Infants born deaf or hard of hearing show adverse changes in how their brains organize and specialize, but exposure to sound and language may help them develop more normally, according to new research.
The study led by two neuroscientists found that infants with sensorineural hearing loss (SNHL) lacked the usual pattern of organization on the brain's left side, which supports language and higher cognitive skills.
The findings also suggest that early auditory stimulation through hearing aids or cochlear implants, along with exposure to language, whether spoken or signed, could help preserve normal brain development.
"The first year of life is a critical window for brain organization," the study said. "If infants miss auditory input or early language exposure during this period, the brain's left and right hemispheres may not develop their usual balance."
The study was spearheaded by Professor Heather Bortfeld of the University of California, Merced, and Professor Haijing Niu of Beijing Normal University. It was published in Science Advances.
The study examined 112 infants aged 3 to 9 months, including 52 with congenital hearing loss and 60 with typical hearing. Using a noninvasive imaging method called functional near-infrared spectroscopy (fNIRS), the researchers tracked how efficiently different regions of the brain communicated.
They found both groups had strong "small-world" network organization, a sign of efficient brain function. But unlike typically hearing infants, those with SNHL did not develop stronger left-hemisphere specialization, which is normally linked to early language and cognitive growth.
The difference was most pronounced in infants with moderate to profound hearing loss, while those with mild loss retained some normal patterns of left-hemisphere activity.
Brain asymmetry — the tendency of certain functions to concentrate in one hemisphere — supports the development of language, reasoning and memory. In an infant who hears normally, the left hemisphere becomes dominant for processing speech and symbolic communication within the first months of life.
This specialization can falter when auditory or language input is missing. Previous research shows that deaf infants who have deaf parents and grow up with sign language still develop normal left-hemisphere organization, demonstrating that language access, not sound alone, can drive healthy neural growth.
"Early exposure — whether through cochlear implants, hearing aids or sign language — is essential," the study said. "The brain needs structured input to build the networks that will later support communication and learning."
The authors emphasized that intervention should begin as early as possible, ideally within the first few months of life — the brain's period of maximum plasticity. Providing a rich linguistic environment can help reinforce neural pathways that otherwise might weaken or reorganize abnormally, they said.
While the study offers strong evidence of how hearing loss affects the infant brain, it only observed infants at one point in time. The researchers plan to follow children over longer periods to see whether early hearing and language interventions can normalize brain asymmetry and support later language and cognitive outcomes.
They also called for studies that combined fNIRS with magnetic resonance imaging and and electroencephalograms to map how sound, language and cognition interact in early development.
"This work reframes hearing loss as a brain-development issue, not just an ear issue," the study said. "We now know that timely access to sound and language is key to keeping the brain's communication networks on track."
