HOW SOUND AFFECTS DEVELOPMENT IN HUMANS

At 25 weeks of gestation, human foetuses can begin to respond to auditory stimuli, meaning that most foetuses can hear well before they are born during a period of critical brain development when neural connections are first laid down. Indeed, babies are born able to recognise their mother’s voices, and exposure to ambient sound in utero has been linked to healthy brain development.

But while it’s more straightforward to study species that lay eggs – which can be moved, manipulated, and measured relatively easily – it’s harder to determine how sound affects babies in the womb.

One approach is to study infants who are born prematurely. Although an incubator in the neonatal intensive care unit (NICU) is vastly different from a womb, researchers can measure how babies respond to their environments while they are still within their 40 weeks of development.

Amir Lahav, formerly a paediatric neuroscientist at Harvard Medical School, US, came to this realisation in 2007, when his thenwife gave birth to twins prematurely at 25 weeks. “I went to the NICU as a parent for the first time, and I was shocked by the amount of noise, by the alarms and monitors and wires and trash cans,” he recalls. He approached the head of neonatal care and suggested an unofficial study – Lahav wanted to record his wife’s voice, transform the audio to mimic how it might sound in utero, and play it to his twins. While the results were preliminary and did not include controls, “the medical team was amazed how my kids skipped every possible complication they would anticipate for babies born that early”, including breathing problems, sepsis, brain haemorrhaging, and death.

Based on that outcome, Lahav and his colleagues designed another experiment, this time with 40 premature infants. Four times a day for a month, newborns heard either muffled, “wombified” recordings of their mothers’ voices and heartbeats, or the ambient sound of a bustling NICU. Afterward, the team imaged the infants’ brains using cranial ultrasonography during a routine health check. Compared with the controls, the babies who heard maternal sounds had significantly larger auditory cortexes, an area of the brain involved in hearing and language development. The results “show the benefits of maternal sounds on the brain, at least structurally”, says Lahav.

Geneva University developmental neuroscientist Petra Hüppi is investigating how sounds early in development affect the infant brain – specifically, she’s looking at connections between regions such as the amygdala, hippocampus, and orbito-frontal cortex. To do so, she uses music, which activates multiple regions involved with auditory, sensory and emotional processing. “Music has a particular effect on humans that is distinct from the response to language and voices,” she says. “It’s still not fully understood what it is.”

In 2020, Hüppi partnered with awardwinning composer Andreas Vollenweider to create music for babies, as chosen by babies. Vollenweider brought a veritable orchestra into the NICU and played each instrument for the infants when they were waking up, falling asleep, or active in their incubators. Based on visual observations and measurements of the babies’ heart rates and eye movements, the team created soundscapes of what the infants liked best – primarily harp, snake flute and bells.

Hüppi and her collaborators then split a cohort of 30 premature NICU infants into two groups, half of whom heard the soundscapes five times a week and half of whom received the usual standard of care, and used magnetic resonance imaging (MRI) to compare their brain development to 15 fullterm babies. At the end of the experiment, the brains of NICU babies who heard the music more closely matched those of babies born at full term than did the brains of the premature controls: the music-exposed babies’ white matter was more fully developed, their amygdalas were larger, and the connections between regions in the brain that process acoustic and emotional stimuli were stronger.

The NICU environment, both Lahav and Hüppi agree, warrants further study. Sound could partly explain why children born prematurely have a higher incidence of behavioural or attention-related issues such as ADHD, autism, aggression, or anxiety. For NICU babies who spend weeks in an incubator,

“the primary stimulation is noise,” Lahav says. As a result, he adds, “the brain learns that noise is the most important thing in life,” possibly making it harder to tune out background noise and focus on the task at hand.