Newborns whose mothers spoke multiple languages are more sensitive to sounds

It is known that babies in the womb hear and learn speech, at least in the third trimester. For example, newborns already prefer their mother's voice, recognize a story that was told to them many times during pregnancy, and distinguish their mother's native language.

Until now, however, it was unknown how developing fetuses learn language when their mothers speak to them in multiple languages. This is quite common: there are 3.3 billion bilinguals in the world (43% of the population), and in many countries bilingualism or multilingualism is the norm.

"We have shown that exposure to monolingual or bilingual speech has different effects on the 'neural coding' of pitch and vowel sounds in newborns: that is, how information about these aspects of speech is initially acquired by the fetus," said Dr Natalia Gorina-Caret, a researcher at the Institute of Neurosciences at the University of Barcelona and one of the first authors of the new study, published in Frontiers in Human Neuroscience.

"At birth, infants of bilingual mothers appear to be more sensitive to a wider range of acoustic variations in speech, whereas infants of monolingual mothers appear to be more selectively tuned to the one language in which they were exposed."

The study was conducted in polyglot Catalonia, where 12% of the population regularly speaks both Catalan and Spanish. The researchers recruited mothers of 131 newborns (including two sets of twins) at the Sant Joan de Déu Children's Hospital in Barcelona.

Of these mothers, 41% responded in the questionnaire that they spoke exclusively Catalan (9%) or Spanish (91%) during pregnancy, including conversations with the growing fetus. The remaining 59% were bilingual (at least 20% of the time in the second language): either Spanish and Catalan, or a combination of one of these languages with languages such as Arabic, English, Romanian or Portuguese.

"Languages differ in the temporal aspects of speech, such as rhythm and accentuation, as well as in pitch and phonetic information. This means that the offspring of bilingual mothers are probably immersed in a more complex acoustic environment than the offspring of monolingual mothers," says Dr. Carles Esera, a professor at the same institute and one of the corresponding authors.

The researchers placed electrodes on the infants' foreheads to measure a specific type of electrophysiological brain response, the frequency-following response (FFR), to repeated playback of a carefully selected 250-millisecond-long sound stimulus consisting of four stages: an /o/ vowel, a transition, an /a/ vowel at a constant pitch, and an /a/ rising in pitch.

"The contrasting vowels /o/ and /a/ are part of the phonetic repertoire of both Spanish and Catalan, which partly explains their choice," explains one of the study's first authors, Dr. Sonia Arenilas-Alcón, from the same institute. "Low-frequency sounds such as these vowels are also transmitted quite well through the womb, unlike mid- and high-frequency sounds, which reach the fetus in a distorted and weakened form."

FFR measures how accurately the electrical signals produced by neurons in the auditory cortex and brainstem mimic the sound waves of a stimulus. A clearer FFR is evidence that the brain has been more effectively trained to perceive that particular sound. For example, FFR can be used as a measure of auditory learning, language experience, and musical training.

The authors showed that the FFR for the production of the sound /oa/ was more distinct, that is, better defined and with a higher signal-to-noise ratio, in neonates of monolingual mothers than in neonates of bilingual mothers.

These results suggest that the brains of fetuses with monolingual mothers have learned to become maximally sensitive to the pitch of one language. In contrast, the brains of fetuses with bilingual mothers seem to have become sensitive to a wider range of pitch frequencies, but without generating a maximal response to any one of them. Thus, there may be a trade-off between efficiency and selectivity in pitch learning.

"Our data show that prenatal language exposure modulates the neural encoding of speech sounds as measured at birth. These results highlight the importance of prenatal language exposure for the encoding of speech sounds at birth and provide new insights into its effects," Esera said.

Corresponding author Dr Jordi Costa Faidella, an associate professor at the same institute, cautioned: "Based on our results, we cannot make any recommendations for multilingual parents. The sensitive period for language acquisition continues long after birth, and so postnatal experience can easily overshadow the initial changes that occurred in the womb. Further research into how the bilingual language environment modulates sound encoding during the first years of life will shed more light on this issue."