In Babies’ Brains, White Matter Is Crucial—and Conversational Turns Make It Grow

Most of us instantly recognize the term “gray matter” as a synonym for the brain. Mention “white matter” and you may get some blank looks. However, in the geography of the central nervous system, white matter, or myelin, deserves at least equal billing.

Myelin is the fatty insulation that protects axons, the transmission lines of the nervous system that shoot information-bearing electrical impulses to various parts of the body (which is a much-abbreviated explanation of a fantastically complex process). Myelin helps these electrical impulses to travel efficiently along the axon. This is critical for effectively transmitting information throughout the brain with exquisitely precise timing.

Babies are born with brains full of axons located right where they need to be for various functions, such as hearing, seeing and movement. White-matter pathways associated with language are also present at birth, but their myelin continues to develop for many years after birth. By examining myelin development, scientists have discovered that these neural connections don’t simply grow, they are cultivated by their environments.

Parental input has been considered a key environmental factor for infants’ language development, as shown by a wealth of behavioral research. But few studies have looked at how parents’ verbal interactions with babies affect the physical development of their brains. Given the critical growth in children’s language-related activities in their first two years of life, a better understanding of what’s going on in their brains at this time is badly needed.

Thanks to a long-term intervention study of infant language-learning, researchers at the University of Washington’s Institute for Learning & Brain Science (I-LABS) have a trove of LENA-device home recordings of child vocalizations and parent-child verbal interactions taken at regular intervals throughout babies’ first 24 months.

For their recent study on the effect of language experience on white-matter development, researchers invited all the families back to the lab for an MRI session when the children were around 2 years old. The MRIs imaged the white matter in the toddlers’ dorsal language system, a brain network that is tied to expressive language development and long-term language ability. They found that the frequency of parents’ verbal interactions with their infants, specifically conversational turns, uniquely predicted myelin density in this system.

A sleeping baby is prepared for the MRI. Julia Mizrahi / I-LABS, taken at UW’s Bio-Molecular Imaging Center (BMIC).

“Conversational turns” are the back and forth between adult and child that can occur even before the child has actual words, a call and response that speaks “connection” in every utterance. In their study described in a paper published March 1 in The Journal of Neuroscience, researchers found that parent-infant conversational turns link to white-matter growth (myelination) at age 2 and suggest that early interactive language experiences uniquely contribute to brain development associated with long-term verbal and cognitive ability. The more back and forth between babies and parents, the greater the growth of the brain in areas critical to language ability and sensory-motor integration that affect the child’s ability to learn language and build vocabulary. These effects carry through early childhood and predict cognitive and linguistic ability into adolescence.

In other words, conversational turns are a very big deal, and MRIs show it.

Not Words Alone

I-LABS researcher Dr. Elizabeth Huber, the paper’s first author, says the studies establish that the growth in white matter isn’t related simply to the amount of language a child is exposed to—the number of words that wash over a child—but the amount of high-quality verbal interaction they have with the adults in their lives. The effects of these interactions were apparent as early as six months, when the child is not yet speaking but vocalizes (“babbling”) and the parent vocalizes back.

“Conversational experience as early as 6 months is predicting what the brain looks like at age 2 years,” Huber says. “It was striking to me how early and potentially long-lasting these effects are.”

It is impossible to overestimate the importance of those early years. White-matter pathways develop at their most dramatic rate during these years, though they continue to develop through adolescence. Language exposure during this window has been linked not only to vocabulary building but to multiple aspects of children’s cognitive development. Being exposed to conversational contingency—meaning interactions that acknowledge each other’s presence and take note of what’s happening in their shared physical environment (Do you see that kitty? How does a kitty go?)—encourages shared and sustained attention. If the adult is focusing on something and draws the child’s attention to it, the child is then able to focus on that thing distinct from everything else in the environment. Maybe not for long, but conversational contingency builds the muscle.

Conversational turns have been shown to stimulate more and higher-quality vocalizations from infants, including making sounds that are more consistent with the speech sounds and patterns of the adults around them (phonology). If you keep sharing conversational turns with your child in your Deep South accent, it’s a fair bet that their baby talk will have a drawl.

Through this conversational give and take, babies learn to listen and adjust their vocalizations in response to another person, a critical ability in all human interactions.

So Much More to Learn

Huber stresses that this research really has just begun. The current study was limited to native English speakers and families without known risk factors such as lower social economic status or a family history of dyslexia. The sample size was relatively small, and future work will look at larger and more diverse samples, including a larger control group of families that didn’t take part in an enriched language intervention.

“Right now, we’re really excited about the idea of adding brain scans with 6-month-old, or even younger, infants,” Huber says. “Can we already see these effects (on white matter) at a much younger age? Or is there something special about what’s happening in the brain around 2 years, as toddlers are starting to really use language to communicate in a more sophisticated way? Are there incremental changes in the white matter that connect to what an infant is currently experiencing, or do environmental effects show up at certain points in development more strongly than others? What we see right now is that conversational turns in infancy predict white-matter density in the 2-year-old, but that raises a lot of follow-up questions.”

Another area that’s ripe for research, Huber says, is looking at the effects of environmental factors such as poverty or trauma, which can interrupt the brain’s development, and potential ways to mitigate that interruption. The human brain is incredibly flexible, she says, and if there is some kind of a deficiency, researchers wonder if there are ways that deficiency can be mitigated.

It’s important to avoid thinking that all is lost if a child isn’t exposed to rich conversational interactions in their earliest years, Huber says. People working two jobs and giving their all to keep a roof over their heads and food on the table might not have as much time as they’d like to spend with their children.

“The rich early experience seems to be really important,” she says. “There are moments in development where we’re particularly sensitive to certain aspects of our environments, and where it’s easier to learn certain skills. So, for example, it’s harder to master a second language if you didn’t hear it or have some exposure as a very young child. I studied Spanish for years in college, but I speak it with a heavy Kansas accent, and I have to stop and search for words.

“At the same time, it isn’t as simple as saying, ‘If you have this amount or type of interaction at this exact age, you will excel in learning language, and otherwise you won’t.’ Children learn in different ways, and there is still lots of flexibility to learn and adapt, even later in life.

“Ultimately though,”Huber adds, “it’s exciting to me to think that we are starting to understand more about what matters for different aspects language development. If we can help parents and children so that a given child is coming into school on strong footing, that can make a difference for a child’s whole life going forward.”


Language Experience during Infancy Predicts White Matter Myelination at Age 2 Years The Journal of Neuroscience; Authors: Elizabeth Huber, Neva M. Corrigan, Vasily L. Yarnykh, Naja Ferjan Ramírez and Patricia K. Kuhl

LENA  is a technology for measuring talk with children, a critical factor in early brain development. LENA uses a small wearable device — often referred to as a “talk pedometer” — combined with cloud-based software to deliver detailed feedback on adults’ interactive talk with children.

K.C. Compton worked as a reporter, editor and columnist for newspapers throughout the Rocky Mountain region for 20 years before moving to the Kansas City area as an editor for Mother Earth News. She has been in Seattle since 2016, enjoying life as a freelance and contract writer and editor.

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