senses develop strategically
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 CEO-like 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.
Babies remember
“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 0 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.
Let’s look at when your baby’s senses—touch, sight, hearing, smell, balance, taste—start to function as you transit through pregnancy.
Touch
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, and 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.
Sight
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.
Hearing
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 or a different story. 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. 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.
