Teenagers who seem to stop listening to their parents are the very image of youth rebelliousness, but new brain research tells us that something more interesting—and completely unconscious—is going on.

In addition to any oppositional tendencies that youths may start to show around the age of puberty, they are also experiencing a previously undiscovered shift in the neurobiology of the brain. This shift, a normal and “adaptive” one that supports the inevitable separation that takes place between children and their parents, disposes teens of both sexes to express more interest and engagement with the voices of non-family members than the voice of their own mother.

This is one of the central findings of a study led by 2018 BBRF Young Investigator Daniel A. Abrams, Ph.D., and 1998 BBRF Young Investigator Vinod Menon, Ph.D., both of Stanford University. It extends findings they and colleagues made several years ago: that children 12 and under are able to recognize the sound of their mother’s voice with an accuracy exceeding 97%, and that the mother’s voice activates a number of brain areas, as visualized in functional brain imaging scans. These areas, in addition to the auditory cortex, which processes sound, include the reward centers of the brain, emotion-processing regions, and brain networks that enable individuals to determine when incoming information is salient, i.e., particularly worthy of attention.

Regarding the new findings, which compare the results of auditory tests in children with those of youths aged 13-16, Dr. Abrams explains: “Just as an infant knows to tune into her mother’s voice, an adolescent knows to tune into novel voices” of people not in one’s own family. “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.”

Dr. Menon stresses the utility of this transition, from the point of view of becoming an independent person: “The process of a child’s becoming independent is precipitated by an underlying biological signal. That’s what we 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.”

Drs. Abrams, Menon and colleagues say that their study not only helps us understand an important event underlying normal development; it also provides a template for analyzing the patterns of brain activation and development in individuals diagnosed with social and communications difficulties, such as children diagnosed with autism spectrum disorder (ASD).

To make their discovery, the team compared results in 24 children aged 7-12 with those in 22 adolescents aged 13-16. All were recruited from schools near Stanford University, and all were the biological offspring of mothers whose voices were recorded while fMRI imaging and behavioral experiments were conducted.

Each mother was recorded while uttering sentences containing what the team calls nonsense words—words with no meaning that are intended to generate no specific semantic or emotional associations in the minds of the children. The purpose was to gauge how accurately the children recognized their mother’s voices, discriminating them from the voices of two unfamiliar women who were recorded uttering the same words; and to assess, via neuroimaging, what parts of the brain were activated when the children heard and identified their own mother’s voice as compared with the voices of the women they did not know (who were also mothers, but strangers to the children).

These tests revealed that in response to their mother’s voice, as compared with the voices of unfamiliar women, younger children showed greater neural activity in the nucleus accumbens of the reward circuit and the ventromedial prefrontal cortex (vmPFC) of the brain’s default mode network, which is associated with social valuation. It was the reverse in adolescents, who showed greater neural activation in response to nonfamiliar voices compared with those of their own mothers. The team considers this result to be the neural correlate of changes in social orientation that occur during adolescent development.

The results in the older children were consistent with and echoed observations that have previously been made about adolescents: that they show a heightened sensitivity to novelty in reward systems; and they are more sensitive to social signals than they were earlier in life. “Tuning in to perceptual aspects of social information such as human vocal stimuli during adolescence may serve as a critical precursor for increased higher-order social cognitive processing, including understanding the perspectives and intentions of others during interactions,” the team wrote in its paper appearing in the Journal of Neuroscience.

The mother’s voice, being unique and biologically salient, is associated by infants and young children with social and language learning, and is engaged by increased neural selectivity in reward processing regions of the brain. The new study, says the team, is the first to demonstrate that adolescents show the opposite effect, with increased neural sensitivity in reward regions for the voices of people who are not only not their own mothers but entirely outside the family circle.