Eye, Brain, and Vision

Far out in the periphery of the retina, receptive-field centers are made up of
thousands of receptors and can have diameters ofi 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 antago-
nistic 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.

THE PHOTORECEPTORS
It was many years before much progress was made in the physiol-
ogy of the receptors, bipolars, horizontal cells, or amacrine cells. There are
many reasons for this: vascular pulsations bedevil our attempts to keep micro-
electrodes in or close to single cells; receptors, bipolars, and horizontal cells do
not fire impulses, so that recording the much smaller graded potentials re-
quires intracellular techniques; and it is hard to be certain which of the cell
types our electrode is in or near. We can circumvent some of these problems by
choosing just the right animal: retinas of cold-blooded vertebrates survive
when taken out of the eye and bathed in oxygenated salt water, and eliminat-
ing the blood circulation eliminates arterial pulsations; the mudpuppy (a kind
of large salamander) has very large cells, easy to record from; fish, frogs,
turtles, rabbits, and cats all have special advantages for one or another kind of
study, so that many species have been used in the study of retinal physiology.
The problem with using so many species is that the details of the organization
of the retinas can differ markedly from one species to the next. Moreover, our
knowledge of the primate retina, one of the most difficult to record from, has
until recently had to depend largely on inferences from the results pooled from
these other species. But progress in primates is accelerating as the technical
difficulties are overcome.
In the past few years, our understanding of the way in which a rod or cone
responds to light has dramatically increased, so much so that one has the
feeling of at last beginning to understand how they work.
Rods and cones differ in a number of ways. The most important difference
is in their relative sensitivity: rods are sensitive to very dim light, cones require
much brighter light. I have already described the differences in their distribu-
tion throughout the retina, the most notable being the absence of rods in the
fovea. They differ in shape: rods are long and slender; cones are short and
tapered. Both rods and cones contain light-sensitive pigments. All rods have
the same pigment; cones are of three types, each type containing a different
visual pigment. The four pigments are sensitive to different wavelengths of
light, and in the case of the cones these differences form the basis of our color
vision.
The receptors respond to light through a process called bleaching. In this
process a molecule of visual pigment absorbs a photon, or single package, of
visible light and is thereby chemically changed into another compound that
absorbs light less well, or perhaps differs in its wavelength sensitivity. In virtu-
ally all animals, from insects to humans and even in some bacteria, this recep-
tor pigment consists of a protein coupled to a small molecule related to vitamin
A, which is the part that is chemically transformed by light. Thanks largely to
the work in the 1950s of George Wald at Harvard, we now know a lot about
the chemistry of bleaching and the subsequent reconstitution of visual pig-
ments.
Most ordinary sensory receptors—chemical, thermal, or mechanical—are
depolarized in response to the appropriate stimulus, just as nerves become
depolarized in response to an excitatory stimulus; the depolarization leads to
release of transmitter at the axon terminals. (Often, as in visual receptors, no
impulse occurs, probably because the axon is very short.) Light receptors in
invertebrates, from barnacles to insects, behave in this way, and up to 1964 it
was assumed that a similar mechanism—depolarization in response to light—
would hold for vertebrate rods and cones.
In that year Japanese neurophysiologist Tsuneo Tomita, working at Keio
University in Tokyo, first succeeded in getting a microelectrode inside the
cones of a fish, with a result so surprising that many contemporaries at first
seriously doubted it. In the dark, the potential across the cone membrane was
unexpectedly low for a nerve cell: roughly 50 millivolts rather than the usual
70 millivolts. When the cone was illuminated, this potential increased—the
membrane became hyperpoUrized—just the reverse of what everyone had as-
sumed would happen. In the dark, vertebrate light receptors are apparently
more depolarized (and have a lower membrane potential) than ordinary resting
nerve cells, and the depolarization causes a steady release of transmitter at the
axon terminals, just as it would in a conventional receptor during stimulation.
Light, by increasing the potential across the receptor-cell membrane (that is,
by hyperpolarizing it), cuts down this transmitter release. Stimulation thus
turns the receptors off, strange as that may seem. Tomita's discovery may help
us to understand why the optic-nerve fibers of vertebrates are so active in the
dark: it is the receptors that are spontaneously active; presumably, many of the
bipolar and ganglion cells are simply doing what the receptors tell them to do.
In the ensuing decades, the main problems have been to learn how light
leads to hyperpolarization of the receptor, especially how bleaching as little as
a single molecule of visual pigment, by a single photon of light, can lead, in the
rod, to a measureable change in membrane potential. Both processes are now
reasonably well understood. Hyperpolarization by light is caused by the shut-
ting off of a flow of ions. In darkness, part of the receptor membrane is more
permeable than the rest of the membrane to sodium ions. Consequently, so-
dium ions continually flow into the cell there, and potassium ions flow out
elsewhere. This flow of ions in the dark, or dark current, was discovered in 1970
by William Hagins, Richard Penn, and Shuko Yoshikami at the National Insti-
tute of Arthritis and Metabolic Diseases in Bethesda. It causes depolarization
of the receptor at rest, and hence its continual activity. As a result of the
bleaching of the visual pigment in response to light, the sodium pores close,
the dark current decreases, and the membrane depolarization declines—the cell
thus hyperpolarizes. Its rate of activity (that is, transmitter release) decreases.
Today, as a result of the work of Evgeniy Fesenko and co-workers at the
Academy of Sciences in Moscow, Denis Baylor at Stanford University, King-
Wai Yau of the University of Texas, and others, we are much closer to under-
standing the linkage between the bleaching and the closing of the sodium
pores. For example, it had been very hard to imagine how the bleaching of a
single molecule could lead to the closing of the millions of pores that the
observed potential changes would require. It now appears that the pores of the
receptor are kept open by molecules of a chemical called cyclic guanosine
monophosphate, or cGMP. When the visual pigment molecule is bleached a
cascade of events is let loose. The protein part of the bleached pigment mole-
cule activates a large number of molecules of an enzyme called transducin; each
of these in turn inactivates hundreds of cGMP molecules, with consequent
closing of the pores. Thus as a result of a single pigment molecule being
bleached, millions of pores close off.
All this makes it possible to explain several previously puzzling phenomena.
First, we have long known that a fully dark-adapted human can see a brief
flash of light so feeble that no single receptor can have received more than i
photon of light. Calculations show that about six closely spaced rods must be
stimulated, each by a photon of light, within a short time, to produce a visible
flash. It now becomes clear how a single photon can excite a rod enough to
make it emit a significant signal.
Second, we can now explain the inability of rods to respond to changes in
illumination if the light is already bright. It seems that rods are so sensitive that
at high illumination levels—for example, in sunlight—all the sodium pores are
closed, and that any further increase in light can have no further effect. We say
the rods are saturated.
Perhaps a few years from now students of biology will regard this entire
story of the receptors as one more thing to learn—I hope not. To appreciate
fully its impact, it helps to have spent the years wondering how the receptors
could possibly work; then suddenly, in the space of a decade or less of spectac-
ular research, it all unfolds. The sense of excitement still has not subsided.