Brain development begins with the formation and closure of the neural tube, the earliest nervous tissue that looks like a fat earthworm stretched out along the entire back of the embryo. The neural tube forms from the neural plate, which begins forming just sixteen days after conception. This plate lengthens and starts folding up, forming a groove at around eighteen days, which then begins fusing shut into a tube around twenty-two days post-conception. By 27 days, the tube is fully closed and has already begun its transformation into the brain and spinal cord of the embryo.
One of the most sensitive periods in brain development occurs at the very beginning, when the neural tube is closing. If, during this fourth week after conception, the tube fails to seal in the head end of the embyro, a defect known as anencephaly results. Anencephaly means, “lack of a cerebral cortex,” and is always fatal. If the tube fails to seal at its lower end, the defect is known as spina bifida. In spina bifida, part of the spinal cord may develop outside the spine, where it is highly vulnerable to damage. Spina bifida varies in severity from being totally symptomless to highly disabling, with problems including paralysis, sensory loss, and loss of bladder or bowel function.
Fortunately, NTDs can now be detected prenatally with good accuracy. Even better news comes from the recent discovery that the B vitamin, folic acid, can prevent some 60 percent of NTDs from ever developing in the first place. To be most effective, women should take a 400 microgram folic acid supplement every day, beginning about one month before conception and continuing until at least the end of the first trimester of pregnancy. (This is the amount of folic acid present in most multivitamins sold over the counter. Supplements of up to 1000 micrograms–equivalent to 1 milligram–per day are considered safe during pregnancy.) Most grains, breads and cereals sold in the U.S. are now fortified with folic acid in quantities estimated to raise the average woman’s consumption by 100 micrograms per day.
Generally speaking, the central nervous system (which is composed of the brain and the spinal cord) matures in a sequence from “tail” to head. In just the fifth week after conception, the first synapses begin forming in a fetus’s spinal cord. By the sixth week, these early neural connections permit the first fetal movements–spontaneous arches and curls of the whole body–that researchers can detect through ultrasound imaging. Many other movements soon follow–of the limbs (around eight weeks) and fingers (ten weeks), as well as some surprisingly coordinated actions (hiccuping, stretching, yawning, sucking, swallowing, grasping, and thumb-sucking). By the end of the first trimester, a fetus’s movement repertoire is remarkably rich, even though most pregnant women can feel none of it. (Most women sense the first fetal movements around eighteen weeks of pregnancy.)
The second trimester marks the onset of other critical reflexes: continuous breathing movements (that is, rhythmic contractions of the diaphragm and chest muscles) and coordinated sucking and swallowing reflexes. These abilities are controlled by the brainstem, which sits above the spinal cord but below the higher, more recently-evolved cerebral cortex. The brainstem is responsible for many of our body’s most vital functions–heart rate, breathing, and blood pressure. It is largely mature by the end of the second trimester, which is when babies first become able to survive outside the womb.
Last of all to mature is the cerebral cortex, which is responsible for most of what we think of as mental life–conscious experience, voluntary actions, thinking, remembering, and feeling. It has only begun to function around the time gestation comes to an end. Premature babies show very basic electrical activity in the primary sensory regions of the cerebral cortex–those areas that perceive touch, vision, and hearing–as well as in primary motor regions of the cerebral cortex. In the last trimester, fetuses are capable of simple forms of learning, like habituating (decreasing their startle response) to a repeated auditory stimulus, such as a loud clap just outside the mother’s abdomen. Late-term fetuses also seem to learn about the sensory qualities of the womb, since several studies have shown that newborn babies respond to familiar odors (such as their own amniotic fluid) and sounds (such as a maternal heartbeat or their own mother’s voice). In spite of these rather sophisticated abilities, babies enter the world with a still-primitive cerebral cortex, and it is the gradual maturation of this complex part of the brain that explains much of their emotional and cognitive maturation in the first few years of life.
Many factors can influence fetal brain development, but most healthy pregnant women do not need to radically alter their lifestyles in order to promote optimal brain development. Good nutrition is important, since brain growth–like the growth of the rest of the fetus’ body–is influenced by the quality of a pregnant woman’s diet. Alcohol and cigarettes should be avoided, since these can impair the formation and wiring of brain cells. Some chemicals and forms of radiation are potentially harmful to fetal brain development, but most need concern only women exposed through their occupations–that is, those who work on farms or in factories, laboratories, hospitals, dry-cleaning stores, or other sites that expose them to dangerous chemicals, radiation, or infections.
Infections pose perhaps the greatest risk to the developing fetus’ brain. Many seemingly harmless infections can seriously interrupt fetal development, including the formation and wiring of brain cells. Fortunately, most women are already immune to the most dangerous of these–rubella (which causes German measles) and varicella virus (cause of chicken pox). Other potentially harmful infections include cytomegalovirus (CMV), toxoplasmosis, and several sexually transmitted diseases (syphilis, gonorrhea, and genital herpes). Prenatal testing and treatment can minimize the risk of some of these, but generally speaking, pregnant women can best protect their babies’ brains by practicing strict hygiene: wash your hands frequently, avoid sick friends and co-workers, watch out for sloppy kisses, and don’t share food or drinks with anyone–even your own toddlers!
Although it has already undergone an amazing amount of development, the brain of a newborn baby is still very much a work-in-progress. It is small–little more than one-quarter of its adult size–and strikingly uneven in its maturity. By birth, only the lower portions of the nervous system (the spinal cord and brain stem) are very well developed, whereas the higher regions (the limbic system and cerebral cortex) are still rather primitive.
The lower brain is therefore largely in control of a newborn’s behavior: all of that kicking, grasping, crying, sleeping, rooting, and feeding are functions of the brain stem and spinal cord. Even the striking visual behavior of newborns–their ability to track a bold moving object, like a red ball of string, or to orient to Mom or Dad’s face—is thought to be controlled by visual circuits in the brain stem. When pediatricians conduct a series of reflex tests on the newborn, they are primarily assessing the function of these lower neural centers. These reflexes include the doll’s eye maneuver (the baby’s eyes stay focused forward when his head is turned to one side), the “Moro” or startle response (baby splays out arms and then slowly closes them in response to a sudden movement or feeling of falling), and even the remarkable stepping reflex (the baby “walks” when you hold him up with feet touching a flat surface).
The human brain takes time to develop, so nature has insured that the neural circuits responsible for the most vital bodily functions—breathing, heartbeat, circulation, sleeping, sucking, and swallowing—are up and running by the time a baby emerges from the protective womb. The rest of brain development can follow at a more leisurely pace, maximizing the opportunity for a baby’s experience and environment to shape his emerging mind.
Parents are another important part of the developmental equation. Infants prefer human stimuli–your face, voice, touch, and even smell–over everything else. They innately orient to people’s faces and would rather listen to a speech or singing than any other kind of sound.
Just as newborn babies are born with a set of very useful instincts for surviving and orienting to their new environment, parents are equally programmed to love and respond to our babies’ cues. Most adults (and children) find infants irresistible, and instinctively want to nurture and protect them. It is certainly no accident that the affection most parents feel towards their babies and the kind of attention we most want to shower them with—touching, holding, comforting, rocking, singing and talking to—provide precisely the best kind of stimulation for their growing brains. Because brain development is so heavily dependent on early experience, most babies will receive the right kind of nurturing from their earliest days, through our loving urges and parenting instincts.
In spite of all the recent hype about “making your baby smarter,” scientists have not discovered any special tricks for enhancing the natural wiring phase in children’s brain development. Normal, loving, responsive caregiving seems to provide babies with the ideal environment for encouraging their own exploration, which is always the best route to learning.
The one form of stimulation that has been proven to make a difference is language: infants and children who are conversed with, read to, and otherwise engaged in lots of verbal interaction show somewhat more advanced linguistic skills than children who are not as verbally engaged by their caregivers. Because language is fundamental to most of the rest of cognitive development, this simple action—talking and listening to your child—is one of the best ways to make the most of his or her critical brain-building years.
While babies come into the world with some very useful survival reflexes, they are still strikingly helpless, in large part because the cerebral cortex is still quite immature. As the highest, most recently evolved part of the brain, the cerebral cortex is responsible for all of our conscious thoughts, feelings, memories, and voluntary actions.
Although all of the neurons in the cortex are produced before birth, they are poorly connected. In contrast to the brain stem and spinal cord, the cerebral cortex produces most of its synaptic connections after birth, in a massive burst of synapse formation known as the exuberant period. At its peak, the cerebral cortex creates an astonishing two million new synapses every second. With these new connections come a baby’s many mental milestones, such as color vision, a pincer grasp, or a strong attachment to his parents.
By two years of age, a toddler’s cerebral cortex contains well over a hundred trillion synapses. This period of synaptic exuberance varies in different parts of the cerebral cortex: it begins earlier in primary sensory regions, like the visual cortex or primary touch area of the cortex, while it takes off somewhat later in the temporal and frontal lobes, brain areas involved in higher cognitive and emotional functions. Nonetheless, the number of synapses remains at this peak, over-abundant level in all areas of the cerebral cortex throughout middle childhood (4-8 years of age). Beginning in the middle elementary school years and continuing until the end of adolescence, the number of synapses then gradually declines down to adult levels.
This pattern of synaptic production and pruning corresponds remarkably well to children’s overall brain activity during development. Using PET imaging technology, neuroscientists have found dramatic changes in the level of energy use by children’s brains over the first several years of life—from very low at birth, to a rapid rise and over-shoot between infancy and the early elementary school years, followed by a gradual decline to adult levels between middle childhood and the end of adolescence. In other words, children’s brains are working very hard, especially during the period of synaptic exuberance that corresponds to the various critical periods in their mental development (see above).
Besides synapse formation and pruning, the other most significant event in postnatal brain development is myelination. Newborns’ brains contain very little myelin, the dense impermeable substance that covers the length of mature brain cells and is necessary for clear, efficient electrical transmission. This lack of myelin is the main reason why babies and young children process information so much more slowly than adults—why it might take a toddler a minute or more to begin responding to a request such as “Joey, bring Mommy the teddy bear.” Myelination of the cerebral cortex begins in the primary motor and sensory areas—regions that receive the first input from the eyes, ears, nose, skin, and mouth—and then progresses to “higher-order,” or association regions that control the more complex integration of perception, thoughts, memories, and feelings. Myelination is a very extended process: although most areas of the brain begin adding this critical insulation within the first two years of life, some of the more complex areas in the frontal and temporal lobes continue the process throughout childhood and perhaps well into a person’s 20s. Unlike synaptic pruning, myelination appears to be largely “hard-wired.” Its sequence is very predictable in all healthy children, and the only environmental factor known to influence it is severe malnutrition.
Yes, but they are subtle, and a product of both nature and nurture.
Neuroscientists have known for many years that the brains of men and women are not identical. Men’s brains tend to be more lateralized—that is, the two hemispheres operate more independently during specific mental tasks like speaking or navigating around one’s environment. For the same kinds of tasks, females tend to use both their cerebral hemispheres more equally. Another difference is size: males of all ages tend to have slightly larger brains, on average, than females, even after correcting for differences in body size.
Electrical measurements reveal differences in boys’ and girls’ brain function from the moment of birth. By three months of age, boys’ and girls’ brains respond differently to the sound of human speech. Because they appear so early in life, such differences are presumably a product of sex-related genes or hormones. We do know that testosterone levels rise in male fetuses as early as seven weeks of gestation, and that testosterone affects the growth and survival of neurons in many parts of the brain. Female sex hormones may also play a role in shaping brain development, but their function is currently not well understood.
Sex differences in the brain are reflected in the somewhat different developmental timetables of girls and boys. By most measures of sensory and cognitive development, girls are slightly more advanced: vision, hearing, memory, smell, and touch are all more acute in female than male infants. Girl babies also tend to be somewhat more socially-attuned—responding more readily to human voices or faces, or crying more vigorously in response to another infant’s cry—and they generally lead boys in the emergence of fine motor and language skills.
Boys eventually catch up in many of these areas. By age three, they tend to out-perform girls in one cognitive area: visual-spatial integration, which is involved in navigation, assembling jigsaw puzzles, and certain types of hand-eye coordination. Males of all ages tend to perform better than females on tasks like mental rotation (imagining how a particular object would look if it were turned ninety degrees) while females of all ages tend to perform better than males at certain verbal tasks and at identifying emotional expression in another person’s face. (It is important to emphasize that these findings describe only the average differences between boys and girls. In fact, the range of abilities within either gender is much greater than the difference between the “average girl” and the “average boy.” In other words, there are plenty of boys with excellent verbal skills, and girls with excellent visual-spatial ability. While it can be helpful for parents and teachers to understand the different tendencies of the two sexes, we should not expect all children to conform to these norms.)
Genes and hormones set the ball rolling, but they do not fully account for sex differences in children’s brains. Experience also plays a fundamental role. Consider, for example, the “typical” boy, with his more advanced spatial skills; he may well prefer activities like climbing or pushing trucks around—all of which further hone his visual-spatial skills. The “typical” girl, by contrast, may gravitate more toward games with dolls and siblings, which further reinforce her verbal and social skills. It is not hard to see how initial strengths are magnified—thanks to the remarkable plasticity of young children’s brains—into significant differences, even before boys and girls begin preschool.
But this remarkable plasticity also provides parents and other caregivers with a wonderful opportunity to compensate for the different tendencies of boys and girls. For example, it is known that greater verbal interaction can improve young children’s language skills. So the “typical boy” may especially benefit from a caregiver who engages him in lots of conversation and word play. On the other hand, the “typical girl” may benefit more from a caregiver who engages her in a jigsaw puzzle or building a block tower—activities that encourage her visual-spatial integration. The point is not to discourage children from sex-typical play (since pushing trucks or playing with dolls are great activities for any young child), but to supplement those activities with experiences that encourage the development of many competences.