. So how—or rather why—does that illusion work?

Well, like a lot of optical illusions, the Ponzo Illusion plays upon how we perceive size. The mind, Ponzo postulated, relies on background context to judge the sizes of things that it’s looking at. Remove or distort that context, and ultimately your brain can be tricked into making misjudgments.

In the Ponzo Illusion, it’s the forced perspective created by the underlying lines that causes all the problems.

By overlaying two identical lines over a diminishing series of converging lines, like train tracks, the Ponzo Illusion tricks our brain into presuming that the upper of the two lines must be longer, because it appears—due solely to its background—to somehow be “in the distance.” So to be of anywhere near the same size to the lower lines, the top line must be larger because we perceive it as being “further away.”

It isn’t, of course, because we’re looking at a flat picture. But because of our perception of the forced perspective in the background, we get confused and make a misjudgment.

Other optical illusions that rely on context to trick us include the famous Ebbinghaus circles...:

...and the equally famous Müller-Lyer illusion, involving another set of two identical lines, this time equipped with “tails” that appear to distort the lines’ lengths.

Muller-Lyer illusion, an optical illusion of two lines

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