The Ponzo illusion is a
geometrical-optical illusion Geometrical-optical illusions are visual illusions, also optical illusions, in which the geometrical properties of what is seen differ from those of the corresponding objects in the visual field.
Geometrical properties
In studying geometry one con ...
that was first demonstrated by the
Italian
Italian(s) may refer to:
* Anything of, from, or related to the people of Italy over the centuries
** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom
** Italian language, a Romance language
*** Regional Ita ...
psychologist
Mario Ponzo
Mario Ponzo (; June 23,1882 – January 9, 1960) was an Italian academic psychologist. He was also the Professor Emeritus of Psychology at the University of Rome La Sapienza, University of Rome as well as the Honorary President of the Italian Soci ...
(1882–1960) in 1911. He suggested that the
human mind judges an object's size based on its background. He showed this by drawing two identical lines across a pair of converging lines, similar to
railway
Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a pre ...
tracks. The upper line looks longer because we interpret the converging sides according to
linear perspective
Linear or point-projection perspective (from la, perspicere 'to see through') is one of two types of graphical projection perspective in the graphic arts; the other is parallel projection. Linear perspective is an approximate representation, ...
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 – a farther object would have to be longer than a nearer one for both to produce retinal images of the same size.
One of the explanations for the Ponzo illusion is the "
perspective hypothesis
Perspective may refer to:
Vision and mathematics
* Perspectivity, the formation of an image in a picture plane of a scene viewed from a fixed point, and its modeling in geometry
** Perspective (graphical), representing the effects of visual persp ...
", which states that the perspective feature in the figure is obviously produced by the converging lines ordinarily associated with distance, that is, the two oblique lines appear to converge toward the horizon or a
vanishing point
A vanishing point is a point on the image plane of a perspective drawing where the two-dimensional perspective projections of mutually parallel lines in three-dimensional space appear to converge. When the set of parallel lines is perpendicul ...
. Another is the "
framing-effects hypothesis", which says that the difference in the separation or gap of the horizontal lines from the framing converging lines may determine, or at least contribute to the magnitude of the distortion.
The Ponzo illusion is one possible explanation of the
Moon illusion, with objects appearing "far away" (because they are "on" the horizon) appearing bigger than objects "overhead". However, some have argued that explaining one perception ("appears far away") in terms of another ("appears bigger") is problematic scientifically, and that there are probably complex internal processes behind these illusions.
The Ponzo illusion also occurs in touch and with an auditory-to-visual
sensory-substitution device. However, prior visual experience seems mandatory to perceive it as demonstrated by the fact that congenitally blind subjects are not sensitive to it.
The Ponzo illusion has also been used to demonstrate a dissociation between vision-for-perception and vision-for-action (see
Two-streams hypothesis
The two-streams hypothesis is a model of the neural processing of vision as well as hearing. The hypothesis, given its initial characterisation in a paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct visu ...
). Thus, the scaling of grasping movements directed towards objects embedded within a Ponzo illusion is not subject to the size illusion.
In other words, the opening between the index finger and thumb is scaled to the real not the apparent size of the target object as the grasping hand approaches the object.
Cross-cultural differences in susceptibility to the Ponzo illusion have been noted, with non-Western and rural people showing less susceptibility.
Other recent research suggests that an
individual's receptivity to this illusion, as well as the
Ebbinghaus illusion
The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. Named for its discoverer, the German psychologist Hermann Ebbinghaus (1850–1909), the illusion was popularized in the English-speaking world by Edwa ...
, may be inversely correlated with the size of that individual's
primary visual cortex
The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...
.
References
Further reading
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External links
An interactive illustration of the Ponzo illusion in Roger Shepard's ''Terror Subterra''
{{DEFAULTSORT:Ponzo Illusion
Optical illusions