Eye, Brain, and Vision

RESPONSES OF
LATERAL GENICULATE CELLS
The fibers corning to the brain from each eye pass uninterrupted
through the optic chiasm (from chi, X, the Greek letter whose shape is a cross).
There, about half the fibers cross to the side of the brain opposite the eye of
origin, and half stay on the same side. From the chiasm the fibers continue to
several destinations in the brain. Some go to structures that have to do with
such specific functions as eye movements and the pupillary light reflex, but
most terminate in the two lateral geniculate bodies. Compared with the cere-
bral cortex or with many other parts of the brain, the lateral geniculates are
simple structures: all or almost all of the roughly one and one half million
cells in each geniculate nucleus receive input directly from optic-nerve fibers,
and most (not all) of the cells send axons on to the cerebral cortex. In this
sense, the lateral geniculate bodies contain only one synaptic stage, but it
would be a mistake to think of them as mere relay stations. They receive fibers
not only from the optic nerves but also back from the cerebral cortex, to which
they project, and from the brainstem reticular formation, which plays some
role in attention or arousal. Some geniculate cells with axons less than a milli-
meter long do not leave the geniculate but synapse locally on other geniculate
cells. Despite these complicating features, single cells in the geniculate respond
to light in much the same way as retinal ganglion cells, with similar on-center
and off-center receptive fields and similar responses to color. In terms of visual
information, then, the lateral geniculate bodies do not seem to be exerting any
profound transformation, and we simply don't yet know what to make of the
nonvisual inputs and the local synaptic interconnections.

LEFT AND RIGHT
IN THE VISUAL PATHWAY
The optic fibers distribute themselves to the two lateral geniculate
bodies in a special and, at first glance, strange way. Fibers from the left half of
the left retina go to the geniculate on the same side, whereas fibers from the left
half of the right retina cross at the optic chiasm and go to the opposite genicu-
late, as shown in the figure on page 14; similarly, the output of the two right
half-retinas ends up in the right hemisphere. Because the retinal images are
reversed by the lenses, light coming from anywhere in the right half of the
visual environment projects onto the two left half-retinas, and the information
is sent to the left hemisphere.
The term visual fields refers to the outer world, or visual environment, as
seen by the two eyes. The right visual field means all points to the right of a
vertical line through any point we are looking at, as illustrated in the diagram
on this page. It is important to distinguish between visual fields, or what we see
in the external world, and receptive field, which means the outer world as seen
by a single cell. To reword the previous paragraph: the information from the
right visual field projects onto the left hemisphere.
Much of the rest of the brain is arranged in an analogous way: for example,
information about touch and pain coming from the right half of the body goes
to the left hemisphere; motor control to the right side of the body comes from
the left hemisphere. A massive stroke in the left side of the brain leads to
paralysis and lack of sensation in the right face, arm, and leg and to loss of
speech. What is less commonly known is that such a stroke generally leads also
to blindness in the right half of the visual world—the right visual field—
involving both eyes. To test for such blindness, the neurologist has the patient
stand in front of him, close one eye, and look at his (the neurologist's) nose
with the other eye. He then explores the patient's visual fields by waving his
hand or holding a Q-tip here and there and, in the case of a left-sided stroke,
can show that the patient does not see anything to the right of where he is
looking. For example, as the Q-tip is held up in the air between patient and
neurologist, a bit above their heads, and moved slowly from the patient's right
to left, the patient sees nothing until the white cotton crosses the midline,
when it suddenly appears. The result is exactly the same when the other eye is
tested. A complete right homonymous hemianopia (as neurologists call this
half-blindness!) actually dissects precisely the foveal region (the center of gaze):
if you look at the word was, riveting your gaze on the a, you won't see the s,
and you will only see the left half of the a—an interesting if distressing experi-
ence.
We can see from such tests that each eye sends information to both hemi-
spheres or, conversely, that each hemisphere of the brain gets input from both
eyes. That may seem surprising. After my remarks about touch and pain sen-
sation and motor control, you might have expected that the left eye would
project to the right hemisphere and vice versa. But each hemisphere of the
brain is dealing with the opposite half of the environment, not with the opposite
side of the body. In fact, for the left eye to project to the right hemisphere is
roughly what happens in many lower mammals such as horses and mice, and
exactly what happens in lower vertebrates such as birds and amphibia. In
horses and mice the eyes tend to point outward rather than straight ahead, so
that most of the retina of the right eye gets its information from the right
visual field, rather than from both the left and right visual fields, as is the case
in forward-looking primates like ourselves. The description I have given of the
visual pathway applies to mammals such as primates, whose two eyes point
more or less straight ahead and therefore take in almost the same scene.
Hearing works in a loosely analogous way. Obviously, each ear is capable of
taking in sound coming from either the left or the right side of a person's
world. Like each eye, each ear sends auditory information to both halves of the
brain roughly equally, but in hearing, as in vision, the process still tends to be
lateralized: sound coming to someone's two ears from any point to the right of
where he or she is facing is processed, in the brainstem, by comparing ampli-
tudes and time of arrival at the two ears, in such a way that the responses are
for the most part channeled to higher centers on the left side.
I am speaking here of early stages of information handling. By speaking or
gesturing, someone standing to my right can persuade me to move my left
hand, so that the information sooner or later has to get to my right hemi-
sphere, but it must come first to my left auditory or visual cortex. Only then
does it cross to the motor cortex on the other side.
Incidentally, no one knows why the right half of the world tends to project
to the left half of the cerebral hemispheres. The rule has important exceptions:
the hemispheres of our cerebellum (a part of the brain concerned largely with
movement control) get input largely from the same, not the opposite, half of
the world. That complicates things for the brain, since the fibers connecting
the cerebellum on one side to the motor part of the cerebral cortex on the other
all have to cross from one side to the other. All that can be said with assurance
is that this pattern is mysterious.

LAYERING OF
THE LATERAL GENICULATE
Each lateral geniculate body is composed of six layers of cells
stacked one on the other like a club sandwich. Each layer is made up of cells
piled four to ten or more deep. The whole sandwich is folded along a fore-and-
aft axis, giving the cross-sectional appearance shown in the illustration on this
page.