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 ...

What does the Ponzo illusion tell you about how the visual system works?

In the Ponzo illusion the converging parallel lines tricks the brain into thinking that the image higher in the visual field is farther away, so the brain thinks the image is larger, but the two images hitting the retina are same in size.

Is the Ponzo illusion real?

The Ponzo illusion is an optical illusion where a pair of converging lines distorts the perception of two identically sized lines. Like most visual and perceptual illusions, the Ponzo illusion helps neuroscientists study the way the brain and visual system perceive and interpret images.