Auditory cortex anatomy reflects multilingual phonological experience

Abstract

This study examines whether auditory cortex anatomy reflects multilingual experience, specifically individuals’ phonological repertoire. Using data from over 200 participants exposed to 1–7 languages across 36 languages, we analyzed the role of language experience and typological distances between languages they spoke in shaping neural signatures of multilingualism. Our findings reveal a negative relationship between the thickness of the left and right second transverse temporal gyrus (TTG) and participants’ degree of multilingualism. Models incorporating phoneme-level information in the language experience index explained the most variance in TTG thickness, suggesting that a more extensive and more phonologically diverse language experience is associated with thinner cortices in the second TTG. This pattern, consistent across two datasets, supports the idea of experience-driven pruning and neural efficiency. Our findings indicate that experience with typologically distant languages appear to impact the brain differently than those with similar languages. Moreover, they suggest that early auditory regions seem to represent phoneme-level cross-linguistic information, contrary to the most established models of language processing in the brain, which suggest that phonological processing happens in more lateral posterior superior temporal gyrus (STG) and superior temporal sulcus (STS).

Alternate abstract:

eLife digest

Our brains are dynamic organs that respond to, and are shaped by, our life experiences. Sometimes, this even results in alterations in the shape and size of their various regions.

The auditory cortex is one of the brain areas processing speech. In people who have mastered more than one tongue, it is also tasked with recognising and distinguishing between speech sounds from different languages.

Although previous research suggests that being bilingual can influence brain anatomy, these changes are still poorly understood. How different parts of the auditory cortex are structured, or how they work together has also remained unclear. Kepinska et al. therefore set out to determine if the structure of the brain's auditory cortex is shaped by language experience. To do so, they used an imaging technique known as structural MRI to take detailed pictures of the auditory cortex of over 200 people who each spoke between one and seven languages. In total, 36 different languages were represented across the entire group.

The scans showed that a specific part of the auditory cortex, called the second TTG, was thinner in people who spoke more languages. Further analysis revealed that some of this variation in TTG thickness was associated with the variety of speech sounds present in the languages that participants were familiar with: the more languages someone spoke, and the greater the sound differences between them, the thinner the second TTG. These results suggest that the auditory cortex is shaped by a process called ‘experience-driven efficiency’; in other words, the TTG needs less tissue to do its job in people who have more experience in different languages.

Going forward, Kepinska et al. hope that these findings may help refine our understanding of how the brain adapts to language exposure. This, in turn, could improve both educational interventions and clinical therapies, for example to help people with dyslexia or hearing impairments.