Infant brains show capacity to listen amid competing noise

New research offers the first neurophysiological evidence that infants as young as 7 months can use the spatial location of sound to help separate a target voice from competing background noise. The findings indicate that this key ability that helps people listen in noisy environments is present, though still developing, well before infants fully understand spoken language. 

The phenomenon being studied, known as spatial release from masking, is one that adults typically experience without realizing. Senior study author Bonnie Lau explained: 

“Imagine standing at a noisy party trying to talk with someone nearby. If the surrounding chatter moves farther away, or if you and your conversation partner step aside, it suddenly becomes easier to hear each other even though neither person's voice has changed. That spatial separation between the message and the surrounding noise, the masker, is what makes the difference.”  

Lau is a research assistant professor of otolaryngology at the University of Washington School of Medicine.  

This listening capability has been documented in numerous species, including songbirds and ferrets. Lau and colleagues were studying whether the capability exists in the infant human brain. Such a finding supports the notion that it is a fundamental biological survival strategy. 

Researchers compared electroencephalography (EEG) recordings from 53 infants (ages 7 and 11 months) as well as from 20 adults who served as a mature-brain comparison group. Study subjects heard baby talk from female voices projected through speakers. The voice was presented in three conditions: alone without competing noise; with competing babble noise presented from the same direction; and with the same babble noise spatially separated to the sides.   

The researchers measured how accurately each participant's brain activity could be predicted from the speech signal. This measure does not require a behavioral response and is well suited to infant populations.  

Infants at both 7 and 11 months reliably tracked target speech across all three listening conditions, including in the presence of competing noise. The study authors asserted that this represents the first demonstration of infant speech tracking beyond quiet listening conditions, which had been the focus of prior infant EEG studies. 

"This dataset shows that, from a very young age, the human brain is wired to make use of spatial cues," Lau said. "This is one of the mechanisms that even a 7-month-old brain is able to make use of — an evolutionarily conserved mechanism for sorting out where different sounds are coming from and which ones to attend to." 

Adults demonstrated robust acoustic tracking broadly across their heads. Their tracking of target speech improved significantly when competing noise came from separate sides rather than from the same direction.  

Infants’ acoustic tracking ability was more narrowly confined to the front of their heads. The tracking ability was reduced by noise from the same direction but was restored to near-quiet levels when noise came from a side. Infants’ brain responses also showed a slight delay relative to those of adults. The authors said this finding is consistent with neural maturation during infancy. 

The results, published in the Journal of Neuroscience, have relevance to clinicians and early childhood educators. 

Many elementary classrooms contain clusters or pods of children speaking simultaneously. This group chatter is like the challenge of listening amid same-direction noise  described in the study. Understanding when and how spatial listening abilities mature could suggest ways to improve children’s attention and comprehension in those learning environments. 

In the clinical domain, audiologists and otolaryngologists routinely face decisions about whether to fit infants with one or two hearing aids or cochlear implants. They also need to determine whether to coordinate the devices’ function across both ears to help with localizing sounds and listening in noise.  

Natural hearing depends on the brain integrating input from both ears to localize sound — the same process that underlies effective masking to listen. If the capacity to use spatial cues is present from infancy rather than acquired through experience, clinicians have a reason  to consider coordinated bilateral devices and early intervention strategies, even before infants show understanding of spoken language, the authors suggested. 

The findings also could inform the treatment of hearing loss in one ear (unilateral), a relatively common condition that complicates listening in noisy settings such as classrooms, restaurants or public transit. 

The study’s co-first authors are Farhin Ahmed, postdoctoral fellow, and Katrina Zheng, research assistant, both in the UW Laboratory for Auditory Neuroscience and Development. 

The project was funded by the National Institute on Deafness and Other Communication Disorders (R01 DC022585, R00 DC016640, T32DC005361-21), part of the National Institutes of Health. 

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