Most parents are probably cautious and quiet around a sleeping baby, worried that the slightest sound will jar them from their slumber. Now, there’s even more reason to watch what you say when a sleeping baby is around, because we know that even if it doesn’t wake them up, they can still hear it – and their brains will respond. Even more important, how their sleeping brains respond to emotional speech tells us we need to watch what we say around them when they’re awake, too, because that may shape their brains.
Researchers look at sleeping babies’ brains for entirely practical reasons: if you’re going to stick a normally squirmy, clingy infant into a large and impersonal brain scanner whose accuracy depends on you keeping very, very still, then while they sleep is the time to do it. I can imagine no other way that 20 babies only 6 to 12 months old could provide useful brain data.
That brain data was collected while babies were played some soothing and some not-so-soothing lullabies: gibberish sentences (made up of legitimate English sounds) read in neutral, happy, angry, or “very angry” tones of voice. It took “very angry” to see a clear difference with the brain’s reaction to the speech – probably just because the babies are asleep, and it should take a more dramatic stimulus out there in the world to break through the slumber barrier.
That very angry speech got responses in two very important brain regions. One is the anterior cingulate cortex, which is linked to emotional conflict, and seems to be involved in both physical and emotional pain). The other region is the hypothalamus, which I’ve recently given some attention as the “the brain within the brain” and is a critical player in stress and love. We can only speculate what exactly is going on in these brain regions, because baby brains are wired differently than adult ones and this changes how their brains process emotions. It does tell us that their brains are exquisitely tuned to detect angry speech in their environment, though, even when they appear completely zonked out.
And that detection of angry voices does not seem to be something babies get used to; instead, it seems to be something that they only get more and more sensitive to, the more they hear it. These babies’ mothers were asked about the level of conflict in their relationship with their co-parent, focused on non-physical conflict like shouting and yelling, or “stomping out” in the middle of an argument. The more parent conflict they reported, the more their babies’ brains responded to that angry voice, in both the anterior cingulate cortex and the hypothalamus.
What this will mean years or even just months later is impossible to say; it’s difficult to identify consequences based on just activity alone, because it depends on what we’re doing with the brain region. It does suggest, though, that sensitivity to raised voices and harsh words might be started even younger than most of us thought possible.
Graham AM, Fisher PA, & Pfeifer JH (2013). What sleeping babies hear: a functional MRI study of interparental conflict and infants’ emotion processing. Psychological science, 24 (5), 782-789. PMID: 23538912