Language processing in the brain across diverse languages and speakers

Abstract

A staggering 7,000 languages are spoken and signed around the world. Furthermore, the majority of the world’s population speak two or more languages. Nevertheless, language research –especially in neuroscience – has been limited to primarily monolingual speakers of a couple dozen languages, which makes our understanding of how the language system works incomplete and potentially biased. In my dissertation, I use a precision fMRI approach to paint a more complete picture of language processing in the brain by studying both diverse languages and diverse speakers. Language processing is supported by a set of inter-connected left-lateralized frontal and temporal areas that are jointly referred to as the ‘language network’. First, I asked whether core findings about this network that have come from studies of on native speakers of English and a few other languages generalize to speakers of typologically diverse languages. I found that four key properties of the language network – its i) topography, ii) left-hemispheric lateralization, iii) selectivity for language relative to non-linguistic inputs and tasks, and iv) strong functional connectivity are robustly present across 45 languages spanning 11 language families (Malik-Moraleda, Ayyash et al., 2022, Nature Neuroscience). I further extended these results to constructed languages (conlangs), such as Esperanto and Klingon. Constructed languages differ from natural languages in that they were created much more recently, often by a single individual and for diverse (sometimes esoteric) purposes, and are not shaped by learning and processing pressures, at least not to the same degree. I tested neural responses during the processing of five conlangs in proficient speakers, including Esperanto, Klingon, Na'vi, High Valyrian and Dothraki. All constructed languages recruited the same brain areas as natural languages, suggesting that constructed and natural languages share critical features despite their differences, thus constraining the definition of what constitutes a ‘language’ (Malik-Moraleda et al., 2023, under review). Second, I investigated the neural architecture of bilingual and monolingual speakers. Contra claims that some frontal brain regions in bilingual individuals support both language processing and domain-general executive functions (e.g., Garbin et al, 2010, Coderre et al., 2016), I found no overlap between the language and the executive control networks in either monolingual speakers (replicating past work) or bilingual speakers (Malik-Moraleda et al., in prep). However, I found that bilinguals engaged the executive control network to a greater extent than the monolingual speakers during a demanding spatial working memory task (Malik-Moraleda et al., 2021, Neurobiology of Language). Although this result is in line with the controversial claim that bilinguals exhibit superior executive functions than monolingual speakers (Bialystok, 2001), it remains difficult to link it to bilingual experience specifically, rather than to other features that may differ between these populations. Last, I investigated the language network of polyglots – individuals who have some degree of proficiency in five or more languages. In a group of 34 polyglots, including 16 hyperpolyglots with some knowledge of 10+ languages, I found that all languages—including the participants’ native language, non-native languages of varying proficiency, and even unfamiliar languages consistently engaged all areas of the language network. Moreover, the languages that participants rated as higher-proficiency elicited a stronger response, which plausibly reflects greater engagement of linguistic computations. The exception was the native language, which elicited a lower response than a non-native language of similar proficiency. These results contribute to our understanding of how multiple languages co-exist within a single brain (Malik-Moraleda, Jouravlev et al., 2024, Cerebral Cortex).