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The same words can mean very different things depending on intonation or rhythm. According to a new study, the same brain networks that enable human adults to decipher the emotional content of vocalizations is at play in infants as young as six months old.

Parents are well aware that their children are able to recognize if they’re happy or angry well before they can learn to speak. In adults, emotional content is processed in the frontal and temporal lobes, but it was never clear if such was the case in infants, too. Previous work that relied on MRI machines to scan the brains of infants proved to be challenging because of the highly disturbing noise.

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Researchers at the University of Manchester, UK, solved this problem by employing a non-invasive brain imaging method called functional near-infrared spectroscopy. This brain imaging technique involves measuring blood flow to cortical areas.

In an experiment, infants listened to recorded non-speech vocalizations while sitting in their mothers’ laps. The vocalizations were either angry, happy, or neutral in their emotional content.

The researchers also studied the same mother-infant pairs during normal activities such as floor play, carefully quantifying the mother’s interactions with her child. Specifically, the researchers were interested in the degree to which the mother sought to control her infant’s behavior and how sensitive the infant’s behavior was to the mother’s commands.

Both angry and happy vocalizations were found to activate the same fronto-cortical network as in adults. Angry vocalizations elicited the highest level of activation in this brain region, and increased with the mother’s degree of control. This suggests that caregiving can heighten an infant’s sensitivity to angry vocalizations as well as the stress they produce.

“Brain science shows that babies’ brains are sensitive to different emotional tones they hear in voices. Such tones can cause different activation patterns in the infant’s brain areas which are also known to be involved in processing voices in adults and older children. These patterns also reveal that the early care experienced by babies can influence brain responses so that the more intrusive and demanding their mother, the stronger the brain response of these 6-month-olds is to hearing angry voices,” said Chen Zhao, lead author of the new study published in the journal PLOS ONE.