Ponzo illusion

From New World Encyclopedia
An example of the Ponzo Illusion. Both horizontal lines are the same size.

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. Artists have also utilized the illusion to great effect in their works.


The Ponzo illusion was first published by the Italian psychologist Mario Ponzo (1882-1960) in 1913. Ponzo suggested that the human mind judges an object's size based on its background, and illustrated this idea by drawing two identical lines across a pair of converging lines.


The Ponzo illusion is often illustrated with two converging lines that mimic railroad tracks disappearing into the distance. Two horizontal lines or bars are placed across these "tracks," one higher than the other. When looking at the image, viewers routinely see the upper bar (where the converging lines are closer) as larger than the lower bar. In reality, the two bars are of identical size.


The effect of the Ponzo illusion is often attributed to linear perspective. The upper line looks longer because we interpret the converging sides as parallel lines receding into the distance. In this context, we interpret the upper line as though it were farther away, so we see it as longer. In the three dimensional world, an object located farther away would have to be larger than a nearby object for both to produce retinal images of the same size. This explanation is often referred to as the perspective hypothesis.

The more visual cues present surrounding the two horizontal lines, the more powerful the illusion. The framing effect, where objects that appear to fill an enclosing border appear larger than the same object surrounded by a larger frame, also contributes to the illusion. The difference in the separation or gap of the horizontal lines from the framing converging lines helps to create the sense that the upper bar, which is closer to the bordering lines, is larger than the lower one.

Interestingly enough, the Ponzo illusion also appears to be present in other species. Studies have show that pigeons, rhesus monkeys, and chimpanzees all perceive the Ponzo illusion, although the relative strength of various contributing factors like figure orientation was different among each species.[1]


In addition to providing a means of studying the way the human visual and perceptual systems operate, the Ponzo illusion has also provided a way to help determine how other species perceive information.

Some researchers believe that the Moon illusion is an example of the Ponzo illusion, with trees and houses playing the role of Ponzo's converging lines.[2]


  1. Tadasu Oyama and Takuo Goto. "Editorial: Studies on optical illusions in Japan" Japanese Psychological Research 49 (1), 1-6. Retrieved January 2, 2008.
  2. The Moon looks bigger on the horizon because the air acts like a lens, magnifying it Retrieved January 2, 2008.

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