Eye, Brain, and Vision

                       A TYPICAL NEURAL PATHWAY
    Now that we know something about impulses, synapses, excita-
tion, and inhibition, we can begin to ask how nerve cells are assembled into
larger structures. We can think of the central nervous system—the brain and
spinal cord—as consisting of a box with an input and an output. The input
exerts its effects on special nerve cells called receptors, cells modified to respond
to what we can loosely term "outside information" rather than to synaptic
inputs from other nerve cells. This information can take the form of light to
our eyes; of mechanical deformation to our skin, eardrums, or semicircular
canals; or of chemicals, as in our sense of smell or taste. In all these cases, the
effect of the stimulus is to produce in the receptors an electrical signal and
consequently a modification in the rate of neurotransmitter release at their
axon terminals.
    (You should not be confused by the double meaning of receptor; it initially
meant a cell specialized to react to sensory stimuli but was later applied also to
protein molecules specialized to react to neurotransmitters.)
    At the other end of the nervous system we have the output: the motor neu-
rons, nerves that are exceptional in that their axons end not on other nerve cells
but on muscle cells. All the output of our nervous system takes the form of
muscle contractions, with the minor exception of nerves that end on gland
cells. This is the way, indeed the only way, we can exert an influence on our
environment. Eliminate an animal's muscles and you cut it off completely
from the rest of the world; equally, eliminate the input and you cut off all
outside influences, again virtually converting the animal into a vegetable. An
animal is, by one possible definition, an organism that reacts to outside events
and that influences the outside world by its actions.
The central nervous system, lying between input cells and output cells, is
the machinery that allows us to perceive, react, and remember—and it must be
responsible, in the end, for our consciousness, consciences, and souls. One of
the main goals in neurobiology is to learn what takes place along the way—
how the information arriving at a certain group of cells is transformed and
then sent on, and how the transformations make sense in terms of the success-
ful functioning of the animal.
Although the wiring diagrams for the many subdivisions of the central ner-
vous system vary greatly in detail, most tend to be based on the relatively
simple general plan schematized in the diagram on this page. The diagram is a
caricature, not to be taken literally, and subject to qualifications that I will soon
discuss. On the left of the figure I show the receptors, an array of information-
transducing nerves each subserving one kind of sensation such as touch, vibra-
tion, or light. We can think of these receptors as the first stage in some sensory
pathway. Fibers from the receptors make synaptic contacts with a second array
of nerve cells, the second stage in our diagram; these in turn make contact with
a third stage, and so on. "Stage" is not a technical or widely applied
neuroanatomical term, but we will find it useful.
Sometimes three or four of these stages are assembled together in a larger
unit, which I will call a structure, for want of any better or widely accepted
term. These structures are the aggregations of cells, usually plates or globs,
that I mentioned in Chapter i. When a structure is a plate, each of the stages
forming it may be a discrete layer of cells in the plate. A good example is the
retina, which has three layers of cells and, loosely speaking, three stages. When
several stages are grouped to form a larger structure, the nerve fibers entering
from the previous structure and those leaving to go to the next are generally
grouped together into bundles, called tracts.
You will notice in the diagram how common divergence and convergence
are: how almost as a rule the axon from a cell in a given stage splits on arriving
at the next stage and ends on several or many cells, and conversely, a cell at any
stage except the first receives synaptic inputs from a few or many cells in the
previous stage.
We obviously need to amend and qualify this simplified diagram, but at least
we have a model to qualify. We must first recognize that at the input end we
have not just one but many sensory systems—vision, touch, taste, smell, and
hearing—and that each system has its own sets of stages in the brain. When
and where in the brain the various sets of stages are brought together, if indeed
they are brought together, is still not clear.
In tracing one system such as the visual or auditory from the receptors
further into the brain, we may find that it splits into separate subdivisions. In
the case of vision, these subsystems might deal separately with eye move-
ments, pupillary constriction, form, movement, depth, or color. Thus the
whole system diverges into separate subpathways. Moreover, the subpaths
may be many, and may differ widely in their lengths. On a gross scale, some
paths have many structures along the way and others few. At a finer level, an
axon from one stage may not go to the next stage in the series but instead may
skip that stage and even the next; it may go all the way to the motor neuron.
(You can think of the skipping of stages in neuroanatomy as analogous to what
can happen in genealogy. The present English sovereign is not related to Wil-
liam the Conqueror by a unique number of generations: the number of
"greats" modifying the grandfather is indeterminate because of intermarriage
between nephews and aunts and even more questionable events.)
When the path from input to output is very short, we call it a reflex. In the
visual system, the constriction of the pupil in response to light is an example of
a reflex, in which the number of synapses is probably about six. In the most
extreme case, the axon from a receptor ends directly on a motor neuron, so
that we have, from input to output, only three cells: receptor, motor neuron,
and muscle fiber, and just two synapses, in what we call a monosynaptic reflex
arc. (Perhaps the person who coined the term did not consider the nerve-
muscle junction a real synapse, or could not count to two.) That short path is
activated when the doctor taps your knee with a hammer and your knee
jumps. John Nicholls used to tell his classes at Harvard Medical School that
there are two reasons for testing this reflex: to stall for time, and to see if you
have syphilis.
At the output end, we find not only various sets of body muscles that we can
voluntarily control, in the trunk, limbs, eyes, and tongue, but also sets that
subserve the less voluntary or involuntary housekeeping functions, such as
making our stomachs churn, our water pass or bowels move, and our sphinc-
ters (between these events) hold orifices closed.
We also need to qualify our model with respect to direction of information
flow. The prevailing direction in our diagram on page 24 is obviously from
left to right, from input to output, but in almost every case in which informa-
tion is transferred from one stage to the next, reciprocal connections feed
information back from the second stage to the first. (We can sometimes guess
what such feedback might be useful for, but in almost no case do we have
incisive understanding.) Finally, even within a given stage we often find a rich
network of connections between neighboring cells of the same order. Thus to
say that a structure contains a specific number of stages is almost always an
oversimplification.
When I began working in neurology in the early 1950s, this basic plan of the
nervous system was well understood. But in those days no one had any clear
idea how to interpret this bucket-brigade-like handing on of information from
one stage to the next. Today we know far more about the ways in which the
information is transformed in some parts of the brain; in other parts we still
know almost nothing. The remaining chapters of this book are devoted to the
visual system, the one we understand best today. I will next try to give a
preview of a few of the things we know about that system.

THE VISUAL PATHWAY
We can now adapt our earlier diagram on page 6 to fit the special
case of the visual pathway. As shown in the illustration at the top of the facing
page, the receptors and the next two stages are contained in the retina. The
receptors are the rods and cones; the optic nerve, carrying the retina's entire
output, is a bundle of axons of the third-stage retinal cells, called retinal gan-
glion cells. Between the receptors and the ganglion cells are intermediate cells,
the most important of which are the bipolar cells. The optic nerve proceeds to a
way station deep in the brain, the lateral geniculate body. After only one set of
synapses, the lateral geniculate sends its output to the striate cortex, which
contains three or four stages.
You can think of each of the columns in the diagram above as a plate of cells
in cross section. For example, if we were to stand at the right of the page and
look to the left, we would see all the cells in a layer in face-on view. Each of
the columns of cells in the figure represents a two-dimensional array of cells, as
shown for the rods and cones in the diagram to the side.