Neural Encoding of Continuous Attended Speech in Monolingual and Bilingual Listeners
Creators: name: Olguin Chau, Andrea Katerina
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: attention
bilingualism
neural encoding
cortical entrainment
attention to speech
continuous speech
electroencephalography
typological similarity
early bilinguals
late bilinguals
Representational Similarity Analysis
selective attention
auditory attention
selective attention to speech
language similarity
second language age of acquisition

description: Humans are capable of learning multiple languages without major difficulty, especially at an early age. While this brings obvious advantages such as intercultural communication and enhanced career prospects, bilingualism has also been linked to changes to selective attention and inhibition of unwanted information. Although behavioural differences between monolinguals and bilinguals on tasks of selective attention remain controversial, the experience of learning and using a second language undoubtedly represents a major environmental demand that can impact the way the brain selects and processes information. Here, I investigate how bilingualism influences the neural mechanisms of selective attention to speech, and whether this is further affected by the age of acquisition of a second language, and typological similarity between the two languages. These questions are addressed in four experiments by investigating the neural encoding of continuous attended speech under different types of linguistic and non-linguistic interference in monolinguals, Spanish-English and Dutch-English early bilinguals, and Spanish-English late bilinguals. In a dichotic-listening paradigm that was kept constant across the four experiments, participants attended to a narrative in their native language presented to one ear, while ignoring interference presented to the other ear. Four different types of interference were presented to the unattended ear: a different narrative in their native language (Native-Native), a narrative in a language unknown to the listener (Native-Unknown), a well-matched non-linguistic acoustic interference (Native-Musical Rain), and no interference. The neural activity was recorded by a dense array 128-channel EEG system and cross-correlated with the speech envelopes for both attended and unattended streams for each participant group separately. Results were also directly compared across the groups using multivariate Representational Similarity Analysis (RSA). Monolingual results indicated that there was significantly more robust neural encoding for the attended envelopes than the ignored ones across all conditions. The type of interference significantly modulated the encoding of attended speech, with the strongest encoding seen when the interference was in the same known language and weakest when the interference was non-linguistic noise. Equivalent analyses on Spanish-English and Dutch-English early bilinguals also showed stronger neural encoding for attended than for unattended speech. However, in contrast to the results seen in monolinguals, the type of distractor did not modulate the strength of encoding of the attended stream in either of the bilingual groups. Results also showed a subtle difference in the neural encoding of attended and unattended speech between bilinguals who speak typologically similar languages (Dutch-English) and those who speak a combination of typologically less similar languages (Spanish-English). Experiment 4 tested the same effects on late Spanish- English bilinguals. Consistent with the early bilingual results, late bilinguals also showed stronger neural encoding for attended than for unattended speech and no enhancement of the attended signal depending on the type of interference. However, late bilinguals dissociated native language interference with a different time-course from early bilinguals, suggesting further modification of the top-down mechanisms of selective attention. Importantly, all these effects were observed in the absence of significant behavioural differences between the groups. Taken together, results indicate that bilingualism modulates the neural mechanisms of selective attention, while still providing the basis for optimal behavioural performance. This modulation is further shaped by the age of acquisition and the typological similarity of a bilingual’s two languages, which reflects life-long experience with resolving competition between more or less similar competitors. Findings from all four experiments are interpreted in the context of theories of selective attention and bilingualism.