A Sensory Trigger
A baby does what he can to attract and hold his mother's attention. A young son's distinctive cry, his unique scent and the way he curls his fingers around his mother's are just a handful of the sensations that shower down on her highly sensitized nervous system. The infant creates a rich environment that stimulates the mother, pushing her brain into a higher gear.
Of all the senses, smell—olfaction—plays the largest role in reproduction. Females rely on their sense of smell from the very beginning to help them select their mates all the way through to the weaning of their young, during which scents act as a form of communication between mother and child. An extreme example of the power of smell is known as the Bruce effect, a phenomenon in which certain scents induce abortions in pregnant rodents. If a female's mate disappears after conception and an interloper starts hanging around, the new male's smell will inhibit the production of key hormones, causing the female's pregnancy to abort. Otherwise, chances are high that the interloper would end up killing and eating the pups, thereby obtaining a high-protein meal and removing a rival's genes in the bargain. In a kind of “Sophie's choice” for rodents, the female is basically making a cold calculation—better to lose the young as embryos than as pups.
Because of our limited ability to peer into human brains, rodents help us approximate the changes that are taking place inside mothers such as Liz. What we have seen so far is that the mammalian brain possesses a dramatic ability to shape-shift when life demands it. During a rat's pregnancy, for example, we know that the olfactory system starts churning out new neurons. The theory is that the extra neurons allow moms to become more adept at processing the cues hidden in infant odors. Indeed, mothers distinguish themselves quite obviously in how they react to smells. Whereas virgin female rats find the odors of infants noisome, once they become pregnant those smells attract them. Human mothers also demonstrate these effects, as psychologist Alison Fleming of the University of Toronto Mississauga and her colleagues reported. They found that mothers are much more likely to rate their infants' odors as pleasant, as compared with nonmothers.
To transform women's perceptions of smells, the olfactory system may rely on a region known as the medial amygdala, suggests neurobiologist Michael Numan of Boston College and his colleagues. This brain area could be acting as a hub for the olfactory system, with information arriving here to be processed for emotional content. The olfactory tweaks may aid in solidifying the mother-child bond by making babies' odors alluring. Before she had her first child, Liz had avoided the smells of children, even those to whom she was related. But with the birth of her son, she discovered she had no problem stuffing her nose into his diaper to determine if he needed a change.