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One obvious way to decide between these alternatives is to address the question head on and simply record from a newborn cat or monkey. If learning is necessary for the wiring up to occur, then we should fail to find any of the rich specificity that we see in adult animals. A lack of specificity would nevertheless not decide the issue, because we could then ascribe the lack of connections either to immaturity—still-incomplete genetically programmed wiring—or to lack of experience. On the other hand, finding such specificity would argue against a learning mechanism. We did not expect the experiments with kittens to be easy, and they weren't. Kittens arc visually very immature at birth and make no use at all of their eyes before about the tenth day, when the eyes open. At that time, even the media of the eye, the transparent substances between the cornea and the retina, are far from clear, so that it is impossible to get a clear image on the retina. The immature visual cortex indeed responded sluggishly and somewhat unpredictably and was on the whole a far cry from a normal adult visual cortex; nevertheless we found many clearly orientationspecific cells. The more days that elapsed between birth and recording, the more the cells behaved like adult cells: perhaps because the media were clearer and the animal more robust but perhaps because learning had occurred. Interpretations differed from one set of observers to another.
The most convincing evidence came from newborn monkeys. The day after it is born, a macaque monkey is visually remarkably mature: unlike a newborn cat or human, it looks, follows objects, and takes a keen interest in its surroundings. Consistent with this behavior, the cells in the neonate monkey's primary visual cortex seemed about as sharply orientation-tuned as in the adult. The cells even showed precise, orderly sequences of orientation shifts (see the graph on this page). We did see differences between newborn and adult animals, but the system of receptive-field orientation, the hallmark of striate cortical function, seemed to be well organized.
Compared with that of the newborn cat or human, the newborn macaque monkey's visual system may be mature, but it certainly differs anatomically from the visual system of the adult monkey. A Nissl-stained section of cortex looks different: the layers are thinner and the cells packed closer. As Simon LeVay first observed, even the total area of the striate cortex expands by about 30 percent between birth and adulthood. If we stain the cortex by the Golgi method or examine it under an electron microscope, the differences are even more obvious: cells typically have a sparser dendritic tree and fewer synapses.
Given these differences, we would be surprised if the cortex at birth behaved exactly as it does in an adult. On the other hand, dendrites and synapses are still sparser and fewer a month before birth. The nature-nurture question is whether postnatal development depends on experience or goes on even after birth according to a built-in program. We still are not sure of the answer, but from the relative normality of responses at birth, we can conclude that the unresponsiveness of cortical cells after deprivation was mainly due to a deterioration of connections that had been present at birth, not to a failure to form because of lack of experience.
The second major question had to do with the cause of this deterioration. At first glance, the answer seemed almost obvious. We supposed that the deterioration came about through disuse, just as leg muscles atrophy if the knee or ankle is immobilized in a cast. The geniculate-cell shrinkage was presumably closely related to postsynaptic atrophy, the cell shrinkage seen in the lateral geniculates of adult animals or humans after an eye is removed. It turned out that these assumptions were wrong. The assumptions had seemed so selfevident that I'm not sure we ever would have thought of designing an experiment to test them. We were forced to change our minds only because we did what seemed to us at the time an unnecessary experiment, for reasons that I forget.

 

 

 

 

 

 

 



We sutured closed both eyes, first in a newborn cat and later in a newborn monkey. If the cortical unresponsiveness in the path from one eye arose from disuse, sewing up both eyes should give double the defect: we should find virtually no cells that responded to the left or to the right eye. To our great surprise, the result was anything but unresponsive cells: we found a cortex in which fully half the cells responded normally, one quarter responded abnormally, and one quarter did not respond at all. We had to conclude that you cannot predict the fate of a cortical cell when an eye is closed unless you are told whether the other eye has been closed too. Close one eye, and the cell is almost certain to lose its connections from that eye; close both, and the chances are good that the control will be preserved. Evidently we were dealing not with disuse, but with some kind of eye competition. It was as if a cell began by having two sets ofsynaptic inputs, one from each eye, and with one pathway not used, the other took over, preempting the territory of the first pathway, as shown in the drawing below.

 

 

 

 

 

 

 


Such reasoning, we thought, could hardly apply to the geniculate shrinkage because geniculate cells are monocular, with no obvious opportunities for competition. For the time being we could not explain the cell shrinkage in the layers corresponding to the closed eye. With binocular closure, the shrinkage of geniculate cells seemed less conspicuous, but it was hard to be sure because we had no normal layers to use as a standard of comparison. Our understanding of this whole problem did not move ahead until we began to use some of the new methods of experimental anatomy.

   
 




We suppose a cortical cell receives input from two sources, one from each eye, and that covering one eye has the effect of weakening the connections from that eye and strengthening the connections from the other one.

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In a newborn macaque monkey, the cortical cells seem about as sharply tuned for orientation as in adult monkeys, and the sequences are about as orderly.
The Japanese macaque monkey, Macaca fiiscata, the largest of all macaques, lives on the ground and in trees in northern Japan.
It is protected by its thick grey-brown coat.
 
 
 
 
 

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The day after its birth, a macaque monkey is looking about, fixating on objects, taking a keen interest in his environment. Humans and cats show this degree of visual maturity only many weeks after birth.