The Delboeuf illusion: the outer 
circle in the left figure 
assimilates to the smaller 
circle it surrounds and the 
inner circle in the right figure 
assimilates to the larger circle 
around it so that the two circles 
to be compared appear unequal 
in size.

    Assimilation implies the 
opposite process—–that is, a 
tendency to perceive an object 
as including or incorporating 
the properties of certain 
background elements. A good 
example is the Delboeuf 
illusion, shown in the 
illustration here. In that 
illustration, the inner circle to 
the right looks larger than the 
outer circle to the left although 
both are the same size. One 
might say that this occurs 
because the inner circle on the 
right assimilates to its larger 
surrounding circle, while the 
outer circle on the left 
assimilates to the smaller 
circle it surrounds. In the case 
of the Ponzo illusion, the upper 
line can be said to assimilate to 
the inducing lines so that it 
tends to "stretch," to appear as 
wide as the distance between 
the converging lines at the 
same height in the field. Thus, 
the effect of contrast is to 
diminish the perceived size of 
the lower line, and the effect 
of assimilation is to increase 
the perceived size of the upper 
line. Note that this explanation 
does not entail either depth 
perception or constancy.