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 geniculate, 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 experience.
We can see from such tests that each eye sends information to both hemispheres or, conversely, that each hemisphere of the brain gets input from both eyes. That may seem surprising. After my remarks about touch and pain sensation 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 amplitudes 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 hemisphere, 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-andaft axis, giving the cross-sectional appearance shown in the illustration on this page.
The six cell layers show clearly in the left lateral geniculate body of a macaque monkey, seen in a section cut parallel to the face. The section is stained to show cell bodies, each of which appears as a dot.
A microscopic cross-sectional view of the optic nerve where it leaves the eye, interrupting the retinal layers shown at the left and right. The full width of the picture is about 2 millimeters. The clear area at the top is the inside of the eye. The retinal layers, from the top down, are optic-nerve fibers (clear), the three stained layers of cells, and the black layer of melanin pigment.
The right visual field extends out to the right almost to 90 degrees, as you can easily verify by wiggling a finger and slowly moving it around to your right. It extends up 60 degrees or so, down perhaps 75 degrees and to the left, by definition, to a vertical line passing through the point you are looking at.