Two neighboring retinal 
ganglion cells receive input 
over the direct path from two 
overlapping groups of receptors. 
The areas of retina occupied by 
these receptors make up their 
receptive-field centers, shown 
face on by the large overlapping 
circles.

    You can see why by glancing 
at the simplified circuit in the 
diagram to the left: the cell 
colored pink and the one 
colored blue receive inputs 
from the overlapping regions, 
shown in cross section, of the 
appropriate colors. Because of 
divergence, in which one cell 
makes synapses with many 
others at each stage, one 
receptor can influence 
hundreds or thousands of 
ganglion cells. It will 
contribute to the receptive-
field centers of some cells and 
to the surrounds of others. It 
will excite some cells, through 
their centers if they are on-
center cells and through their 
surrounds if they are off-center 
cells; and it will similarly 
inhibit others, through their 
centers or surrounds. Thus a 
small spot shining on the 
retina can stir up a lot of 
activity, in many cells.

DIMENSIONS OF RECEPTIVE FIELDS

    My third comment is an 
attempt to relate these events 
in the retina to everyday vision 
in the outside world. Obviously 
our vision completely depends 
on information the brain 
receives from the eyes; all this 
information is conveyed to the 
brain by the axons of retinal 
ganglion cells. The finer the 
detail conveyed by each of 
these fibers, the crisper will be 
our image of the world. This 
fineness of detail is best 
measured not by the overall size 
of receptive fields, but by the 
size of the field centers.