How to Build a Baby's Brain
YOU CANNOT SEE WHAT IS GOING ON INSIDE YOUR newborn's brain. You cannot see the electrical activity as her eyes lock onto yours and, almost instantaneously, a neuron in her retina makes a connection to one in her brain's visual cortex that will last all her life. The image of your face has become an enduring memory in her mind. And you cannot see the explosive release of a neurotransmitter--brain chemical--as a neuron from your baby's ear, carrying the electrically encoded sound of "ma," connects to a neuron in her auditory cortex. "Ma" has now commandeered a cluster of cells in the infant's brain that will, as long as the child lives, respond to no other sound.
You cannot see any of this. But Dr. Harry Chugani can come close. With positron-emission tomography (PET), Chugani, a pediatric neurobiologist at Wayne State University in Detroit, watches the regions of a baby's brain turn on, one after another, like city neighborhoods having their electricity restored after a blackout. He can measure activity in the primitive brain stem and sensory cortex from the moment the baby is born. He can observe the visual cortex burn with activity in the second and third months of life. He can see the frontal cortex light up at 0 to 8 months. He can see, in other words, that the brain of a baby is still forming long after the child has left the womb-not merely growing bigger, as toes and livers and arms do, but forming the microscopic connections responsible for feeling, learning and remembering. For doing, in short, everything that a brain is born to do but that it is born without knowing how to do.
Scientists are just now realizing how experiences after birth, rather than something innate, determine the actual wiring of the human brain. "Only 15 years ago," reports the Families and Work Institute in the just-re-leased study "Rethinking the Brain," "neuroscientists assumed that by the time babies are born, the structure of their brains [had been] genetically determined." But by last year researchers knew that was wrong. Instead, early-childhood experiences exert a dramatic and precise impact, physically determining how the intricate neural circuits of the brain are wired (NEWSWEEK, Feb. 19, 1996). Since then they have been learning how those experiences shape the brain's circuits.
At birth, the brain's 100 billion or so neurons form more than 50 trillion connections (synapses). The genes the baby carries-from the egg and sperm that made him-have already determined his brain's basic wiring. They have formed the connections in the brain stem that will make the heart beat and the lungs respire. But that's all. Of a human's 80,000 different genes, fully half are believed to be involved in forming and running the central nervous system. Yet even that doesn't come close to what the brain needs. In the first months of life, the number of synapses will increase 20-fold-to more than 1,000 trillion. There simply are not enough genes in the human species to specify so many connections.
That leaves experience--all the signals that a baby receives from the world. Experience seems to exert its effects by strengthening synapses. Just as a memory will fade if it is not accessed from time to time, so synapses that are not used will also wither away in a process called pruning. The way to reinforce these wispy connections has come to be known as stimulation. Contrary to the claims of entrepreneurs preying on the anxieties of new parents, stimulation does not mean subjecting a toddler to flashcards (page 34). Rather, it is something much simpler-sorting socks by color or listening to the soothing cadences of a fairy tale. In the most extensive study yet of what makes a difference, Craig Ramey of the University of Alabama found that it was blocks, beads, peekaboo and other old-fashioned measures that enhance cognitive, motor and language development-and, absent traumas, enhance them permanently.
The formation of synapses (synaptogenesis) and their pruning occurs at different times in different parts of the brain. The sequence seems to coincide with the emergence of various skills. Synaptogenesis begins in the motor cortex at about 2 months. Around then, infants lose their "startle" and "rooting" reflexes and begin to master purposeful movements. At 3 months, synapse formation in the visual cortex peaks; the brain is fine-tuning connections allowing the eyes to focus on an object. At 8 or 9 months the hippocampus, which indexes and files memories, becomes fully functional; only now can babies form explicit memories of, say, how to move a mobile. In the second half of the first year, finds Chugani, the prefrontal cortex, the seat of forethought and logic, forms synapses at such a rate that it consumes twice as much energy as an adult brain. That furious pace continues for the child's first decade of life.
Research on language has shown how "neuroplastic" an infant's brain is, and how that plasticity lessens with age. Patricia Kuhl of the University of Washington studies the "auditory maps" that infants' brains construct out of phonemes (the smallest units of sound in a language, such as "ee" or 'T'). At first, neurons in the auditory cortex are like laborers to whom jobs have not yet been assigned. But as a newborn hears, say, the patter of English, a different cluster of neurons in the auditory cortex is recruited to respond to each phoneme. Each cluster then fires only when a nerve from the ear carries that particular sound, such as "pa" or "ma." If one sound is clearly distinct from another, as "ra" and "la" are in English, then the neurons whose job it is to hear one will lie far from those whose job it is to hear the other. (Kuhl makes noninvasive electrical measurements, through the babies' scalps, to identify which neurons fire in response to a particular sound.) But if the sounds are nearly identical, as "ra" and "la" are in Japanese, then the two sets of neurons are so close that the baby will have trouble distinguishing the two phonemes. By 12 months, an infant's auditory map is formed. He will be unable to pick out phonemes he has not heard thousands of times for the simple reason that no cluster of neurons has been assigned the job of responding to that sound. And the older he gets, the more he will struggle to learn a new language: fewer unassigned neurons are available for the job of hearing new phonemes.
Experience counts in building vocabulary, too, and at a very young age. The size of a toddler's vocabulary is strongly correlated with how much a mother talks to the child, reports Janellen Huttenlocher of the University of Chicago. At 20 months, children of chatty mothers averaged 131 more words than children of less talkative mothers; at 2 years, the gap had more than doubled, to 295 words. "The critical factor is the number of times the child hears different words," says Huttenlocher. The effect holds for the complexity of sentence structure, too, she finds. Mothers who used complex sentences (those with dependent clauses, such as "when..." or "because...") 40 percent of the time had toddlers who did so 35 percent of the time; mothers who used such sentences in only 10 percent of their utterances had children who did so only 5 percent of the time.
ONLY "LIVE" LANGUAGE, not television, produced these vocabulary- and syntax-boosting effects. Why doesn't all the gabbing on TV stimulate language development? Huttenlocher suspects that "language has to be used in relation to ongoing events, or it's just noise." That may hold for other sorts of cognition, too. Information embedded in an emotional context seems to stimulate neural circuitry more powerfully than information alone. A child will more readily learn the concept of "more" if it refers to the happy prospect of more cookies, and "later" if it is attached to a frustrating wait for a trip to the playground, than if the word is presented in isolation from things the baby cares about. There is nothing mysterious about this: adults form a memory much more readily if it has emotional content (how did you hear that the space shuttle had exploded?) than if it doesn't (what's the difference between a sine and a cosine?). Causality, a key component of logic, is also best learned through emotion: if I smile, Mommy smiles back. A sense that one thing causes another forms synapses that will eventually support more abstruse concepts of causality. Feelings, concepts and language begin to be linked in this way in the months from 7 through 12.
Another route to brain wiring seems to be tapping into its natural harmonies. In the last year, new studies have nailed down how music affects spatial-temporal reasoning--the ability to see a disassembled picture of, say, a rabbit and mentally piece it back together. Such reasoning underlies math, engineering and chess. In a study published in February in the journal Neurological Research, scientists report how spatial-temporal reasoning in 3- and 4-year-olds was affected by weekly piano lessons. After six months, the budding Horowitzes--all of whom scored at the national average on tests of spatial recognition-scored 34 percent above average on this reasoning skill. None of the other children (who had received computer keyboard and mouse lessons, singing lessons or nothing at all) had improved. What explains the effect? Physicist Gordon Shaw of the University of California, Irvine, suspects that in playing the piano, "you are seeing how patterns work in space and time." When sequential finger and key patterns make melodies, neural circuits that connect positions (keys) to sounds in space and time (the melody) are strengthened. "Music training produces long-term modifications in neural circuitry," says Shaw. What scientists do not know is whether the effects of early music training endure-whether the preschoolers will be math wizards in high school.
The downside of the brain's great plasticity is that it is acutely vulnerable to trauma. "Experience may alter the behavior of an adult," says Dr. Bruce Perry of Baylor College of Medicine, but it "literally provides the organizing framework" for the brain of a child. If the brain's organization reflects its experience, and the experience of the traumatized child is fear and stress, then the neurochemical responses to fear and stress become the most powerful architects of the brain. "If you have experiences that are overwhelming, and have them again and again, it changes the structure of the brain," says Dr. Linda Mayes of the Yale Child Study Center. Here's how:
Trauma elevates stress hormones, such as cortisol, that wash over the tender brain like acid. As a result, regions in the cortex and in the limbic system (responsible for emotions, including attachment) are 20 to 30 percent smaller in abused children than in normal kids, finds Perry; these regions also have fewer synapses.
In adults who were abused as children, the memory-making hippocampus is smaller than in nonabused adults. This effect, too, is believed to be the result of the toxic effects of cortisol.
High cortisol levels during the vulnerable years of zero to 3 increase activity in the brain structure involved in vigilance and arousal. (It's called the locus ceruleus.) As a result the brain is wired to be on hair-trigger alert, explains Perry: regions that were activated by the original trauma are immediately reactivated whenever the child dreams of, thinks about or is reminded of the trauma (as by the mere presence of the abusive person). The slightest stress, the most inchoate fear, unleashes a new surge of stress hormones. This causes hyperactivity, anxiety and impulsive behavior. "The kids with the higher cortisol levels score lowest on inhibitory control," says neuroscientist Megan Gunnar of the University of Minnesota. "Kids from high-stress environments [have] problems in attention regulation and self-control."
Trauma also scrambles neurotransmitter signals, ratcheting up some and depressing others. Since neurotransmitters play key roles in telling growing neurons where to go and what to connect to, children exposed to chronic and unpredictable stress--a mother's boyfriend who lashes out in fury, an alcoholic uncle who is kind one day and abusive the next--will suffer deficits in their ability to learn. "Some percentage of capacity is lost," says Perry. "A piece of the child is lost forever."
That is tragedy enough, of course, but it is made even greater by the loss of what could have been. Babies are born into this world with their brain primed to learn. But they cannot do it alone.
53% of all parents say that they read to their child every day; 55% of parents say they sing to or play music for their child every day.