I suspect light-dark contours 
are the most important 
component of our perception, 
but they are surely not the only 
component. The coloring of 
objects certainly helps in 
defining their contours, 
although our recent work tends 
to emphasize the limitations of 
color in defining forms. The 
shading of objects, consisting of 
gradual light-dark transitions, 
as well as their textures, can 
give important clues 
concerning shape and depth. 
Although the cells we have 
been discussing could 
conceivably contribute to the 
perception of shading and 
texture, we would certainly not 
expect them to respond to 
either quality with 
enthusiasm. How our brain 
handles textures is still not 
clear. One guess is that complex 
cells do mediate shades and 
textures without the help of 
any other specialized sets of 
cells. Such stimuli may not 
activate many cells very 
efficiently, but the spatial 
extension that is an essential 
attribute of shading or texture 
may make many cells respond, 
all in a moderate or weak way. 
Perhaps lukewarm responses 
from many cells are enough to 
transmit the information to 
higher levels.

    Many people, including 
myself, still have trouble 
accepting the idea that the 
interior of a form (such as the 
kidney bean) does not itself 
excite cells in our brain--that 
our awareness of the interior as 
black or white (or colored, as 
we will see in Chapter 8) 
depends only on cells sensitive 
to the borders. The intellectual 
argument is that the perception 
of an evenly lit interior 
depends on the activation of 
cells having fields at the 
borders and on the absence of 
activation of cells whose fields 
are within the borders, since 
such activation would indicate 
that the interior is not evenly 
lit. So our perception of the 
interior as black, white, gray, 
or green has nothing to do with 
cells whose fields are in the 
interior--hard as that may be to 
swallow. But if an engineer 
were designing a machine to 
encode such a form, I think 
this is exactly what he would 
do. What happens at the borders 
is the only information you 
need to know: the interior is 
boring. Who could imagine that 
the brain would not evolve in 
such a way as to handle the 
information with the least 
number of cells?