Bilingual Exposure and Sex Shape Developmental Trajectories of Brain Responses to Speech-Sound Features in Infants.

As the auditory brain becomes functional during the third trimester of pregnancy, both biological and environmental processes begin shaping its maturation, influencing how speech sounds are perceived. Biological factors, such as sex, introduce early genetic differences, while environmental experiences, like bilingualism, modulate the auditory input that infants receive. Although existing research highlights the impact of sex and bilingualism on the development of speech perception, the neural mechanisms remain unclear. In this study, we recorded frequency-following responses longitudinally, at birth, 6 months, and 12 months of age in 73 infants exposed to varying degrees of bilingual input. We modeled the developmental trajectories for neural encoding of voice pitch and speech formant structure, finding significant maturation during the first 6 months, followed by less pronounced change through the first year. Distinct developmental patterns emerged as a function of sex and bilingualism, revealing their influence on neural attunement to key speech-sound features. Female infants exhibited stronger neural encoding of both pitch and formant structure, depicting a distinctive quadratic trajectory that peaked at 6 months. Bilingual exposure notably predicted lower neural pitch encoding values at 6 months, but higher values by 12 months. A positive effect of bilingualism on speech formant encoding was observed throughout the first year. These findings reveal how biological and environmental factors contribute to individual variability in early auditory development and speech acquisition.