Brain Rules for Baby (5 page)

The brain does not develop in isolation, of course. The early embryo temporarily displays gill arches around the fourth week, much like the ones fish have. These soon convert into face muscles and the throat structures that will allow your baby to speak. Your embryo next gets the stub of a tail but soon reverses course and resorbs the structure. There are strong evolutionary roots to our development, and we share this miracle with every other mammal on the planet. Except for one thing.

Those bulges at the end of your embryo’s neural tube will turn into a great big, fat, super-smart brain—about the heaviest brain-per-body mass that exists on the planet. This massive structure is composed of a delicate spider web of cells, crackling with tiny bolts of electricity. Two types of cells are important here. The first type, glial cells, make up 90 percent of the brain cells inside your child’s head. They give the brain its structure and help the neurons correctly process information. It’s a good name; glial is a Greek word for glue. The second type of cell is the familiar neuron. Though they do a lot of your child’s thinking, neurons make up only about 10 percent of the total number of brain cells. That’s probably where we get the myth that you use only 10 percent of your brain.

 

So how do cells turn into brains? Embryonic cells are manufactured into neurons in a process called neurogenesis. This is when the baby would like to be left alone, in the first half of pregnancy. Then, in the second half of pregnancy, the neurons migrate to the region
they eventually will call home and start wiring together. This is called synaptogenesis.

Cell migration reminds me of when tracking bloodhounds are suddenly loosed from the sheriff’s truck to pick up the scent of a criminal. Neurons bolt out of their ectodermal cages, crawling over one another, sniffing out molecular cues, pausing, trying out different pathways, slithering helter-skelter throughout the developing brain. Eventually they stop, having arrived at a destination that may be pre-programmed into their cellular heads. They look around their new cellular digs and try to hook up with the neighbors. When they do, tiny, lively gaps between neural cells are created, called synapses (hence the term synaptogenesis). Electrical signals jump between the naked spaces to allow neural communication. This final step is the real business of brain development.

Synaptogenesis is a prolonged process, for an easily understood reason: It is ridiculously complex. A single neuron has to make an average of 15,000 connections with the locals before its wiring job is over. Some neurons have to make more than 100,000 connections. That means your baby’s brain has to lash together an astonishing 1.8 million new connections per second to make a complete brain. Many of the neurons never complete the process. Like post-sex salmon, they simply die off.

Even given this incredible speed, baby brains never make the birth deadline. About 83 percent of synaptogenesis continues
after
birth. Surprisingly, your baby girl’s brain will not completely finish its wiring until she is in her early 20s. Boys brains may take even longer. In humans, the brain is the last organ to finish developing.

The purpose of that furiously fast (then frustratingly slow) production is to build a functioning brain, one that can receive and respond to inputs. So the questions for prying parents become: What do fetuses
know, and when do they know it? When is your baby capable of sensing, say, taps on your belly?

The developmental principle to remember is this: The brain spends the first half of pregnancy setting up its neuroanatomical shop,
blissfully ignoring most parental involvement.
(I am referring to well-intentioned interference. Drugs, including alcohol and nicotine, clearly can damage a baby’s brain during pregnancy.) The second half of pregnancy is a different story. As brain development moves from mostly neurogenesis to mostly synaptogenesis, the fetus begins to exhibit much greater sensitivity to the outer world. The wiring of cells is much more subject to outside influences—including you—than the act of creating them in the first place.

What is baby’s approach to constructing the brain’s sensory systems? Ask paratroop commanders. They will tell you that successfully fighting a war involves three steps: parachuting into enemy territory, securing hostile real estate, and communicating back to home base. This process gives central command both knowledge about progress and “situational awareness” of what to do next. Something similar happens to sensory systems in the brain as they develop in utero.

Like parachutists securing enemy territory, neurons invade a given region of the brain and establish various sensory bases. Neurons that hook up to the eyes will eventually be used for vision, ears for hearing, nose for smelling. Once their areas are secured, these cells will establish linkages that help them reach out to the perceptual command-and-control structures also growing up within the brain. (In the real world of the brain, there are many central commands.) These CEOLIKE structures, which give us perceptual abilities, are busily capturing territory just like the paratroopers. And they are some of the last areas in utero to wire up properly. This means neurons hooked up to the eyes or ears or nose might receive a busy signal when they try to
report back to their home base. Because of this odd timing, parts of a baby’s brain can respond to sensory stimulation before a baby can actually perceive being stimulated.

But once babies can perceive inputs like sounds and smells, starting around the second half of the pregnancy, they become precisely attuned to them. And they subconsciously remember. Sometimes it’s spooky, as legendary conductor Boris Brott discovered one day.

“It just jumped out at me!” Brott exclaimed to his mother. Brott had been at the podium of a symphony orchestra, conducting a piece of music for the first time, when the cellist began to play. He instantly
knew
he’d heard this piece before. This was no casual reminder of some similar but forgotten work: Brott could predict exactly what musical phrase was coming next. He could anticipate the flow of the entire work during the course of the rehearsal; he knew how to conduct it even when he lost his place in the score.

Freaking out, he called his mother, a professional cellist. She asked for the name of the piece of music, then burst out laughing.
It was the piece she had been rehearsing when she was pregnant with him.
The cello was up against her late-pregnancy mid-abdomen, a structure filled with sound-conducting fluids, fully capable of relaying musical information to her unborn son. His developing brain was sensitive enough to record the musical memories. “All the scores I knew by sight were the ones she had played while she was pregnant with me”, Brott later said in an interview. Incredible stuff for an organ not even zero years old.

This is but one of many examples of how babies in the womb can pick up information from the outer world. As we’ll see, what you eat and smell can influence your infant’s perceptions, too. For a newborn, these things are the familiar comforts of home.

Here’s when your baby’s senses—touch, sight, hearing, smell, balance, taste—start to function as you transit through pregnancy:

One of the earliest senses to come on line is touch. Embryos about 1 month old can sense touch to their noses and lips. The ability spreads quickly; nearly the entire surface of the skin is sensitive to touch by 12 weeks of age.

I swear I could detect this by the time my wife was in the middle of her third trimester with our youngest son. He was quite a mover, and at times I could see what looked like a bulging shark’s fin move across my wife’s belly, swelling, then submerging. Creepy. And cool. Thinking it might be the little guy’s foot, I tried touching the bulge when it appeared one morning. The bulge immediately “kicked” back (!), causing us both to yelp with excitement.

If you try this in the first half of pregnancy, you won’t get any results. Not until about the fifth month after conception can babies truly experience touch in the way you and I might perceive it. That’s when your baby’s brain develops“body maps”—tiny neurological representations of his entire body.

By the beginning of the third trimester, a fetus readily displays avoidance behaviors (trying to swim away, for example, when a needle comes near for biopsy). From this we conclude that babies can feel pain, though it is impossible to measure this directly.

The fetus appears to possess sensitivity to temperature by this time, too. But it’s possible that the wiring diagrams for temperature sensation aren’t fully completed at birth and that they require experience with the outside world to fully develop. In two unrelated child-abuse cases, a French boy and an American girl were kept in isolation for years. Both children had an eerie inability to distinguish between hot and cold. The little girl never dressed appropriately for the weather, even when it was freezing outdoors. The little boy regularly pulled potatoes out of a roaring fire with his bare hands, oblivious to the temperature difference. We don’t know exactly why. We do know that touch remains very important for a baby’s development after birth.

Can babies see in the womb? That’s a tough question to answer, mostly because vision is our most complex sense.

Vision begins developing about four weeks after conception, the fetus forming little eye-dots on either side of her tiny head. Cup-shaped structures within these dots soon emerge, which will form, in part, the lens of the eye. Retinal nerves then snake out from behind these primitive eyes, trying to reach the back of the head and connect to regions that will eventually form the visual cortex. The cells in this cortex have themselves been busy, getting ready to greet these neural travelers and form partnerships. The second and third trimesters are filled with massive neural meet-and-greets in these regions, a fair bit of cell death, and lots of chattering connectivity. At this point, the brain is forming about 10 billion new synapses per day. You’d think a baby would get a migraine!

One result of all this activity is that the neural circuitry necessary to control blinking, dilation of pupils, or tracking moving objects is present before birth. Experiments show that infants just entering the third trimester will move or alter their heart rate, or both, in response to a strong light beamed at the womb. But it takes so long to build adequately functioning circuits that the baby needs more than nine months to finish the job. The brain will continue forming 10 billion synapses a day for almost a year after birth. During that interval, the brain uses external visual experiences to help it finish its internal construction projects.

If you were to tell me that an important scientific fact was going to be discovered using a combination of mouth sucking and reading
The Cat in the Hat,
I would have suggested you change your brand of beer. But in the early 1980s, that’s exactly what happened. During the final six weeks of pregnancy, women in a study were asked to read the Dr. Seuss book out loud twice a day. That’s a lot: Total infant exposure
was about five hours. When the babies were born, they were given a pacifier hooked up to a machine that could measure the strength and frequency of their sucking. Rates of strength and frequency can be used to assess whether an infant recognizes something (a form of pattern-matching). The babies then heard tapes of their mothers reading
The Cat in the Hat,
a different story, or no story at all. Sucking rates and patterns were measured at all points. What the researchers found was astonishing. The babies who had heard Dr. Seuss while in the womb appeared to recognize, and prefer, a tape of their mother reading
The Cat in the Hat.
They sucked their pacifiers in a pattern triggered by her reading that book, but not a different book or no book at all. The babies recognized their previous in-womb auditory experience.

We now know that auditory perception begins at a much earlier age than that of the babies tested in this amazing result. Tissues involved in hearing can be observed just four weeks after conception. Hearing begins with the emergence of two structures that look like miniature saguaro cacti sprouting from either side of your baby’s head. They are called primordial otocysts, and they will form a great deal of your child’s hearing apparatus. Once this territory is established, the next weeks are devoted to setting up house, from internal hairs that look like tiny whiskers to the canals they line, which look just like snail shells.

When do these structures hook up to the rest of the brain, allowing babies to hear? The answer should be familiar by now: not until the beginning of the third trimester. At 6 months, you can supply a sound to a fetus in the womb (mostly clicks) and listen in astonishment as the brain weakly fires back electrical responses! In another month, this crackling call-and-response increases not only in intensity but in speed of reaction. Give it another month or more, and everything has changed. Now you have a pre-term infant who can not only hear and respond but can discriminate between various speech sounds like “ahhhh versus “eeee”, or “ba” versus “bi”. We once again
see this paratrooper pattern of establishing the territory first, then hooking things up to central command.

Babies can hear mom’s voice in the womb by the end of the second trimester, and they prefer it to other voices at birth. They respond especially strongly after birth if mom’s voice is muffled, recreating the sonic environment of the womb. Babies even respond to television shows their mothers watched while still in the womb. One funny test exposed pre-term infants to the opening jingle of a particular soap opera. When these babies were born, they would stop crying the moment they heard that jingle (controls had no such distinguishing response). Newborns have a powerful memory for sounds they encountered while still in the womb in the last part of gestation. Re-exposing them to these comforting familiar sounds after birth is another way to smooth their transition into life on this cold, unfamiliar planet.