Newborns whose mothers spoke multiple languages are more sensitive to sounds

It is known that children 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 repeatedly during pregnancy, and recognize their mother's native language.

However, it was until now unknown how developing fetuses learn to speak when their mothers speak multiple languages to them. 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 voice pitch and vowel sounds in newborns: that is, how information about these aspects of speech is initially acquired by the fetus,” says Dr Natalia Gorina-Kareta, a researcher at the Institute of Neurosciences of the University of Barcelona and one of the first authors of a new study published in Frontiers in Human Neuroscience.

"At birth, newborns of bilingual mothers seem to be more sensitive to a wider range of acoustic variations in speech, while newborns of monolingual mothers seem to be more selective for the single language in which they have been immersed."

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

Researchers placed electrodes on 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 sound stimulus consisting of four stages: vowel /o/, transition, vowel /a/ at a constant height and /a/ rising in height.

“The contrasting vowels /o/ and /a/ are part of the phonetic repertoire of both Spanish and Catalan, which partly explains their choice,” explains study co-first author Dr. Sonia Arenilas-Alcón from the same institute. "Low-frequency sounds like 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 closely 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 the degree of auditory learning, language experience, and musical training.

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

These results suggest that the brains of fetuses from monolingual mothers have learned to become maximally sensitive to the pitch of one tongue. While the brains of fetuses from bilingual mothers appeared to become sensitive to a wider range of pitch frequencies, but without producing a maximum response to any 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, 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 therefore postnatal experiences 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 question."