Maturing brain pathways explain the sudden leap in children’s language skills

Between the ages of three and four, children undergo a major leap in their ability to use grammar, and recent research provides evidence that this behavioral milestone corresponds to the maturation of specific neural pathways. A new study published in Developmental Cognitive Neuroscience suggests that the white matter connections along the upper routes of the brain mature during this time to facilitate the learning of grammar rules. This structural shift helps explain why children’s language abilities expand so rapidly just before they start kindergarten.

Learning a native language requires young children to master a massive vocabulary and figure out the complex rules for combining words. While scientists know a great deal about how the adult brain processes these rules, it remains unclear exactly how the developing brain supports early language acquisition.

In adults, rule-based linguistic processes rely on specific white matter pathways. White matter consists of bundles of nerve fibers that act like communication cables, connecting different brain regions and allowing them to share information rapidly.

These particular language pathways mature relatively late in child development. Scientists designed this study to figure out if these late-maturing pathways already help young children learn grammar rules, or if toddlers rely on completely different, earlier-maturing brain connections to communicate.

“The preschool period (between 3 and 5 years of age) is marked by major leaps in language development, with grammar especially taking off during this phase. While we know quite a lot about the mature language network in the adult brain – specifically that the ‘dorsal route’ seems to be a crucial pathway for processing grammar – we know much less about how the developing brain supports these skills,” said study author Cheslie C. Klein of the Max Planck Institute for Human Cognitive and Brain Sciences.

“This is largely because acquiring MRI data with young children is so challenging. In this study, we were particularly interested in the brain’s ‘wiring’ – the white matter pathways that connect the frontal and temporal regions – which ensures these areas can efficiently work together to accomplish complex cognitive tasks like language. This question was particularly interesting because the dorsal white matter connection that supports grammar in adults matures relatively late compared to other connections within the language network.”

The researchers evaluated a sample of 120 typically developing, monolingual German-speaking children. The group included 47 three-year-olds and 73 four- to five-year-olds. The scientists used magnetic resonance imaging, commonly known as MRI, to safely scan the children’s brains and observe the structural development of their white matter pathways.

During the imaging process, the researchers measured how water molecules moved along the nerve fibers. As children grow, their brain’s communication cables become better insulated and organized, which changes how water diffuses along them. By tracking this movement, scientists can estimate the physical maturity of specific neural highways.

Before the brain scans, the children completed a behavioral language test designed to assess their grammar skills at the word level. The researchers used a picture-based game where children were asked to name the plural form of different nouns. For example, a child would see a picture of one car and hear a spoken description, and then see a picture of three cars and be prompted to say the plural word.

The researchers scored the children’s answers based on whether they applied the correct grammatical rule to form the plural word. Because the German language has multiple different rules for making nouns plural, this task requires children to actively sort and apply grammatical patterns.

Young children often make mistakes when learning these rules, such as adding the wrong ending to a word. These errors actually provide evidence that a child is actively attempting to apply a newly learned grammatical rule rather than simply repeating memorized words.

For the four- to five-year-olds, the test also included made-up words. This allowed researchers to see if the older children could apply grammar rules to completely new words they had never heard before. The scientists then mathematically compared these behavioral test scores to the maturation of specific fiber pathways in the children’s brains.

Specifically, the researchers focused on the dorsal routes, which are neural pathways located in the upper part of the brain. They looked at one dorsal pathway extending to Broca’s area, a brain region that handles grammar rules in adults.

They also examined a second dorsal pathway that connects auditory regions to the premotor cortex. The premotor cortex is an area that helps translate sounds into physical mouth movements for speech.

Finally, they evaluated a lower brain route called the ventral pathway, which processes word meanings and memory retrieval. To ensure their findings were specific to language development, the scientists also tracked a completely unrelated neural pathway that controls general body movement. This served as a baseline control measure for the experiment.

The data revealed developmental differences between the three-year-olds and the four- to five-year-olds. In the older group, higher scores on the plural word test were directly associated with the structural maturity of both dorsal pathways. This indicates that four- and five-year-olds use the upper brain route to process sound-to-motor speech movements as well as complex grammar rules.

In contrast, the researchers found no relationship between grammar abilities and these specific white matter pathways in the three-year-old children. The findings suggest that a major qualitative developmental milestone occurs between ages three and four. At this stage, the brain’s upper communication cables become mature enough to actively support a child’s expanding grammar skills.

The researchers also noticed a slight relationship between the lower brain route, the ventral pathway, and grammar scores in the older children. This lower route tends to help with retrieving word meanings from memory. The scientists suspect this pathway helped the older children draw upon real words they already knew to figure out how to pluralize the made-up words in the test.

As expected, the control pathway governing basic body movement showed no connection to the children’s language scores. This confirms that the observed brain changes were specifically related to grammar acquisition, rather than general physical growth.

“The main takeaway is that a qualitative milestone seems to occur between the ages of 3 and 4, when white matter connections via the ‘dorsal route’ (the upper route through the brain) mature to facilitate the acquisition of grammar rules,” Klein told PsyPost. “Caregivers may have noticed how much language abilities improve between ages 3 and 4, and our findings align well with these behavioral milestones.”

The new research builds upon previous findings from the same team of scientists, who earlier mapped the brain’s processing centers, known as gray matter. In their 2023 study, the researchers found that three-year-olds tend to rely on a lower brain region called the temporal lobe to process sentences, but by age four, this activity shifts to the frontal lobe, specifically to an area that handles complex grammar rules in adults. Together, these studies provide evidence that both the brain’s processing regions and the neural highways connecting them experience a synchronized developmental leap to support a child’s rapidly expanding language skills.

“Most striking was how nicely the maturation of these white matter pathways aligned with our previous gray matter findings and the behavioral timeline for grammar acquisition reported in the literature during this time frame,” Klein said.

While the study provides detailed insights into early brain development, it comes with certain limitations. Because three-year-old children did not show a connection between grammar skills and the evaluated pathways, it is still unclear which exact brain structures handle grammar before age four. Scanning the brains of very young children is notoriously difficult, which represents a major hurdle for future research.

Additionally, this study only evaluated word-level grammar, specifically how children pluralize nouns. Because children learn nouns and verbs at different rates, it is possible that the brain processes verb rules differently.

“It is important to note that this is basic research,” Klein noted. “The immediate significance lies in furthering our understanding of how the brain changes during typical language development. Since acquiring grammar is a fundamental language skill, understanding the specific structural network that supports it may also advance our future understanding of developmental delays or atypical language development.”

The researchers suggest that future studies should examine sentence-level grammar and explore whether similar brain pathways help children learn to use verbs. Scientists could also look beyond the core language centers to see how other brain networks interact to support early language learning in young children.

“I would like to acknowledge the tremendous effort required to successfully conduct neuroimaging research with such a young group of participants,” Klein added. “This research would not be possible without the support of families interested in advancing our understanding of the developing human brain.”

The study, “Grammar acquisition in preschool children is related to white matter maturation of the dorsal language network,” was authored by Cheslie C. Klein, Philipp Berger, Charlotte Grosse Wiesmann, and Angela D. Friederici.