(Editor:   Karen Brakke)


The Ponzo illusion, named after Italian artist and psychologist Mario Ponzo (1882-1960), is one of a set of geometrical illusions that produce misjudgment of relative line length. This misjudgment reflects our tendency to perceive two-dimensional geometric illustrations in certain contexts as if they were three-dimensional objects in our environment. The effect is illustrated below.

In the task presented here, two horizontal lines appear either against a plain background or with a set of converging lines that provide context. The participants adjust the length of one of the horizontal lines until they perceive the two lines as being of equal length.

A picture of ponzo data
Figure 1


This experiment contrasts judgments on five control trials with five experimental trials. For the experimental trials, the converging lines background that induces misjudgment of length is present. On control trials, the background is plain. The dependent variable is the difference in the lengths of the upper and lower lines measured in pixels. A positive number indicates that the lower line has been drawn to be longer than the upper line. A negative number means the lower line was drawn shorter than the upper line.

Example Data Analyses

The data for the experimental and control conditions are amenable to analysis with a paired-samples t-test to see how much perceptual distortion was introduced by the converging lines. A bar graph with standard errors can be used to display the data.

If the research participants in the Ponzo experiment have also completed the Poggendorff and Mueller-Lyer experiments, one could correlate errors on the three illusions to see if there is a relationship. In other words, do people who experience a strong Ponzo effect also experience strong Mueller-Lyer and Poggendorff effects?

Data Format and Download

The data file includes the following variables for each participant: UserID; ClassID; Gender, Age, Time, Date, Absent and Present. The Absent and Present columns contain the error (reported in pixels) for the two different conditions. The Absent condition contains data from the control condition or plain background. The Present Condition column contains data from the experimental condition or lined background.

Sample data appears below:

Sample data image from the Ponzo experiment
Figure 2


We are susceptible to illusions such as Ponzo because they mimic perceptual cues that are important to navigating in the real world. Size perception, for example, depends on the eye's interpretation of depth cues. Thus, the pattern of lines seen in the Ponzo experimental trials often suggests a railroad track or sidewalk, and perceiving the horizontal lines to be at different distances from the observer is accurate and adaptive in our three-dimensional environment.

We sometimes, however, experience geometric illusions in our real world. Consider, for example, the "moon illusion", which makes the moon appear larger when it is near the horizon rather than high in the sky. You may be able to think of other illusions that you have encountered as well.


Brislin, R. W. (1974). The Ponzo illusion: Additional cues, age orientation and culture.  
		Journal of Cross-Cultural Psychology, 5, 139-161.

Diaz, L. F., & Delay, E. R. (1992). Response confidence and the Ponzo illusion.  Perceptual 
		and Motor Skills, 74, 265-266.