The eye and the interpretative brain

Description and explanation

The Ponzo effect was first demonstrated by the Italian psychologist Mario Ponzo (1882–1960) in 1911. He demonstrate that by placing two identically-sized lines across two converging lines, similar to railway tracks, a geometrical-optical visual illusion happen.
The influence of linear perspective cues (two converging lines) causes the perceptual rescaling of stimulus size of one of the two identically-sized lines.

For me, the only plausible explanation for Ponzo effect is that what we “see” with our eyes is not what we “perceive” with our brain.
In fact, in absence of context we see the two identically-sized lines as egual, indeed what they are.
On the other side, in presence of context, we perceive the entire patterns of lines, not merely as individual components but as complex abstract idea.

In fact the misinterpretation of the real size of the two identically-sized lines is only happening in presence of an environment that contain converging lines.

The more visual cues we present on the surrounding, the more the perception distorts the interpretation of the size of the two identically-sized lines. This happen on both binocular and monocular vision.

Experiment to test your explanation

My objective was to determine how size rescaling takes place and to analyse whether the influence of linear perspective cues causes misinterpretation and consequent perceptual rescaling of stimulus size.
In the experiment I tested how the perceived size of the two identically-sized Dinosaurs (figure 1) changed when linear perspective cues are  presented in the surroundings (figures 2 and 3) and what changes under binocular and  monocular viewing conditions.

I found differences between viewing conditions in the perceived size of the two identically-sized Dinosaurs when linear perspective cues were presented (figure 2). Especially the perceptual rescaling of size augment exponential with the presence of many linear perspective cues (figure 3).
I did not notice any difference under the monocular compared to the binocular viewing condition. 

I conclude that the perceptual field (what an organism perceives) in a 2-D space (such as in a photograph or drawing) is organised by the interpretation of the stimulus elements in terms of their shapes and relative locations in the background, based on our memory on previous experience and knowledge of the 3-D world.

What impact might it have on concepts of visual perception?

The Ponzo effect provide the opportunity to identify and learn about the assumptions that our visual system are based on its long-term adaptation to our visual environment.

The perceptual interpretation of our environment (3-D world) depends on a number of factors: the actual size of the object and the distance the object is from the eye and the size constancy.
With the Ponzo effect is possible to provide a way of explaining the way our brain apply size constancy to perceive lines, angles, and object sizes, in a 2-D space (as in a photograph or drawing). It demonstrates how pictorial depth cues exert different effects on the perception of size.

Author: Marzia Cosenza - February 2021