A similar effect is illustrated by 
the figure shown here. A shape 
is seen for which much of the 
contour is not present. To 
perceive this shape, one must 
reverse figure and ground since 
at the outset the colored 
regions, being surrounded by 
white, are favored as figure. 
This effect is referred to as that 
of illusory contours. While it 
was first reported years ago, the 
current interest was ignited by 
striking black-and-white 
figures such as those created by 
Gaetano Kanizsa. In those cases 
as well as in the figure seen 
here, not only does one see 
contours that are not present, 
but the figure thus defined 
acquires a lightness different 
from that of the surrounding 
screen, which has the same 
physical reflectance. In this 
illustration, the white figure 
looks whiter than the white 
screen.

    The explanation of this effect 
is in dispute. Based on research 
I conducted with Richard 
Anson, my own view is that 
perceiving the figure with its 
illusory contour is an elegant 
solution to the problem of what 
such a retinal image might 
represent. That stimulus after 
all is ambiguous, and at first 
one may tend to perceive the 
isolated colored fragments. But 
the illusory figure percept 
integrates all fragments into 
one coherent solution and 
accounts both for alignment of 
some of the contours with one 
another and for the 
incompleteness of various 
fragments. Thus, in the figure 
at left, instead of perceiving 
four colored pies with sectors 
missing, one perceives a solid 
white rectangle covering four 
colored spheres.