In research which reframes hearing loss as a brain-development issue, not just an ear issue, neuroscientists have found infants born deaf or hard of hearing show adverse changes in how their brains organise and specialise.
But the study also indicated exposure to sound and language may help them develop more normally.
It found infants with sensorineural hearing loss (SNHL) lacked the usual pattern of organisation on the brain’s left side, which supports language and higher cognitive skills.
Findings suggested 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 organisation,” the researchers 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 Heather Bortfeld, Professor of Psychological Sciences and Cognitive and information Sciences at the University of California, Merced, and Professor Haijing Niu of Beijing Normal University. It was published in Science Advances on 15 October 2025.
The examined 112 infants aged three to nine months, including 52 with congenital hearing loss and 60 with typical hearing.
Using a non-invasive 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 organisation, a sign of efficient brain function. But unlike typically hearing infants, those with SNHL did not develop stronger left-hemisphere specialisation, which is normally linked to early language and cognitive growth.
The difference was most pronounced in babies with moderate to profound hearing loss, while those with mild loss retained some normal patterns of left-hemisphere activity.
They said brain asymmetry – the tendency of certain functions to concentrate in one hemisphere – supported 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.
But this specialisation could falter when auditory or language input was missing.
Previous research found deaf infants with deaf parents who grow up with sign language still develop normal left-hemisphere organisation, 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 researchers said. “The brain needs structured input to build the networks that will later support communication and learning.”
The authors said 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 could help reinforce neural pathways that otherwise might weaken or reorganise abnormally, they added.
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 if early hearing and language interventions can normalise brain asymmetry and support later language and cognitive outcomes.
They called for studies that combined fNIRS with magnetic resonance imaging 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.”
