We Are Our Brains (5 page)

FIGURE 4.
An anencephalic newborn, a child born with most of its brain missing. To establish whether the mother's brain or the child's brain initiated the birth process, we studied the birth of 150 anencephalic babies. Their births were usually either extremely premature or extremely overdue, the normal precise timing of birth (around the fortieth week of pregnancy) having been completely disrupted. The birth process itself took twice as long as normal, and the placenta took three times as long to appear. In normal circumstances, a baby's brain determines the moment of birth and speeds up the birth process.

A woman's brain starts being programmed for maternal behavior right from the onset of pregnancy. Hormones alter the brain in ways that are reinforced after the birth by the presence of the child. The changes in the mother's brain are long-term, perhaps even permanent. Adults sometimes lament that the umbilical cord attaching them to their mothers has never really been cut, while mothers can be burdened by concern about their offspring well after they've grown up. If something happens to their child, some will claim they felt a premonition the previous day—which is true, for the simple reason that they worry about their children on a daily basis.

During pregnancy the pituitary gland secretes the hormone prolactin, which prompts nesting behavior. There's an urge to clean the house and paint the baby's room. Once, when I went to check on my laboratory rats as a PhD student, I was sure that someone had swapped my cages with adult male rats for ones with pregnant rats. Each of the rats had built an enormous nest of sawdust. They were, in fact, my male rats—they had built the nests because the previous day I'd given them a dose of prolactin. Similarly, during his stay in Amsterdam's Wilhelmina Gasthuis hospital, a male patient with a pituitary gland tumor that produced prolactin was never happier than when helping staff clean the bedside lockers.

At the end of pregnancy, both the mother's and the child's brain cells produce oxytocin and release it into the bloodstream. This hormone has many functions. Doctors use oxytocin to induce labor, and some women are given nasal spray containing oxytocin to boost milk release after giving birth. Its role at the end of pregnancy is to
stimulate and speed up labor. The mother's brain secretes more oxytocin at night, when the uterus is most sensitive to the hormone, which encourages labor to start while the body is at rest. During labor, extra oxytocin is released when the child's head presses down on the cervix. This signal is passed on to the mother's brain via the spinal cord, triggering the release of more oxytocin to heighten labor. If a woman is given an epidural to offset labor pains, the signal no longer reaches her brain, causing her pituitary gland to release less oxytocin. In such cases women often need an oxytocin drip to restore the strength of their contractions.

After birth, oxytocin ensures that the mother secretes enough milk. When the child sucks at the nipple, it stimulates its mother's brain to release oxytocin, causing milk to be expressed from the mammary gland. After a time, it's sufficient for the child to cry to trigger this reflex. Such a strong dose of oxytocin is produced that milk spurts from the breasts—a potentially embarrassing state of affairs in company. Farmers have known about this reflex for centuries, having seen how milk will spurt from the cow's udders when they come into the cowshed rattling a milk bucket.

Studies are increasingly revealing the importance of oxytocin in many forms of social interaction. One of the new names given to it is the “bonding hormone” because of the tie it creates between mother and child. This process starts in late pregnancy, when oxytocin levels surge, peaking during birth. When delivery is by Caesarean section that peak does not occur, which might explain why the mother's brain reacts less strongly to the child's crying and maternal behavior takes longer to manifest itself. Nursing and playing with the child causes the mother's brain to be calmed by oxytocin, which also stimulates her warm interaction and close bond with the child. A mother who hasn't formed a close attachment to her child doesn't experience a rise in oxytocin levels when she plays with it. Children who grew up in an orphanage have lower levels of oxytocin in their blood than children who grew up in a family. Even three years after being adopted, children who were neglected early in their development
don't experience a normal surge of oxytocin during affectionate bodily contact with their carers. In other words, their ability to bond is impaired on a long-term basis, sometimes even permanently. A recent study of women who had been emotionally neglected or physically or sexually abused showed greatly reduced oxytocin levels in their cerebral fluid, causing concern that their problems would be passed on to the next generation. Oxytocin also inhibits the stress axis. When girls between the ages of seven and twelve underwent the stressful experience of giving a talk to a group of strangers, reassuring input from their mothers led to the release of oxytocin. It made no difference whether the child was cuddled or simply reassured by a telephone conversation with the mother.

These findings seem to suggest that it's possible to inhibit that excessive motherly concern that can be so annoying to adult children. Maternal behavior in apes was experimentally blocked with a substance that inhibited oxytocin's effect on the brain. It would seem to be the perfect medication for mothers who simply can't accept the fact that their children can function perfectly well independently. Unfortunately, however, the substance reduces apes' interest not only in their offspring but also in sex.

Thirty years ago, our research group investigated the effect of oxytocin on the brain and behavior. We created oxytocin antibodies, staining them so that they would show up in the brain, and looked for the locations where oxytocin is produced and secreted. We found extensive networks of brain cells and axons containing oxytocin in a number of brain structures (
fig. 5
). Those fibers made contact with other neurons, transferring oxytocin to them in the form of a chemical messenger. Examination with an electron microscope revealed that transmission locations looked just like the synapses (cell contact points) where other chemical messengers are passed from one neuron to another (
fig. 6
). These form the basis for oxytocin's behavioral effects.

Different social contexts prompt the release of oxytocin in specific parts of the brain, triggering different types of behavior. It has been
identified as the messenger of affection, generosity, tranquility, trust, and attachment. Oxytocin has also been found to suppress fear by affecting the amygdala, the center of fear and aggression. When affectionate social interaction like hugging takes place, not only do oxytocin levels in the blood rise but more oxytocin is also released in the brain. It's also the messenger that tells your brain when you've had enough to eat. Oxytocin affects not only maternal behavior but also adult relationships, influencing our responses to social stress and sexual contact. As a result it has been dubbed the “love hormone.” Yet again, this is nothing new to farmers, who have been aware of its effects for centuries, long before anyone had ever heard of the hormone. When an orphan lamb is placed with a foster mother, the farmer stimulates the sheep's vagina and uterus, causing oxytocin to be released and the ewe to bond with the strange lamb.

The brain also produces a closely related substance: vasopressin. Just like oxytocin, vasopressin plays a crucial role in maternal behavior, including maternal aggression prompted by threats to offspring. It's also involved in other aspects of social behavior, like pair bonding. Men with a tiny variation in a DNA building block for the vasopressin receptor (the protein that receives vasopressin's message in the brain) are twice as likely to experience marital difficulties and divorce and twice as likely to be unfaithful. If men are shown an image of a strange man after being given vasopressin, they perceive the facial expression as unfriendly, making them more hostile. In the case of women, exactly the opposite occurs. Vasopressin makes them more likely to approach strangers, because they are better at distinguishing friendly features. It's tempting to imagine how society might be improved by giving men a whiff of oxytocin and women a whiff of vasopressin.

A link has recently been found between autism and malfunctions in the brain's vasopressin and oxytocin systems. People with autism often find it hard to interpret other people's emotions and intentions from their gestures and facial expressions or to feel empathy. Some don't understand what is going on, for instance, when another child
cries, or can't detect emotion in a person's voice. Temple Grandin, a professor of animal science with autism, has described her emotional circuitry simply as being disconnected. Indeed, abnormal blood levels of vasopressin and oxytocin have been found in people with autism. They also display small genetic variations in the proteins that are the brain's vasopressin and oxytocin receptors. Conversely, their ability to “read” emotions and predict behavior through facial cues improves when they are given oxytocin. It also helps them to detect emotion in someone's voice and to grasp the emotional significance of intonation. So both oxytocin and vasopressin can play a part in autism, but to regard these two chemical messengers as the “social brain” is a huge oversimplification. Many more messengers and brain structures are involved in social behavior.

There would seem to be considerable scope for capitalizing on our knowledge of the effects of these substances. Psychological experiments with games involving financial payment have shown a link between high oxytocin levels and trust in others, including strangers. That trust remains even if you get cheated a few times. The commercial potential was immediately spotted, and you can now buy “Liquid Trust” online, little bottles of oxytocin that you spray on your clothes to induce confidence in your partner, boss, co-workers, and customers. Since you can only expose people to very tiny amounts in this way, this is at best a placebo (acting via the person who has sprayed the compound on his clothes), if not a downright rip-off.

It's also debatable whether a direct dose of oxytocin in the form of nasal spray can replicate the normal brain processes. After all, the brain produces very precise and limited amounts of oxytocin at a very specific location in response to certain circumstances. Just inhaling the substance may have a completely different effect. And that's actually a general problem when treating disorders of the brain. You can't replace the highly specialized functions of nerve cell systems just by administering their messengers, any more than you can replace a calculator by means of the figures that it produces.

FIGURE 5.
The localization of oxytocin and vasopressin in the brain. The two hormones are produced in two regions of the hypothalamus, the paraventricular nucleus (PVN) and the supraoptic nucleus (SON), and released into the bloodstream as neurohormones in the posterior pituitary. Oxytocin causes contractions of the uterus during labor and contractions in the mammary gland during suckling. Vasopressin regulates the body's retention of water by acting on the kidneys. Oxytocin and vasopressin are also transported to many brain areas that we know of (indicated here with abbreviations) and to as yet unknown brain areas, and are released in those locations from synapses as neurotransmitters (chemical messengers).

We all know cases of mothers who just haven't managed to sever the umbilical cord. Their children might be long grown up, but they are
constantly concerned about them and want to know exactly what they are doing, even when they're on the other side of the globe. The tie between mother and child remains a special one. No matter what country he fights for, a wounded soldier on a battlefield will always call for his mother, not his father.

In the case of chimpanzees, females are responsible for teaching cultural skills. So I always thought that a father's role was confined to fertilization, the moment at which less than half the child's DNA has to be delivered, a job that can be done in a few minutes. We fathers could then hide behind the newspaper and leave the child's care and upbringing to the mother. But it turns out that fathers can't get off that lightly. The animal kingdom provides examples of paternal behavior that replicate every aspect of maternal behavior. There is even a male bat that produces milk!

Humans do occupy a special place in the animal world in terms of their focus on the family. The family is the building block of our society, which is not the case with great apes like chimpanzees or bonobos. It's not so much pair formation that's unusual—you see that in gibbons, birds, and voles—but in those species families live isolated from each other. The family-based society is unique to our species. As far back as two million years ago, our ancestors gave birth to offspring that weighed twice as much as those of chimpanzees. Since those heavy, helpless babies couldn't be transported easily, shared child care was crucial to ensure sufficient food for the mother and a chance for her to suckle her offspring. Patriarchy—male dominance within the family—is thought to have developed when our ancestors had to exchange the protection of the jungle for a more vulnerable life in the savanna. In those wide-open spaces it was crucial for the male to protect the female and her child. Incidentally, these human ancestors, who walked on their knuckles, ate fruit, hunted, and used tools, didn't leave the jungle of their own volition, as is so often claimed. The jungle disappeared around them as a result of drastic climate change. Vast tracts were gradually transformed into dry savanna. The male's protection of the female and her child had an evolutionary
advantage: Humans were able to reproduce every two to three years, while female chimpanzees, who were solely responsible for their young and therefore had to look after them and feed them for much longer, could reproduce only every six years.