Before I go on to describe the receptors and other retinal cells, I want to
make three additional comments about receptive fields. The first is a general
comment about receptive fields as a concept; the other two comments are
specifically about the receptive fields of retinal ganglion cells: their overlap and
their dimensions.

                                 THE CONCEPT OF
                                   A RECEPTIVE FIELD
Narrowly defined, the term receptive field refers simply to the spe-
cific receptors that feed into a given cell in the nervous system, with one or
more synapses intervening. In this narrower sense, and for vision, it thus refers
simply to a region on the retina, but since Kuffler's time and because of his
work the term has gradually come to be used in a far broader way. Retinal
ganglion cells were historically the first example of cells whose receptive fields
had a substructure: stimulating different parts of the receptive fields gave qual-
itatively different responses, and stimulating a large area resulted in cancella-
tion of the effects of the subdivisions rather than addition. As presently used,
receptive field tends to include a description of the substructure, or if you prefer,
an account of how you have to stimulate an area to make the cell respond.
When we speak of "mapping out a cell's receptive field", we often mean not
simply demarcating its boundaries on the retina or the screen the animal is
looking at, but also describing the substructure. As we get deeper into the
central nervous system, where receptive fields tend to become more and more
complex, we will find that their descriptions become increasingly elaborate.
Receptive-field maps are especially useful because they allow us to predict
the behavior of a cell. In the case of retinal ganglion cells, for example, suppose
we stimulate an on-center cell with a long, narrow rectangle of light, just wide
enough to span the receptive-field center, and long enough to go beyond the
whole field, center plus surround. We would predict from the on-center map
on page 10 that such a stimulus would evoke a strong response, since it covers
all the center and only a small fraction of the antagonistic surround. Further-
more, from the radial symmetry of the map we can predict that the magnitude
of the cell's response will be independent of the slit's orientation. Both predic-
tions are confirmed experimentally.

                                THE OVERLAP OF
                                   RECEPTIVE FIELDS
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 on
this page. This is the equivalent of saying that the receptive fields almost
completely overlap.

You can see why by glancing at the simplified circuit in the diagram above:
the cell colored purple and the one colored blue receive inputs from the over-
lapping 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.
We can describe the size of a receptive field in two ways. The more straight-
forward description is simply its size on the retina. This has the disadvantage
of being not very meaningful in the everyday scale of things. Alternatively,
you could measure receptive-field size in the outside world, for example, by
taking its diameter on a screen that an animal faces, but you would then have
to specify how far the screen is from the animal's eyes. The way around this
problem is to express receptive-field size as the angle subtended by the recep-
tive field on the screen, at the animal's eye, as shown in the figure on this page.
We calculate this angle in radians simply by dividing the field diameter by the
screen distance, but I will use degrees: (radians x 180)/3.14. One millimeter on
the human retina corresponds to an angle of about 3.5 degrees. At 54 inches
screen distance, i inch on the screen corresponds to 1degree. The moon and
sun, seen from the earth, are about the same size, and each subtends one-half a
degree.
Receptive fields differ in size from one ganglion cell to the next. In particu-
lar, 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 progres-
sively larger the farther out we go, and meanwhile our acuity falls off progres-
sively.
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 gan-
glion 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.