Receptive fields differ in size 
from one ganglion cell to the 
next. In particular, the centers 
of the receptive fields vary 
markedly and systematically in 
size: they are smallest in the 
fovea, the central part of the 
retina, where our visual 
acuity--our ability to 
distinguish small objects--is 
greatest; they get progressively 
larger the farther out we go, 
and meanwhile our acuity falls 
off progressively.

    In a monkey the smallest 
field centers yet measured 
subtend about 2 minutes of arc, 
or about 10 micrometers (0.01 
millimeters) on the retina. 
These ganglion cells are in or 
very close to the fovea. In the 
fovea, cones have diameters 
and center-to-center spacing of 
about 2.5 micrometers, a figure 
that matches well with our 
visual acuity, measured in 
terms of our ability to separate 
two points as close as 0.5 
minutes of arc. A circle 2.5 
micrometers in diameter on the 
retina (subtending 0.5 minutes) 
corresponds to a quarter viewed 
from a distance of about 500 
feet.

    Far out in the periphery of 
the retina, receptive-field 
centers are made up of 
thousands of receptors and can 
have diameters of 1 degree or 
more. Thus as we go out along 
the retina from its center, 
three items correlate in an 
impressive way, surely not by 
coincidence: visual acuity 
falls, the size of the receptor 
population contributing to the 
direct pathway (from receptors 
to bipolars to ganglion cells) 
increases, and the sizes of 
receptive-field centers 
increase. These three trends 
are clues that help us 
understand the meaning of the 
direct and indirect paths from 
receptors to ganglion cells. The 
strong implication is that the 
center of the receptive field is 
determined by the direct path 
and the antagonistic surround 
by the indirect one, and that 
the direct path sets limits on 
our acuity. To obtain more 
evidence for this conclusion, it 
was necessary to record from 
the other cells in the retina, as 
I will describe in the next 
section.