The enhancement and pruning of neural networks occurs most apparently as the baby begins to develop language. Spoken languages can sound very different from each other. In all, human languages produce about 200 different spoken sounds, called phonemes. Spoken English contains just over one-sixth of those possible sounds.
A Japanese-language keyboard suggests some of the potential complexity of learning language.
Brain scans of newborns reveal that in the first few months of life, their brain recognizes the subtle differences in phonemes other than those spoken at home. Japanese infants easily recognize the difference between the sounds made by the letters R and L. However, as the Japanese language has no sound like the letter L, adults raised speaking Japanese lose their ability to distinguish it from the letter R. Similarly, English speakers learning Spanish as adults struggle to separate the subtle sounds of the letters Band P in spoken Spanish.
But babies are able to tell such differences. That’s why it’s far easier to learn a variety of languages as a child. However, as infant brains focus on processing the auditory signals of their native languages, starting at about age 11 months they lose their ability to differentiate some nonnative phonemes. Children and adults who learn new languages after having undergone “phoneme contraction” speak with an accent.
By the time a baby is three or four months of age, its behavior provides clues to its having reached new milestones in brain development. At that age, individual infants differ widely in their reaction to events and in their patterns of brain activity as measured in EEG scans.
Rs & Ls
JAPANESE WHO BEGIN studying the English language as adults struggle with the sound of the letters Rand L. It’s not the tongue that’s to blame-it’s the brain. Newborns can distinguish all phonemes, or language sounds. Between six months and one year of age, however, children lose the ability to process previously unheard language sounds. Their loss is called phoneme contraction. Since the Japanese language slurs Rand L phonemes, adults who are exposed to the separate sounds in English for the first time cannot hear, or articulate, the difference. It’s the same for English speakers learning Japanese. They can learn the words, but it’s too late for the neuronal circuits to get the sounds exactly right.
A pattern of responses known as behavioral inhibition, which includes shyness and fear when exposed to new people and experiences, occurs in one in five healthy four month olds. Their brains show higher levels of electrical activity in the right frontal lobes. Likewise, older babies who cry upon being separated from their mother have more activity in the prefrontal cortex of their right hemisphere than do children who remain calm when mom disappears from sight.
ALBERT & THE RAT
IN A 1913 manifesto, John B. Watson introduced the term behaviorism, which, he wrote, eliminated the “dividing line between man and brute” in asserting that emotions are determined not by DNA but by external stimuli. Watson built on Ivan Pavlov’s foundation of conditioned stimulus response. Foreshadowing the 1932 publication of Aldous Huxley’s novel Brave New World, Watson theorized that “man and brute” alike can be made to order. He guaranteed, for instance, to rear any of 12 random infants to take on the occupation of his choosing. Yet Watson is remembered most, perhaps, for instilling in an infant boyan irrational fear of all things white and furry.
An 11-month-old called Little Albert plays his part in a famous behaviorist experiment.
In 1919, Watson began to work with 11-month-old Little Albert, conditioning him to fear a white rat. To begin with, Albert liked his pet, trying to touch and even hold it. Watson believed this reflected a curiosity innate in all children. Later, a new stimulus was introduced: When Albert reached for the rat, Watson banged a metal bar with a carpenter’s hammer. Albert fell face-forward on the mattress, whimpering. The rat was shown repeatedly, with gong and without, until Little Albert’s congenital fear of loud noises was transferred to the rat. This phobia, Watson later learned, applied also to white rabbits, dogs, a fur coat, and even a Santa Claus mask. Presumably, Watson wrote, Albert could eventually become unconditioned, but the boy was adopted before further experiments could be performed.
As a baby emerges from the womb, brain development expands to include processing responses to the baby’s new experiences sights, sounds, smells, actions, sensations, and emotions. Networks of neurons, primed to receive new stimuli,compete for survival. It’s a random battle at first, but soon becomes more organized as environmental stimuli strengthen some connections while others wither. If the baby is exposed to a broad vocabulary and a wide range of music, the connections for language and sound recognition grow stronger. If the baby is kept in an environment lacking in toys and visual stimulation, the baby’s analytical powers may be slow to develop.
ESTABLISHING NETWORKS
Defects in infants’ eyes illustrate the sensitivity of a newborn’s brain and the competing neural networks. When a child is born with a cataract in one eye, that eye is deprived of normal vision, and the portion of the brain that processes information from that eye suffers lack of stimulation. The baby’s one normally functioning eye begins to process all visual information.
NEWBORN SIGHT
WE CAN’T KNOW for certain what the world looks like to a newborn; babies don’t answer interviewers’ questions. However, scientists who study the makeup of new-borns’ eyes and test for whether babies will gaze at objects believe that for the first months of life, children lack the ability to see fine lines and a full spectrum of colors. The world probably looks like a blurred, faded photograph as seen through a card-board tube.
New-borns appear to be hardwired for looking at faces. Shortly after birth, infants will look at faces longer than they will look at any other object.
The “use it or lose it” principle starts to work-with a vengeance. Neural connections develop for the good eye but fail to do so for the eye with the cataract. Unless the cataract is removed shortly after birth, the child will remain blind in that eye. Even if the cataract is removed later, the brain has lost its one chance to develop the neural circuitry to process visual signals from the eye; the eyeball may appear healthy, but it cannot communicate with the brain.
If surgery removes the cataract in time, the strong, already existing neural connections of the stronger eye give it a favored place in brain development. In order to make both eyes work with the same acuity, doctors often patch the stronger eye for a few hours every day. That way, for extended periods, all of the neural development for vision is processed via the weaker eye. Its brain circuitry grows stronger by not having to compete all the time with the good eye.
The process of establishing and strengthening connections in the brain to process vision underscores the fact that certain periods are absolutely critical to proper functional development. While the brain retains a measure of plasticity among existing networks, it also seldom offers a second chance for establishing those networks at an early age. In other words, the brain cannot expand and reconnect a neural network that doesn’t exist or one that exists, like a dead-end road, without functional traffic.
The first, and easiest, thing a mother to be can do is to eat for two: This doesn’t mean doubling up on servings it means remembering that the vitamins and minerals from a well-balanced diet not only nourish mom’s brain and body but the brain and body of her developing baby. Pregnant women need proper amounts of folic acid, vitamin B12 (crucial to the functioning of the central nervous system), fatty acids, iron, and other nutrients. She should consult her obstetrician about taking prenatal vitamins, which contain many of these substances and fill in any nutritional gaps in her diet.
Getting plenty of exercise is important to both the mother and her developing baby.
Good nutrition is vital for healthy brain development. Lack of nutrients at crucial moments in fetal brain development leads to a drop or even a halt in the creation of neurons. Babies born after suffering malnutrition often display a smaller brain and have cognitive disabilities. Lack of folic acid (found abundantly in bread, beans, pasta, spinach, and orange juice) raises the chances of a child being born with spina bifida. On the other hand, too much of a good thing can be bad. Overabundance of certain vitamins, including A and D, can cause toxic reactions in the fetal brain. The best advice for a mother to be is to consult her doctor about the best diet for her, one with lots of fresh fruits, leafY green vegetables, legumes, whole grains, and lean meats.
AVOID ALCOHOL
To decrease the chances of neurological defects, moms to be should also avoid many substances that can harm an unborn child’s brain, such as alcohol. In 1899, William Sullivan, a doctor who studied babies born in an English women’s prison, discovered much higher rates of still-births among mothers who drank heavily. He suspected a link between alcohol and fetal health when he noted that mothers who gave birth to babies with severe birth defects in the outside world had healthy babies in prison, where they were denied alcohol.
It would take more than seven decades before researchers at the University of Washington cataloged the recurring patterns of birth defects as fetal alcohol syndrome. When pregnant women drink heavily, their children are at high risk of having a malformed heart and limbs, a smaller brain, reading and math disabilities, hyperactivity, depression, and distinctive facial abnormalities. Mental retardation also is possible. Unfortunately, alcohol’s most devastating impact on a developing fetus occurs early in the pregnancy, when the mother may not even know she is carrying a child. And small amounts in the first trimester cause more damage than greater alcohol consumption later on, apparently because of alcohol’s impact on the migration of developing neurons In the fetal brain. Normally, neurons stop their travels when they reach their intended destinations. The presence of alcohol makes them overshoot and die.
JUST SAY NO
Other substances harmful to adults are even more so to a developing fetus, whose brain is especially sensitive to its chemical environment. Tobacco, illegal drugs such as cocaine, and environmental toxins, all of which do some level of harm to an adult’s body, deliver hammer blows to a developing fetus and can even cause harmful impacts on sperm cells, so men should consider their levels of exposure before trying to start a family. Sperm live for about three months. To minimize the chances of their sperm being adversely affected by alcohol, tobacco, drugs, and toxins, fathers to be should avoid exposure to such harmful substances for 90 days.
Drugs taken by pregnant women can cause abnormalities in the developing fetus.
For pregnant women, tobacco smoke is the most common environmental hazard to a fetus. Nicotine in tobacco causes blood vessels to constrict; an affected fetus gets less blood, and its heart rate decreases. Furthermore, nicotine becomes more concentrated in the fetus’s body than in that of the mother. Like alcohol, nicotine is believed to interfere with neuronal migration, connection, and development. Spontaneous abortion rates nearly double for mothers who smoke. Babies carried to term are more likely to be mentally retarded and have congenital abnormalities.
SEEK OUT HIDDEN RISKS
Toxins harmful to a fetus range from obvious hazards such as the poisons in pesticides to common and seemingly harmless substances such as vitamin A, which in high concentrations (such as in acne medication) harms a fetus’s brain. Lead particles, many over the counter and prescription medicines, x-rays, and some cancer drugs also poison a developing brain.
The jury is out on the possible impact of antidepressants. A pregnant woman’s use of Prozac, a common prescription only treatment for depression, so far has been shown to have no impact on her child’s
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