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Rhythmic Auditory Stimulation of Cortical Tracking of Speech in Basque-Spanish Bilinguals

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Poster B80 in Poster Session B, Tuesday, October 24, 3:30 - 5:15 pm CEST, Espace Vieux-Port

Laura Fernández-Merino1,3, Mikel Lizarazu1, Nicola Molinaro1,2, Marina Kalashnikova1,2; 1Basque Center on Cognition, Brain and Language, Donostia-San Sebastian, Spain., 2Ikerbasque, Basque Foundation for Science, Bilbao, Spain., 3University of the Basque Country UPV/EHU, Donostia-San Sebastian, Spain

Long and short-term musical training leads to benefits in speech processing. Musical stimulation is proposed to influence auditory cortical tracking efficiency, the mechanism through which brain oscillations synchronize to the acoustic temporal structure of external stimuli, resulting in more efficient speech processing in individuals who have received music exposure. However, the precise features of musical stimulation that lead to these benefits are debated. Here, we show how the rhythm structure of the musical signal can guide the temporal dynamics of auditory oscillations phase-aligned to the speech envelope. We assessed this proposal by investigating the effects of prior exposure to rhythmic structure in musical sequences on cortical tracking of speech in Basque-Spanish bilingual adults. Our experimental design took advantage of participants’ bilingual background, enabling the assessment of the effects that language-specific musical rhythms may have on cortical tracking of speech. Participants completed a listening task in Spanish and Basque. Each trial consisted of a melodic sequence immediately followed by a spoken sentence. Three experimental conditions were created by manipulating the beat structure of the melodic sequences: 1) Matching Regular, which reflected and matched the syllabic structure of the sentences, 2) Mis-matching Regular, which reflected a regular rhythm but did not match the syllabic structure of the sentences, and 3) Irregular, which followed an irregular rhythm. Cortical tracking of speech (speech-brain coherence) was calculated as the frequency-specific phase synchronization between the brain signal (recorded using electroencephalography) and the corresponding stimuli envelope weighted by their relative amplitude. In Experiment 1, participants (N=33) showed higher coherence of Spanish speech sequences in the delta band (~2 Hz) in the Matching Regular condition compared to Irregular and Mis-matching Regular conditions. In the theta band (~4 Hz), coherence was higher for the Matching Regular condition compared to Mis-matching Regular. Surprisingly, no effects were observed in Basque. Further inspection of the stimuli suggested that the unexpected cross-linguistic findings in Experiment 1 may be due to the stimuli’s more accurate representation of the prosodic structure of Spanish than Basque (while both are syllable-timed languages, Basque exhibits characteristics of both syllable- and stress-timed languages). To test this, Experiment 2 was conducted on a sample of 19 participants from Experiment 1, using stimuli adapted to the natural characteristics of Basque. Results showed higher coherence for Basque speech sequences, for both delta (~1.6 Hz) and theta (~3.6 Hz) bands, in the Matching Regular compared to Mis-matching Regular and Irregular conditions. Coherence was also higher in the Mis-matching Regular compared to the Irregular condition. Our results demonstrate a rhythm-to-speech benefit in cortical tracking of speech. Listeners benefitted from rhythmic scaffolding that matched the syllabic structure of the subsequent speech. While brain oscillators adapt to a wide range of rhythmic stimuli, the optimal matching between the rhythmic priming (Matching Regular condition) and the upcoming speech leads to a significant improvement of cortical tracking. These findings inform the relationship between the neurobiological mechanisms of cortical tracking and the rhythmic structure of speech and music.

Topic Areas: Speech Perception, Multilingualism

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