Welcome to Your Child's Brain: How the Mind Grows From Conception to College (20 page)

In adults, smell and taste take a backseat to vision and hearing, which provide most of the information we use to navigate through the world. Infants, born with rather poor vision (see
chapter 10
), are more dependent on the chemical senses. They share this dependence with some of the oldest of animals. Over 800 million years ago, certain types of worm started to localize more refined chemical sensation in their front end—in other words, they developed noses. Even more primitive animals such as jellyfish are deaf, blind, and noseless, but can still detect the presence of noxious chemicals applied somewhere to the surface of their bodies.

The flavor sense consists of multiple components in the mouth and nose. What hits the tongue is taste and includes the fundamental components of sweet, salty, sour, bitter, and umami. (
Umami
is a Japanese word meaning “yummy.” It is used to refer to the rich mouthfeel of cooked meat or mushrooms, caused by the presence of
glutamate
, a component of protein also found in tomatoes and many broths.) Combined with taste is the sense of smell, which comes in through the nose and up from the back of the mouth and is far more complex.

Smell conveys most of the nuances we experience in food. You can prove this to yourself using the “jellybean test.” Take a bowl of jellybeans and eat one at a time, first normally, then while holding your nose. While holding your nose, you will find that the candies taste much more similar to one another. The same is true for taking a bite from an apple compared with a potato, though you may not enjoy this test as much.

Smell signals are detected in the nose by a layer of receptor cells (see figure opposite) covered in sticky mucus that helps to catch odors. Each sensory cell expresses one type of odorant receptor protein. (All receptors are made of protein.) There are hundreds of distinct receptors, each with its own preference for a different set of odorant molecules. These sensory cells send their axons in a mad tangle, passing along the way through hundreds of perforations in a thin bone called the
cribriform plate
, which resembles a rooster’s comb. The whole thing detangles to make a perfectly sorted landing pattern in the brain in the
olfactory
bulb
, an oblong structure that lies underneath the frontal part of the brain. By the time the axons arrive, each part of the olfactory bulb receives a “private line” corresponding to one, and only one, type of odor receptor.

So when can babies start to smell things? The olfactory epithelium and bulbs are present by the eleventh week of gestation, toward the end of the first trimester. Then, sometime during the second half of the second trimester (weeks sixteen to twenty-four), the nostrils open up, allowing amniotic fluid to reach the olfactory epithelium. In premature infants, the earliest reported responses to smell occur at the end of this period, at seven months of gestational age. This is possible not only because the nostrils are open but because neurons from the olfactory epithelium have sent axons into the brain, to the olfactory bulb. In turn, neurons in the olfactory bulb send connections to other places in the brain, including the amygdala, which is involved in generating emotional responses, and the
piriform cortex
(a region of the neocortex), which passes the information on to other brain regions (see figure opposite).

To an infant, most smells are initially neutral. Some responses are innate: within twelve hours of birth, infants make faces when exposed to the scent of rotting eggs. Likewise, sugary water triggers an automatic smile: more, please. Generally, though, smells take time to become associated with positive or negative reactions, emotions, and memories. These associations to the world’s rich variety of food, drink, and smells are acquired postnatally. This process is guided by the brain’s mechanisms for associating new smells with already-familiar signals and liked (or disliked) outcomes.

PRACTICAL TIP: GETTING YOUR CHILD TO EAT SPINACH

One in five American toddlers don’t eat even one vegetable per day. Can we teach young children to like these bitter but healthy foods?

Most parents use social approaches, which often work: involving the child in preparing the food, or showing that parents or siblings like the food. Less appreciated are techniques based on the direct experience of flavors.

Just consuming a food multiple times is sufficient to reduce negative reactions. Infant taste is particularly plastic during the first few months. Babies fed a relatively bitter nonmilk (such as soy-based) formula are more tolerant of broccoli as children. And when asked to rank their favorite vegetables, they are more likely to give broccoli a high rating (see
p. 109
). Also, it’s a good idea to eat vegetables during pregnancy, as taste preferences begin to develop in utero.

Combining a new flavor with a well-liked familiar flavor is another powerful way to build a new preference. Researchers have found that the two tastes cannot be given more than nine seconds apart. You can try this approach with your baby: simply mix the two flavors together. For example, babies develop a preference for pure carrot juice after having it mixed with milk. Sweeteners work too. A common way to introduce solid food to babies is to puree it in a blender, an approach that lends itself very well to mixing-and-matching of flavors. This pattern persists throughout life. College students who are given broccoli with sugar will later rate broccoli alone as being more pleasant than cauliflower alone—and will do the converse if they are given sweetened cauliflower. Coffee drinkers often initially add sugar or milk but eventually take it black. Learning can even be negative: pairing a flavor with bitter quinine, found in tonic water, can reduce liking.

One approach we don’t recommend is offering dessert as a reward for finishing dinner. The urge to consume foods that contain calories is a powerful motivator, as confirmed both by our everyday experience and by behavioral experiments. But when kids eat dessert right after a meal, something odd happens: their preference for foods eaten earlier decreases. Why?

Recall that our brains want us to like foods that are high in calories. And the gut detects calorie content many minutes after we eat. So, because the calories from foods eaten earlier are still being processed when dessert arrives, the earlier foods actually encourage a preference for the taste of dessert rather than the reverse.

One solution to this problem would be to give dessert before the new flavor—ideally within nine seconds. There’s a converse problem if you give dessert too early: your kid’s not likely to be hungry come spinach time. If you’re going to use this approach, we recommend serving dessert more than thirty minutes after the meal—or offering a bite or two right at the time of the meal!

The developing brain’s first teachers for food preference are the tongue (taste) and gut (nutrient content). In the tongue, molecules that trigger one of the five basic taste sensations bind to receptor molecules in a taste bud, which then generates a chemical signal inside the sensory neuron that bears the receptor molecules. Each neuron, taste bud, and axon is a little communication line that is assigned the identity of
sweet
,
salty
, and so on by virtue of where it connects. These labeled lines convey basic information about chemicals found in foods.

Scientists used to think that sweetness taste buds were found only in one part of the tongue, but now we know that each type of taste bud is found all over the tongue. Receptor cells form in the tongue very early, during week eight of gestation. By week thirteen, taste buds are present throughout the mouth and are connected to nerves that go to the brain. By the time the axons are functionally connected to brain structures, they are hooked up and interpreted correctly.

Taste signals are translated into electrical impulses that are conveyed along the neurons’ axons to the
nucleus solitarius
, a cluster of brainstem neurons (see
figure
). The nucleus solitarius is an important station not just for taste information but also for other visceral signals, including the presence of fat in food. Other organs besides the gut also send signals: the cardiovascular system, liver, and lungs. The nucleus solitarius’s many jobs include generating the gag reflex, coughing, and responses relating to breathing, digestion, and the heart. A notable reflex in this general category is the
gastrocolic response
, in which the eating of food, especially if it is fatty, helps trigger defecation after about half an hour (in babies and grown-ups). This reflex can be useful in making diaper-change time more predictable.

These labeled lines for taste and calorie content serve an important evolutionary purpose. Starting early in life, everyone strongly prefers sweet tastes and calorie-rich foods. The benefit of a sweet tooth is obvious: sweet things are usually loaded with valuable calories. Glutamate is similarly useful, as is salt. Bitter and sour tastes are rejected. In all cases, these experiences stimulate powerful teaching signals that tell the brain that something good or bad has happened. These teaching signals are conveyed through the nucleus solitarius onward to the thalamus, striatum, and neocortex.

DNA does not just encode receptors; it also carries instructions telling each cell which receptor protein it should make. One example that has been studied carefully is taste neurons that detect sweetness. These neurons all make the same sweet receptor molecule, a protein dedicated to the job of binding to sugar. Researchers have applied genetic engineering techniques to make mice that contain the DNA sequence for a receptor for morphine (which mice normally can’t taste) in the exact location where the sequence for the sweet receptor would normally be found. Mice with this genetic alteration can’t taste sugar, but they can taste chemicals related to morphine—and consume them avidly, as if they were sweet. This is true even for amounts of a morphinelike drug that are far too small to be addictive. This result suggests that the surrounding DNA acts as a signpost that
the receptor at that location should be hooked up to brain wiring that carries the message “It’s sweet, and I want more.”

In contrast to having just one sweet receptor, our genome has multiple receptors for other tastes—including dozens that are sensitive to bitter chemicals. Babies instinctively regard bitter flavors as unpleasant. Many toxic chemicals are bitter and are often found in plants. Bitterness is a natural signal to get us to spit these things out, but paradoxically, we can teach our brains to enjoy bitter flavors. This flexibility is useful, since it allows your child’s brain to adapt to a food that has nutritional value but happens to contain one of the chemicals found in other bitter, less healthy substances that occur naturally. With practice, we can learn to like tonic water, coffee, and broccoli—and children should like broccoli (see
Practical tip: Getting your child to eat spinach
). We train our brains to accept particular bitter flavors by using rewards (pairing them with liked flavor, calorie content, or social approval) and reject other bitter flavors using penalties (if they lead to physical illness or social disapproval).