What different theories are there to explain the Ponzo Illusion? How can it be compared to other illusions without be-littling it?

-- Kerry Lavelle (kerrylavelle@hotmail.com), March 04, 2002

Answers

I believe the standard explanation is that the angled lines on the outside are interpreted "automatically" as being parallel lines that extend into the distance. Thus, the upper of the two horizontal lines is interpreted as being further away than the lower one, but since it subtends the same visual angle as the lower, it must be longer than the lower one "objectively." There's a cute java applet at http:/ /www.sandlotscience.com/Distortions/Ponzo_java.htm where you can play with it a bit. There's also an explanation connecting it to the moon illusion at http://www.unmuseum.org/exmo on.htm.

-- Christopehr Green (christo@yorku.ca), March 04, 2002.

The Ponzo phenomenon is also related to the well-known Mueller-Lyer illusion (arrows with end pointing either outwards or inwards). The prevailing explanation appears to be that context is an important determinant of perception. Lines that are flanked with oblique lines create a 'distorted' perception.

-- Casper Hulshof (c.d.hulshof@utwente.nl), October 08, 2002.