How baby cries bore into mom’s brain

Here I am, fresh off of my second maternity leave ready to serve up lots of juicy fresh science about babies. And I would love to do that, if only I were sleeping more at night. With her intoxicating baby aroma, squishy face and sweet little coos, our newest little daughter is irresistible by day. Night is another story altogether. And it’s a sad one.

Our tale begins and ends with her cries wrenching me from a dead sleep over and over again. Sometimes I lie in bed for a split second, deluding myself into thinking that maybe this time she’ll go back to sleep. That pause is long enough for me to notice all the ways her cries affect me: Pounding heart, sweaty hands and feet, and most importantly, a single-minded, maniacal focus on that sound.

Evolution didn’t give babies many ways to communicate, but the method they have, crying, sure gets the job done. So I read with interest an April 23 study in Nature that explains one way in which baby cries sledgehammer a mother’s brain.

Upon hearing a lost pup’s cries, mother mice promptly go and fetch the wayward pup by the scruff of its neck. But this behavior has to be learned — a first-time mom isn’t as attuned to the sounds of her pups’ cries. As she gets the hang of that whole mothering thing, the momma mouse’s brain gets better at picking the sound of a distant crying pup out of the background din. These pup cries bore their way into the mother’s brain in an interesting way, the researchers found.

In the brain, the hormone oxytocin primes certain nerve cells to better hear an infant in distress, the new study suggests. This hormone has wide effects in the brain and body: It’s involved in birth, breast-feeding and social bonding. (You may have heard about the recently discovered oxytocin-induced human-dog lovefest.)

When Robert Froemke of the New York University School of Medicine and colleagues delivered oxytocin directly to a small patch of neural real estate — the left auditory cortex, which is involved in hearing and interpreting sounds — pupless females began acting like experienced mothers, retrieving another mom’s pups even though they’d never heard their cries before.

The research identifies a neat loop: Baby cries, oxytocin floods the mother’s brain, sound-sending neurons get better at recognizing cries, and mother grows more responsive to baby’s cry. All in all, it’s a perfect example of babies using what they got to get what they need.

Of course, the usual caveats apply. The study was done with mice, and it’s not clear how the results would apply to human babies and mothers. But there’s good evidence that mothers’ brains are intensely attuned to their babies.

I found one particular example illuminating: While in a brain scanner, 18 men and women were subjected to hearing a hungry baby cry. In women’s brains, the sound interrupted the normal brain activity that’s associated with awake resting, jolting them out of their quiet daydreaming. The brains of men, in contrast, “carry on without interruption,” the researchers wrote in a 2013 NeuroReport paper.

It’s a small study, but my N=2 observations corroborate the results. The slightest little whimper from our new baby wakes me up and sets my heart racing. My husband, however, often stays sleeping like a baby. Maybe it’s time for me to stop thinking that he’s faking it. 

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