The responses to diffuse 
light--in this case, on to red, 
off to blue or green, and no 
response to white--make it 
clear that such a cell must be 
registering information about 
color. But the responses to 
appropriate white borders and 
the lack of response to diffuse 
light make it clear that the cell 
is also concerned with black-
and-white shapes. We call these 
center-surround color-
opponent cells "type 1".

    The lateral geniculate body 
of the monkey, we recall from 
Chapter 4, consists of six 
layers, the upper four heavily 
populated with small cells and 
the lower two sparsely 
populated with large cells. We 
find cells of the type just 
described in the upper, or 
parvocellular, layers. Type 1 
cells differ one from the next 
in the types of cone that feed 
the center and surround 
systems and in the nature of 
the center, whether it is 
excitatory or inhibitory. We can 
designate the example in the 
diagram on the facing page as 
"r+g-". Of the possible subtypes 
of cells that receive input from 
these two cone types, we find 
all four: r+g-, r-g+, g+r-, g-r+. A 
second group of cells receives 
input from the blue cone, 
supplying the center, and from 
a combination of red and green 
cones (or perhaps just the green 
cone), supplying the surround. 
We call these "blue-yellow", 
with "yellow" a shorthand way 
of saying "red-plus-green".

    We find two other types of 
cells in the four dorsal layers. 
Type 2 cells make up about 10 
percent of the population and 
have receptive fields consisting 
of a center only. Throughout 
this center, we find red-green 
opponency in some cells, blue-
yellow in others. The centers of 
these type 2 cells tend to be 
large, several times larger than 
the centers of type 1 cells. The 
other 15 percent or so of cells 
in the four upper geniculate 
layers, and all the cells in the 
two lower (magnocellular) 
layers, are center-surround but 
show no such color 
preferences; it is as if their 
field centers and surrounds 
received the same relative 
contributions from the three 
cone types. We refer to these 
cells as broad-band, and in the 
upper layers we call them type 3 
cells.

    All these findings are 
remarkably compatible with 
Hering's model: we have two 
classes of color-opponent cells, 
one red-green, the other 
yellow-blue, and a third 
showing no color opponency at 
all but a broad-band spatial 
opponency instead. What 
seemed not to fit any theory was 
the spatial organization of the 
opponent-color, or type 1 cells. 
You might think, at first 
glance, that this organization 
would have something to do 
with color contrast, with the 
tendency for one color, say 
blue, to look more vivid if 
surrounded by another, say 
green, or for a gray piece of 
paper to look yellowish if 
surrounded by blue. But a 
moment's thought will 
convince you that type 1 cells 
can hardly be useful for that 
kind of color contrast: the r+ 
center-g- surround cell just 
described, far from being 
strongly excited by a red spot 
surrounded by green, gives 
little or no response, because 
one effect cancels the other--
the reverse of what would seem 
to be required for color 
contrast.