The Ponzo Illusion was discovered by Mario Ponzo (1882 - 1960), an Italian psychologist. The Ponzo Illusion was first published in the book Intorno ad alcune illusioni nel campo delle sensazioni tattili, sull'illusione di Aristotele e fenomeni analoghi in 1910.

The Ponzo Illusion is one among a number of illusions where a central aspect of a simple line image—e.g. the length, straightness, or parallelism of lines—appears distorted by other aspects of the image—e.g. other background/foreground lines, or other intersecting shapes. These are sometimes called ‘geometrical-optical illusions’. You can search for other such illusions in the Illusions Index.

There have been a number of attempts to explain how the Ponzo Illusion works. One prominent example is the ‘linear perspective’ explanation, according to which foreshortening is responsible for the illusory effect (see Gillam 1973). Foreshortening is a technique used in perspective to create the illusion of an object receding strongly into the distance, by for example, drawing it smaller and higher-up on the page. Another prominent example is the ‘misapplied constancy scaling’ explanation (an explanation also employed by some to account for other geometrical visual illusions such as the Müller-Lyer or Vertical-Horizontal illusions). According to this explanation, the linear perspective mechanism is activated such that the higher yellow line is perceived as being further away than the lower, but, in addition, a misapplied size constancy mechanism in the visual system makes the higher yellow line appear longer (see Gregory 1963, 1968). More recently, it has been proposed that the illusion is caused by a ‘tilt constancy’ mechanism. Tilt constancy is the mechanism that enables perception of the stable orientation of objects despite changes in retinal or bodily orientation on the part of the perceiver (for discussion of this in comparison to the other two explanations, see Prinzmetal et al. 2001).

The Ponzo Illusion is interesting because it is relevant to debates about modularity, cognitive penetration, and the nature of experience. To explain: on the hypothesis that the mind is modular, a mental module is a kind of semi-independent department of the mind which deals with particular types of inputs, and gives particular types of outputs, and whose inner workings are not accessible to the conscious awareness of the person—all one can get access to are the relevant outputs. So, in the case of the Ponzo Illusion, a standard way of explaining why experience of the illusion persists even though one knows that one is experiencing an illusion is that the module, or modules, which constitute the visual system are ‘cognitively impenetrable’ to some degree—i.e. their inner workings and outputs cannot be influenced by conscious awareness. For a general discussion of cognitive penetration, see Macpherson (2012).

Philosophers have also been interested in what illusions like the Ponzo Illusion can tell us about the nature of experience. For example, in the case of experiencing the Ponzo Illusion, it would seem to be that the one can know that the yellow lines are the same length whilst at the same time experience them as of different lengths. If so, then this might count against the claim the perceptual states are belief like, because if perceptual states were belief like then, when experiencing the Ponzo Illusion one would simultaneously believe that the lines were, and were not, the same length. This would seem to entail that one was being irrational, because one would simultaneously be holding contradictory beliefs. But it seems highly implausible that one is being irrational when under going this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).

Finally, the Ponzo Illusion is also cited in debates about the ‘carpentered world hypothesis’—which is the thesis that those in ‘carpentered environments’ which predominantly contain right-angles (e.g. many urban environments) experience the world differently, as demonstrated by differences in susceptibility to visual illusions. In the case of the Ponzo Illusion, some have put forward evidence which might support the view that those raised in environments which are less rectilinear are less susceptible to the illusion (see Shiraev & Levy 2015). Whether such cross-cultural differences are real remains disputed.