If the depth-processing 
theory is incorrect or 
inadequate to explain the 
Ponzo illusion, what other 
explanation might be offered? 
Another general theory of 
geometrical illusions draws 
upon the opposing concepts of 
contrast and assimilation. 
Contrast is the tendency to 
perceive an object’s properties 
by comparing them with the 
properties of the object’s 
background or context and 
exaggerating these differences. 
We have encountered some 
examples of this in earlier 
chapters. The effect of a white 
or black background on the 
appearance of a gray region is a 
case in point: White makes the 
gray appear darker and black 
makes it appear lighter. Frame 
of reference affects the 
perception of size in a similar 
way. The contrast between an 
object and a large frame of 
reference seems to decrease the 
object’s size. In the Ponzo 
illusion, the lower line looks 
small in contrast with the large 
empty space at either end of it. 
The upper line looks large in 
contrast to the small space at 
either end of it. Therefore, the 
lower line should appear 
smaller than the upper line.