THE OVERLAP OF RECEPTIVE 
FIELDS
The receptive fields of two 
neighboring retinal ganglion 
cells will usually overlap. The 
smallest spot of light we can 
shine on the retina is likely to 
influence hundreds of ganglion 
cells, some off center and some 
on center. The spot will fall on 
the centers of some receptive 
fields and on the surrounds of 
others.

    My second comment 
concerns the important 
question of what a population of 
cells, such as the output cells 
of the retina, are doing in 
response to light. To understand 
what ganglion cells, or any 
other cells in a sensory system 
are doing, we have to go at the 
problem in two ways. By 
mapping the receptive field, we 
ask how we need to stimulate to 
make one cell respond. But we 
also want to know how some 
particular retinal stimulus 
affects the entire population of 
ganglion cells. To answer the 
second question we need to 
begin by asking what two 
neighboring ganglion cells, 
sitting side by side in the 
retina, have in common.

    The description I have given 
so far of ganglion-cell receptive 
fields could mislead you into 
thinking of them as forming a 
mosaic of nonoverlapping little 
circles on the retina, like the 
tiles on a bathroom floor. 
Neighboring retinal ganglion 
cells in fact receive their 
inputs from richly overlapping 
and usually only slightly 
different arrays of receptors, as 
shown in the diagram to the 
left. This is the equivalent of 
saying that the receptive fields 
almost completely overlap.