Babies can discern a voice's emotional state by 7 months old, and they use adult-like mental processes to do it, suggests a new study.
The finding underscores the evolutionary importance of emotional communication -- "survival of the fittest" could, in part, be survival of the best empathizer.
Previous research has shown that babies develop sound preferences before they are even born. Newborns as young as a couple days old prefer voices to random sounds, and can distinguish their own mother's voice from that of another female's.
But the tactics of newborns are different than the mental processes used by adults, said the current study's lead researcher Tobias Grossmann of Birkbeck College in London working in collaboration with colleagues at the Max Planck Institute for Human Cognitive and Brain Sciences in Germany.
Newborns use the "baby teeth" of the brain to process sound until adult tools develop.
To determine when adult processes come into use, researchers examined infants 4 and 7 months of age. The babies sat on their caregivers' laps in a dim room, while audio clips of voices and non-vocal sounds were played over a loudspeaker.
With the help of a comfy and technologically cutting-edge hat, their brains were monitored with near-infrared spectroscopy (NIRS), a non-invasive technique that measures the amount of oxygen being used by different areas of the brain. Just as legs need more oxygen when running, areas of the brain increase their oxygen intake when exercising by, say, processing a sound. NIRS captures these changes in oxygen concentration in a localized manner, allowing researchers to see which brain region is doing the work.
The brains of the 7-month-old infants, but not those of the 4-month-olds, responded to voice clips like adult brains, using the same regions of the brain as adults. This suggests that between 4 and 7 months, adult-like processes for attending voices start taking over, Grossmann said.
Next, the researchers added emotion to the vocal clips by using strings of emotionally-loaded words said with inflection. Again, the older infant brains reacted like adult brains.
A small region within the temporal lobe, which is associated with hearing, responded more strongly to furious voices than any other sound. But the frontal cortex, which is involved with higher-order functions such as decision-making and planning, was more attuned to joyful tones than angry ones.
"Happy speech may be processed deeper than other types of speech," Grossmann said.
The finding might give insight into the value of baby talk, also called "motherese," suggest the researchers. Motherese, when compared with adult conversation, is generally slow in tempo, rises and falls in pitch and, most importantly for this study, has cheery inflections. Motherese has previously been found to aid an infant's mental development, especially the learning of language.
"A happy tone of voice does something special to the baby's brain," Grossman said.
The ability to prioritize sounds of both rage and joy was likely selected for by evolution. Anger signals a potential threat, while attending to friendly voices might help us learn about the world, form attachments and find advantages in relatively stable conditions, Grossmann said. More research is needed to firmly link the brain responses measured in this study to behavior, he noted.
Being able to discern emotions, he said, "may have enabled the development of more sophisticated social processes." And it was likely critical to our success as a species.
The study is detailed in the March 25 issue of the journal Neuron.