These graphs plot the 
sensitivity of a cell (measured, 
for example, by the response to 
a constant very small spot of 
light) against retinal position 
along a line AA' passing through 
the receptive-field center. For 
an r+ center-g- surround cell, a 
small red spot gives a narrow 
curve and a small green spot, a 
much broader one. The lower 
graph plots the responses to 
light such as white or yellow 
that stimulates both of the 
opponent systems, so that the 
two systems subtract. Thus the 
red cones dominate in the 
center, which gives on 
responses, whereas the green 
cones dominate in the 
surround, which yields off 
responses.

Both the red cones and the 
green cones feed in from a 
fairly wide circular area, in 
numbers that are maximal in 
the center and fall off with 
distance from the center. In the 
center, the red cones strongly 
predominate, and with distance 
their effects fall off much more 
rapidly than those of the green 
cones. A long-wavelength small 
spot shining in the center will 
consequently be a very 
powerful stimulus to the red 
system; even if it also 
stimulates green cones, the 
number, relative to the total 
number of green cones feeding 
in, will be too small to give the 
red system any competition. 
The same argument applies to 
the center-surround cells 
described in Chapter 3, whose 
receptive fields similarly must 
consist of two opponent 
circular overlapping areas 
having different-shaped 
sensitivity-versus-position 
curves. Thus the surround is 
probably not annular, or donut 
shaped, as was originally 
supposed, but filled. With these 
opponent-color cells in 
monkeys, it is supposed--
without evidence so far--that 
the surrounds represent the 
contributions of horizontal 
cells.