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.
CONCEPT OF A
Narrowly defined, the term receptive field refers simply to the specific 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 qualitatively different responses, and stimulating a large area resulted
in cancellation of the effects of the subdivisions rather than addition. As presently
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.
Furthermore, 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 predictions are confirmed experimentally.
THE OVERLAP OF RECEPTIVE
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
the cell colored purple 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
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.
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
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
straightforward 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 receptive 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 particular, the centers of the receptive fields vary markedly and systematically
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
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.