The next step was to look at 
the receptive fields of these 
cells by using small spots of 
colored light, as Torsten Wiesel 
and I did in 1966, instead of 
diffuse light. For most of De 
Valois' opponent-color cells, 
the receptive fields had a 
surprising organization, one 
that still puzzles us. The cells, 
like Kuffler's in the cat, had 
fields divided into antagonistic 
centers and surrounds; the 
center could be "on" or "off". 
In a typical example, the field 
center is fed exclusively by red 
cones and the inhibitory 
surround exclusively by green 
cones. Consequently, with red 
light both a small spot and a 
large spot give brisk responses, 
because the center is 
selectively sensitive to long-
wavelength light and the 
surround virtually insensitive; 
with short-wavelength light, 
small spots give little or no 
response and large spots 
produce strong inhibition with 
off responses. With white light, 
containing short and long 
wavelengths, small spots evoke 
on responses and large spots 
produce no response.

In a typical type 1 receptive 
field, the center receives 
excitatory input from red 
cones; the surround, inhibitory 
input from green cones.

    Although our first 
impression was that such a cell 
must be getting input from red 
cones in the center region and 
green cones in the surround, it 
now seems probable that the 
total receptive field is a 
combination of two overlapping 
processes, as illustrated in the 
figure to the left.