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  • A new study suggests that sound may influence fetal development earlier than previously thought, potentially before the ear canal even opens.
  • The researchers found that deaf newborn mice had different brain patterns than mice with regular hearing. These changes occurred around a week earlier than anticipated, based on current developmental knowledge.
  • In the study, exposure to sound also encouraged more diverse cortical connections in newborn mice than mice housed in silent enclosures. And the mice not exposed to sound developed similar brain patterns to deaf mice.
  • If accurate, these findings could help uncover new ways to identify and potentially treat hearing disorders and other sensory conditions earlier in life.

Many people expecting a child wonder whether they should introduce their developing baby to sounds such as music.

For this reason, scientists have been working to reveal the intricacies of how parts of the brain that process sensory information develop.

In a new study, researchers set out to assess the impact of sound signals on subplate neurons. These are among the first to develop in the brain, but they die off before birth or shortly afterward.

The team also wanted to see whether changes in sound signals would alter brain circuits earlier than previously expected.

“As scientists, we are looking for answers to basic questions about how we become who we are,” says senior study author Dr. Patrick Kanold, a professor of biomedical engineering at Johns Hopkins University.

“Specifically, I am looking at how our sensory environment shapes us and how early in fetal development this starts happening,” he adds.

In the future, these findings may make it easier to track, assess, and potentially correct irregular brain activity and patterns associated with congenital sensory conditions.

The study, conducted by researchers from Johns Hopkins, in Baltimore, MD, and the University of Maryland, in College Park, appears in Science Advances.

Some research suggests that a fetus may start to hear noises such as bodily sounds at around 18 weeks of pregnancy. Fetal hearing should be fully developed between weeks 26 and 29.

From this point on, a developing baby may react to outside noises by moving, or their pulse may increase. In response to sudden or loud sounds, they may also cry or become startled.

After around 32 weeks of gestation, developing babies may begin to recognize some vowel sounds and music. Music with a beat that mimics the human heartbeat, around 60 beats per minute, may be the most beneficial.

But researchers are still not exactly sure when a fetus develops the ability to fully process sensory information such as sound or how sound exposure influences development.

This makes it harder to diagnose and treat congenital sensory disorders.

The uncertainty may also be frustrating for people looking for additional ways to nurture their developing babies. Researchers have been trying to figure out how exposure to sounds such as music or speaking shapes fetal development for decades.

Some studies show that early exposure to music may help encourage early socioemotional development and sound perception.

Research also suggests that prenatal exposure to music and noise may increase the thickness and growth rate of cells in parts of the brain that process sensory information. Other research supports the idea that developing babies may be able to recognize voices that they hear frequently.

The new study, in newborn mice, suggests that fetal development may be affected by sound earlier than previously thought.

The findings also indicate that fetal sound exposure may affect the number, diversity, and strength of very early neural connections, which may shape important aspects of neurodevelopment.

“In these mice, we see that the difference in early sound experience leaves a trace in the brain, and this exposure to sound may be important for neurodevelopment,” says Dr. Kanold.

The team’s new work explores the impact of sound exposure on subplate neurons in the cortex — the white portion of the brain that processes sensory information, among its other vital functions.

Subplate neurons are among the first brain cells to develop. These cells die off before birth and in the following months.

Before they die, subplate neurons create connective bridges so that sensory information can travel between the thalamus and middle cortex. The thalamus is the region of the brain that transmits sensory signals to the cortex for interpretation.

In previous work with ferrets, Dr. Kanold and his team found that subplate neurons are the first type of cortical neuron capable of receiving sound signals.

In the first part of the new study, the researchers discovered that genetically engineered week-old deaf mice had more subplate and other cortical neural connections than mice without hearing loss that had been raised in a regular environment. These changes developed around a week earlier than expected.

Previously, researchers thought sensory signals could only change cortical connections after neurons from the thalamus had reached out to connect with the cortex, which coincides with the opening of the ear canal.

“When neurons are deprived of input, such as sound, the neurons reach out to find other neurons, possibly to compensate for the lack of sound,” Dr. Kanold explains.

“This is happening a week earlier than we thought it would and tells us that the lack of sound likely reorganizes connections in the immature cortex.”

– Dr. Patrick Kanold

Next, the team tested whether additional sound exposure would impact early neural development. To do this, they kept some 2-day-old mice with regular hearing in a quiet enclosure and others in a quiet enclosure with a small speaker that beeped.

The mouse pups kept in the enclosure with the beeping had stronger connections between subplate and cortical neurons than the mice not exposed to the beeping. But, as the researchers note, this difference was less significant than that between the deaf mice and those with regular hearing.

The mouse pups exposed to the beeping also had greater diversity in the type of subplate-cortical connections than the pups kept in quiet enclosures. The mice with regular hearing who were kept in quiet enclosures also had similar subplate-cortical connections to the genetically engineered deaf mice.

Confirming these findings will require replication on a larger scale and in human subjects. And in the future, the research team plans to assess how sound shapes brain development later on.

The team’s ultimate goal is to identify the role and significance of fetal sound exposure in development. Specifically, the researchers aim to discover more about brain pattern changes related to congenital deafness to help fit cochlear implants in infants.

Another goal is to develop biomarkers to identify problems involving subplate neuron connection abnormalities. They also plan to explore brain patterns in infants born prematurely.