To test this theory, 
investigators have used 
variations of the Ponzo illusion 
that eliminate a sense of 
perspective. In one test, they 
separated the two test lines, 
each within its own inducing 
background, since continuity 
of the inducing lines is not 
essential to the explanation. 
All that remains are the 
appropriate distances between 
the ends of the test lines and 
the contours of the inducing 
pattern. By and large, these
experimental variations do 
yield an illusion that is 
analogous to the Ponzo 
illusion. But the effect is 
generally smaller. One possible 
conclusion is that contrast and 
assimilation are indeed 
determining factors in the 
Ponzo illusion but not the only 
ones. Thus, it is possible that 
this illusion is the joint result 
of both contrast and 
assimilation and depth 
processing, a possibility to 
which I return at the end of the 
chapter.