If you make a spot 
progressively larger, the 
response improves until the 
receptive-field center is filled, 
then it starts to decline as more 
and more of the surround is 
included, as you can see from 
the graph to the left. With a spot 
covering the entire field, the 
center either just barely wins 
out over the surround, or the 
result is a draw. This effect 
explains why 
neurophysiologists before 
Kuffler had such lack of 
success: they had recorded from 
these cells but had generally 
used diffuse light--clearly far 
from the ideal stimulus.

As we stimulate a single on-
center retinal ganglion cell 
with ever larger spots, the 
response becomes more 
powerful, up to a spot size that 
depends on the cell--at most 
about 1 degree. This is the 
center size. Further 
enlargement of the spot causes 
a decline, because now the spot 
invades the antagonistic 
surround. Beyond about 3 
degrees there is no further 
decline, so that 3 degrees 
represents the total receptive 
field, center plus surround.