Between the ages of three and four, children make a huge leap in their ability to use grammar. Recent studies provide evidence that this behavioral milestone corresponds to the maturation of specific neural pathways. New research published in Developmental cognitive neuroscience It has been suggested that white matter connections along upper brain pathways mature during this period and facilitate learning of grammatical rules. This structural change helps explain why children’s language skills develop so rapidly just before entering kindergarten.
Learning their native language requires young children to acquire a huge vocabulary and understand complex rules for combining words. Although scientists know a lot about how the adult brain processes these rules, exactly how the developing brain supports early language acquisition remains unclear.
In adults, rule-based language processes rely on specific white matter pathways. White matter is made up of bundles of nerve fibers that act like communication cables, connecting different brain regions and allowing them to share information quickly.
These specific language pathways mature relatively late in a child’s development. Scientists designed the study to find out whether these late-onset pathways already help young children learn grammar rules, or whether young children rely on entirely different early-developing brain connections to communicate.
“The preschool period (ages 3 to 5) is characterized by a major leap in language development, and grammar specifically begins at this stage. We know quite a bit about the adult brain’s mature language network, but we know that the ‘dorsal pathway’ is particularly important for processing grammar. “This appears to be an important pathway. Little is known about how the developing brain supports these skills,” said study author Chesley C. Klein of the Max Planck Institute for Human Cognitive and Brain Sciences.
“This is primarily because it is very difficult to obtain MRI data in young children. In this study, we were particularly interested in the ‘wiring’ of the brain, the white matter pathways that connect the frontal and temporal regions, which allow these regions to work together efficiently to accomplish complex cognitive tasks such as language. This question was particularly interesting because the dorsal white matter connections that support grammar in adults mature relatively late compared to other connections within the language network.”
Researchers evaluated a sample of 120 typically developing, monolingual, German-speaking children. This group included 47 3-year-olds and 73 4- to 5-year-olds. Scientists used magnetic resonance imaging, commonly known as MRI, to safely scan the children’s brains and observe the structural development of white matter pathways.
During image processing, the researchers measured how water molecules moved along the nerve fibers. As children grow, the brain’s communication cables become better insulated and organized, changing the way water spreads 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 grammatical skills at the word level. The researchers used a picture-based game that asked children to name the plural forms of various nouns. For example, a child may see a picture of one car, hear an audio description, then see a picture of three cars and be prompted to say multiple words.
The researchers scored the children’s answers based on whether they applied the correct grammar rules to form plural words. German has several different rules for making nouns plural, so this task requires children to actively categorize and apply grammatical patterns.
Young children often make mistakes when learning these rules, such as misspelling the endings of words. These errors actually provide evidence that the child is actively trying to apply newly learned grammar rules, rather than simply repeating memorized words.
For 4- to 5-year-olds, the test also included coined words. This allowed the researchers to see if older children could apply grammatical 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 child’s brain.
Specifically, the researchers focused on the dorsal pathway, a neural pathway located at the top of the brain. They investigated one dorsal pathway that extends to Broca’s area, a brain area that deals with grammatical rules in adults.
They also investigated a second dorsal pathway that connects the auditory area to the premotor cortex. The premotor cortex is an area that helps convert sound into physical mouth movements for speech.
Finally, they evaluated a lower brain route called the ventral pathway, which processes word meaning and memory retrieval. To confirm that their findings were specific to language development, the scientists also traced completely unrelated neural pathways that control movement throughout the body. This served as the baseline control measure for the experiment.
The data revealed developmental differences between 3-year-olds and 4- to 5-year-olds. In the older group, higher scores on the multiword test were directly associated with structural maturation of both dorsal pathways. This shows that 4- and 5-year-olds are using upper brain routes to process sound-to-movement movements and complex grammatical rules.
In contrast, the researchers found no association between 3-year-olds’ grammatical abilities and these specific white matter pathways. The findings of this study suggest that major qualitative developmental milestones occur between the ages of 3 and 4 years. At this stage, the brain’s upper communication cables are mature enough to actively support the child’s expanding grammar skills.
The researchers also noticed a small relationship between lower brain pathways, ventral pathways, and grammar scores in older children. This sub-route tends to be useful for retrieving word meanings from memory. Scientists think this route may have helped older children figure out how to pluralize the words created in the test by drawing on real words they already knew.
As expected, the control pathways governing basic body movements showed no association with children’s language scores. This confirms that the observed brain changes are related to the acquisition of grammar specifically, rather than physical growth in general.
“The main takeaway is that a qualitative milestone appears to occur between the ages of 3 and 4, when white matter connections through the ‘dorsal pathway’ (the upper pathway through the brain) mature to facilitate the acquisition of grammatical rules,” Klein told PsyPost. “Caregivers may notice how language skills improve between ages 3 and 4, and our findings align well with these behavioral milestones.”
The new study builds on previous findings by the same team of scientists who previously mapped the brain’s processing center known as gray matter. In a 2023 study, researchers found that 3-year-olds tend to rely on a lower region of the brain called the temporal lobe to process sentences, but by the age of 4, this activity shifts to the frontal lobe, specifically the area that deals with complex grammar rules in adults. Taken together, these studies provide evidence that both the brain’s processing regions and the neural highways that connect them are experiencing synchronized developmental leaps to support children’s rapidly expanding language skills.
“What was most striking was that the maturation of these white matter pathways matched nicely with previous gray matter findings and behavioral timelines of grammar acquisition reported in the literature during this period,” Klein said.
Although this study provides detailed insight into early brain development, it has certain limitations. The exact structure of the brain that processes grammar before the age of 4 is still unknown, as no association between grammar skills and the assessed pathways was shown in 3-year-olds. Scanning the brains of young children is notoriously difficult, creating a major hurdle for future research.
Additionally, this study only assessed word-level grammar, specifically how children pluralize nouns. Because children learn nouns and verbs at different rates, their brains may process verb rules differently.
“It’s important to remember that this is basic research,” Klein said. “The immediate importance lies in furthering our understanding of how the brain changes during typical language development. Grammar acquisition is a fundamental language skill, so understanding the specific structural networks that support it may also advance our future understanding of developmental delays and atypical language development.”
The researchers suggest that future studies should examine sentence-level grammar and investigate whether similar brain pathways help children learn how to use verbs. Scientists will also be able to see how other brain networks, beyond the core language centers, interact to support early language learning in young children.
“We would like to acknowledge the tremendous effort required to successfully conduct neuroimaging studies with such a young group of participants,” Klein added. “This research would not have been possible without the support of families interested in advancing our understanding of human brain development.”
The study, “Grammar acquisition in preschool children is associated with white matter maturation of the dorsal language network,” was authored by Cheslie C. Klein, Philipp Berger, Charlotte Grosse Wiesmann, and Angela D. Friederici.
