Teen brains no longer find moms’ voices uniquely rewarding starting around age 13, and start to tune into unfamiliar voices more, according to new research.

For the study in the Journal of Neuroscience, the researchers used functional MRI brain scans to give the first detailed neurobiological explanation for how teens begin to separate from their parents.

“Just as an infant knows to tune into her mother’s voice, an adolescent knows to tune into novel voices,” says lead author Daniel Abrams, clinical associate professor of psychiatry and behavioral sciences at Stanford University.

“As a teen, you don’t know you’re doing this. You’re just being you: You’ve got your friends and new companions and you want to spend time with them. Your mind is increasingly sensitive to and attracted to these unfamiliar voices.”

In some ways, teens’ brains are more receptive to all voices—including their mothers’—than the brains of children under 12, the researchers discovered, a finding that lines up with teenagers’ increased interest in many types of social signals.

However, in teenage brains, the reward circuits and the brain centers that prioritize important stimuli are more activated by unfamiliar voices than by those of their mothers. The brain’s shift toward new voices is an aspect of healthy maturation, the researchers say.

“A child becomes independent at some point, and that has to be precipitated by an underlying biological signal,” says senior author Vinod Menon, a professor of psychiatry and behavioral sciences. “That’s what we’ve uncovered: This is a signal that helps teens engage with the world and form connections which allow them to be socially adept outside their families.”

Listening to your mom

The team previously found that, in the brains of children 12 and under, hearing Mom’s voice triggers an explosion of unique responses: A study published in 2016 showed that kids can identify their moms’ voices with extremely high accuracy and that the special sound of Mom cues not just the brain’s auditory-processing areas, but also many areas not triggered by unfamiliar voices, including reward centers, emotion-processing regions, visual processing centers, and brain networks that decide which incoming information is salient.

“The mother’s voice is the sound source that teaches young kids all about the social-emotional world and language development,” says Percy Mistry, co-lead author and a research scholar in psychiatry and behavioral sciences. “Fetuses in utero can recognize their mother’s voice before they’re born, yet with adolescents—even though they’ve spent even more time with this sound source than babies have—their brains are tuning away from it in favor of voices they’ve never even heard.”

“When teens appear to be rebelling by not listening to their parents, it is because they are wired to pay more attention to voices outside their home.”

The new study built on the previous study, adding data from teenagers 13 to 16.5 years of age. All participants had an IQ of at least 80 and were being raised by their biological mothers. They did not have any neurological, psychiatric, or learning disorders.

The researchers recorded the teens’ mothers saying three nonsense words, which lasted just under a second. Using nonsense words ensured that the participants would not respond to the words’ meaning or emotional content. Two women unfamiliar with the study subjects were recorded saying the same nonsense words. Each teenage participant listened to several repetitions of the nonsense-word recordings by their own mother and the unfamiliar women, presented in random order, and identified when they heard their mom. Just like younger children, teens correctly identified their mothers’ voices more than 97% of the time.

The researchers then placed the teens in a magnetic resonance imaging scanner, where they listened again to the voice recordings. They also listened to brief recordings of household sounds, such as a dishwasher running, to allow the researchers to see how the brain responds to voices versus other non-social sounds.

Voices connect us

The researchers found that among teenagers, all voices elicited greater activation in several brain regions compared with younger children: the voice-selective superior temporal sulcus, an auditory processing area; salience processing regions that filter which information is important; and the posterior cingulate cortex, which is involved in aspects of autobiographical and social memory.

Brain responses to voices increased with teens’ age—in fact, the relationship was so strong researchers could use the voice-response information in teens’ brain scans to predict how old they were.

What distinguished teenagers from younger children was that unfamiliar voices elicited greater activity than Mom’s voice in the nucleus accumbens of the reward-processing system and in the ventromedial prefrontal cortex, a region involved in assigning value to social information.

The switch toward unfamiliar voices happened in these brain centers between 13 and 14 years of age, and there was no difference between boys and girls.

The research will help study what happens in the brains of adolescents with autism and other conditions that affect how they tune into voices and other social stimuli. Younger kids with autism don’t have as strong a brain response to their mothers’ voices as typically developing children, the researchers have found.

The team is excited to have uncovered the foundations of teens’ ability to tune into new people, an important part of humans’ overall engagement with voices. The fact that the brain is so attuned to voices makes intuitive sense—just ask anyone who has ever felt an emotional jolt at hearing a friend’s or family member’s voice after a long time, the researchers say.

“The voices in our environment are this incredibly rewarding sound source that allow us to feel connected, included, part of a community and part of a family,” Abrams says. “Voices are really what connect us.”

Children’s social interactions undergo a major transformation during adolescence. “Our findings demonstrate that this process is rooted in neurobiological changes,” Menon says. “When teens appear to be rebelling by not listening to their parents, it is because they are wired to pay more attention to voices outside their home.”

The National Institutes of Health, the Brain and Behavior Research Foundation, the Singer Foundation, and the Simons Foundation/SFARI, and Stanford’s department of psychiatry and behavioral sciences supported the work.

Source: Stanford University