A new study published in Science Advances reveals that infants born with sensorineural hearing loss (SNHL) show significant disruptions in how their brains organize and specialize during the first months of life.
The findings highlight how the absence of early auditory input can alter brain network asymmetry—one of the key hallmarks of normal cognitive and language development.
The research team, led by Professor Heather Bortfeld of the University of California, Merced, and Professor Haijing Niu of Beijing Normal University, analyzed brain activity in 112 infants aged 3 to 9 months, including 52 born with congenital SNHL and 60 with typical hearing. Using a noninvasive imaging method known as functional near-infrared spectroscopy (fNIRS), the team examined how efficiently different regions of each infant’s brain communicated with one another.
“Auditory exposure plays a crucial role in shaping brain development,” the researchers said, “but little is known about whether and how an initial lack of auditory exposure might disrupt the development of functional network lateralization.”
Early Disruptions in Brain Asymmetry
The researchers found that both infants with and without hearing loss showed what are known as “small-world” network characteristics—an indicator of efficient overall brain organization. However, infants with typical hearing developed a clear left-hemisphere dominance associated with language and higher cognitive skills, while those with SNHL did not.
“Unlike typically developing controls, who showed an age-related leftward lateralization of network efficiencies, SNHL infants did not exhibit the emergence of hemispheric asymmetry,” the researchers reported.
This difference became especially apparent in the frontal region of the brain. In normally hearing infants, left-hemisphere network efficiency increased steadily with age, reflecting a developmental shift toward specialization for language processing. In contrast, infants with SNHL showed no such age-related increase.
“The disrupted development of network global and local efficiencies in the left hemisphere ultimately led to a developmental deficiency such that no leftward asymmetry emerged in infants with congenital SNHL”
The findings suggest that even mild hearing loss can affect how brain circuits organize. “Lateralization of frontal efficiency was preserved in SNHL infants with mild hearing loss but declined significantly with increasing severity,” they wrote, indicating that the degree of auditory deprivation closely correlates with the degree of neural disruption.
Experience Shapes the Infant Brain
The study underscores that early sensory and linguistic input are key drivers of brain development. According to the researchers, hemispheric specialization—where the left side of the brain takes on language and symbolic reasoning—normally emerges within the first months of life. Without access to sound or language, this process falters.
“Without auditory input to support these maturational mechanisms, functional differentiation between the hemispheres may be delayed or even permanently altered,” they noted.
Importantly, the study’s authors stress that it is not just sound, but language exposure itself—spoken or signed—that helps maintain normal neural growth. “Although SNHL restricts access to sound, it does not necessarily hinder language development,” they wrote, pointing out that deaf infants who are exposed to sign language from birth typically show normal left-hemisphere organization.
“Having or not having access to language has a distinct and profound impact on both brain and behavioral development,” the researchers said.
These findings reinforce that the brain’s early development depends on structured, meaningful input—whether through hearing aids, cochlear implants, or visual-language exposure.
Severity Matters
The study also explored how the extent of hearing loss influences brain development. Infants with mild hearing loss retained typical left-hemisphere specialization, while those with moderate-to-profound loss showed a marked reduction or absence of asymmetry.
“As hearing impairment increased, nodal efficiency asymmetry in the frontal regions significantly decreased, indicating reduced leftward lateralization,” the researchers found.
This relationship between auditory input and neural organization highlights why early detection and intervention are critical. The authors note that early-life brain development is a “critical period” when the neural circuits underlying communication are especially malleable—and vulnerable.
“Adequate auditory exposure is vital for this aspect of early brain development,” the study said. “This aligns with evidence that congenital hearing loss disrupts the development of inter-regional connectivity, which may result in delayed or atypical language acquisition.”
Looking Ahead: Intervention and Long-Term Outcomes
While the study provides strong evidence of altered brain organization in infants with hearing loss, the authors caution that their research only captured a single point in time. They plan to follow infants longitudinally to determine whether early intervention—through hearing technology or language exposure—can normalize these developmental patterns.
“Longitudinal studies are necessary to determine whether the observed disruptions in cerebral lateralization persist, worsen, or resolve over time,” the researchers wrote.
They also suggest combining fNIRS with other imaging tools such as MRI and EEG to gain a more comprehensive view of how sensory and language experiences shape early brain development.
Ultimately, the findings challenge the perception of hearing loss as a purely auditory issue. As the researchers concluded:
“This work reframes hearing loss as a brain-development issue, not just an ear issue. We now know that timely access to sound and language is key to keeping the brain’s communication networks on track.”
Citation
Liu, G., Zhou, X., Hu, Z., Liu, Y., Huo, E., Bortfeld, H., Dong, Q., Chen, C., Liu, H., & Niu, H. (2025). Developmental alterations in brain network asymmetry in 3- to 9-month infants with congenital sensorineural hearing loss. Science Advances, 11(41), eadx1327. https://doi.org/10.1126/sciadv.adx1327
