The Corridor Illusion
In the corridor illusion, the 
three cylinders look unequal in 
size.

Each illusion figure that we 
have considered so far seems 
tailor-made to illustrate one 
particular theory. The Ponzo 
illusion nicely illustrates a 
misapplied constancy (or 
depth-processing) theory; the 
Poggendorff illusion fits a 
misperception-of-angle theory 
(although it also can be 
explained in terms of depth 
processing); and the Müller-
Lyer illusion is a good example 
of an assimilation (or 
incorrect-comparison) theory. 
No one theory seems to do 
justice to all these illusions. 
How, then, can we explain the 
illusion of extent here, 
versions of which have been 
known for many years as the 
perspective illusion and which 
has lately appeared in many 
textbooks? A version such as 
this one appeared in a book by
James Gibson and is now 
referred to as the corridor
illusion. The illusory 
difference in the perceived 
size of the cylinders is quite
striking, greater than in most
geometrical illusions of extent.

    The corridor illusion reveals 
a greater effect of constancy 
(Emmert’s law, to be precise) 
than can usually be obtained in 
representational pictures. The 
illusory effect is usually 
explained in terms of depth 
processing. Alternatively, one 
might want to explain it in 
terms of contrast and 
assimilation or, otherwise 
expressed, in terms of stimulus 
relations. The proportion of 
each cylinder to the height of 
the corridor wall adjacent to it 
differs appreciably for the 
three cylinders. This was one 
explanation we considered for 
the Ponzo illusion, which is 
quite similar to the corridor 
illusion.