If the border between two 
regions is formed by a line, as 
in an outline drawing, one 
region can be figure and the 
other, ground.  These relations 
can also be reversible, with the 
border always assigned to the 
figure.  Of course, a contour 
that is just a line in itself is 
neither convex nor concave, 
but this feature is still ascribed 
to whatever region is perceived 
as figure. The two possible 
figural percepts can be so
different from one another as 
shapes, despite their 
dependence on an identical 
contour, that we may predict 
that if observers organize the 
pattern one way at one time, 
the pattern as a whole will not 
be recognized if it is organized 
the opposite way at a later time. 
Rubin did this experiment with 
just this result.