DEPRIVATION AND DEVELOPMENT
Up to now we have been thinking of the brain as a fully formed, mature machine. We have been asking how it is connected, how the parts function in terms of everyday situations, and how they serve the interests of the animal.
But that leaves untouched an entirely different and most important question:
How did the machine get there in the first place?
The problem has two major components. Much of the brain's development has to go on in the mother's uterus, before the animal is born. A glance at the brain of a newborn human tells us that although it has fewer creases and is somewhat smaller than the adult brain, it is otherwise not very different. But a glance can hardly tell us the whole story, because the baby is certainly not born knowing the alphabet or able to play tennis or the harp. All these accomplishments take training, and by training, we surely mean the molding or modification of neuronal circuits by environmental influences. The ultimate form of the brain, then, is a result of both prenatal and postnatal development. First, it involves a maturation that takes care of itself, depends on intrinsic properties of the organism, and occurs before or after the time at which birth happens to occur; second, it involves postnatal maturation that depends on instruction, training, education, learning, and experience—all more or less synonymous terms.
Prenatal development is a gargantuan subject; I know little about it and certainly will not attempt to describe it in any detail here. One of the more interesting but baffling topics it deals with is the question of how the individual nerve fibers in a huge bundle find their proper destinations. For example, the eye, the geniculate, and the cortex are all formed independently of each other: as one of them matures, the axons that grow out must make many decisions. An optic-nerve fiber must grow across the retina to the optic disc, then along the optic nerve to the chiasm, deciding there whether to cross or not; it must then proceed to the lateral geniculate body on the side it has selected, go to the right layer or to the region that will become the right layer, go to just the right part of that layer so that the resulting topography becomes properly ordered, and finally it must branch and the branches must go to the correct parts of the geniculate cells—cell body or dendrite. The same requirements apply to a fiber growing from the lateral geniculate body to area 17 or from area 17 to area 18. Although this general aspect ofneurodevelopment is today receiving intense study in many laboratories, we still do not know how fibers seek out their targets. It is hard even to guess the winner out of the several major possibilities, mechanical guidance, following chemical gradients, or homing in on some complementary molecule in a manner analogous to what happens in immune systems. Much present-day research seems to point to many mechanisms, not just to one.
This chapter deals mainly with the postnatal development of the mammalian visual system, in particular with the degree to which the system can be affected by the environment. In the first few stages of the cat and monkey visual path—the retina, geniculate, and perhaps the striate, or primary visual, cortex—an obvious question is whether any plasticity should be expected after birth. I will begin by describing a simple experiment. By about 1962 some of the main facts about the visual cortex of the adult cat were known: orientation selectivity had been discovered, simple and complex cells had been distinguished, and many cortical cells were known to be binocular and to show varying degrees of eye preference. We knew enough about the adult animal that we could ask direct questions aimed at learning whether the visual system was malleable. So Torsten Wiesel and I took a kitten a week old, when the eyes were just about to open, and sewed shut the lids of one eye. The procedure sounds harsh, but it was done under anesthesia and the kitten showed no signs of discomfort or distress when it woke up, back with its mother and littermates. After ten weeks we reopened the eye surgically, again under an anesthetic, and recorded from the kitten's cortex to learn whether the eye closure had had any effect on the eye or on the visual path.
Before I describe the results, I should explain that a long history of research in psychology and of observations in clinical neurology prompted this experiment. Psychologists had experimented extensively with visual deprivation in animals in the 1940s and 1950s, using behavioral methods to assess the effects.
A typical experiment was to bring animals up from birth in complete darkness. When the animals were brought out into the light, they turned out to be blind or at least very defective visually. The blindness was to some extent reversible, but only slowly and not in most cases completely.
Paralleling these experiments were clinical observations on children born with cataracts. A cataract is a condition in which the lens of the eye becomes milky, transmitting light but no longer permitting an image to form on the retina. Cataracts in newborns, like those in adults, are treated by removing the lenses surgically and compensating by fitting the child with an artificial lens implant or with thick glasses. In that way, a perfectly focused retinal image can be restored. Although the operation is relatively easy, ophthalmologists have been loath to do it in very young infants or babies, mainly because any operation at a very early age carries more risk statistically, although the risk is small.
When cataracts were removed, say at an age of eight years, and glasses fitted.
the results were bitterly disappointing. Eyesight was not restored at all: the child was blind as ever, and profound deficits persisted even after months or years of attempts to learn to see. A child would, for example, continue to be unable to tell a circle from a triangle. With hopes thus raised and dashed, the child was generally worse off, not better. We can contrast this with clinical experience in adults: a man of seventy-five develops cataracts and gradually loses sight in both eyes. After three years of blindness the cataracts are removed, glasses fitted, and vision is completely restored. The vision can even be better than it was before the cataracts developed, because all lenses yellow with age, and their removal results in a sky of marvelous blue seen otherwise only by children and young adults.
It would seem that visual deprivation in children has adverse effects of a sort that do not occur at all in adults. Psychologists commonly and quite reasonably attributed the results of their experiments, as well as the clinical results, to a failure of the child to learn to see or, presumably the equivalent, to a failure of connections to develop for want of some kind of training experience.
Amblyopia is a partial or complete loss of eyesight that is not caused by abnormalities in the eye. When we sewed closed a cat's or monkey's eye, our aim was to produce an amblyopia and then to try to learn where the abnormality had arisen in the visual path. The results of the kitten experiment amazed us. All too often, an experiment gives wishy-washy results, too good to dismiss completely but too indecisive to let us conclude anything useful. This experiment was an exception; the results were clear and dramatic. When we opened the lids of the kitten's eye, the eye itself seemed perfectly normal: the pupil even contracted normally when we shined a light into it. Recordings from the cortex, however, were anything but normal. Although we found many cells with perfectly normal responses to oriented lines and movement, we also found that instead of about half of the cells preferring one eye and half preferring the other, none of the twenty-five cells we recorded could be influ^ enced from the eye that had been closed. (Five of the cells could not be influenced from either eye, something that we see rarely if ever in normal cats.)
Compare this with a normal cat, in which about 15 percent of cells are monocular, with about 7 percent responding to the left eye and 7 percent to the right.
The ocular-dominance histograms for the cat, shown in the top graph on the
The slit shape of the pupil found in many nocturnal animals such as this cat presumably allows more effective light reduction than a circular pupil.