Temporal Structure of Music Improves the Cortical Encoding of Speech.

Long- and short-term musical training has been proposed to improve the efficiency of cortical tracking of speech, which refers to the synchronization of brain oscillations and the acoustic temporal structure of external stimuli. Here, we study how musical sequences with different rhythm structures can guide the temporal dynamics of auditory oscillations synchronized with the speech envelope. For this purpose, we investigated the effects of prior exposure to rhythmically structured musical sequences on cortical tracking of speech in Basque-Spanish bilingual adults (Experiment 1; N = 33, 22 female, Mean age = 25 years). We presented participants with sentences in Basque and Spanish preceded by musical sequences that differed in their rhythmical structure. The rhythmical structure of the musical sequences was created to (1) reflect and match the syllabic structure of the sentences, (2) reflect a regular rhythm but not match the syllabic structure of the sentences, and (3) follow an irregular rhythm. Participants' brain responses were recorded using electroencephalography, and speech-brain coherence in the delta and theta bands was calculated. Results showed stronger speech-brain coherence in the delta band in the first condition, but only for Spanish stimuli. A follow-up experiment including a subset of the initial sample (Experiment 2; N = 20) was conducted to investigate whether language-specific stimuli properties influenced the Basque results. Similar to Experiment 1, we found stronger speech-brain coherence in the delta and theta bands when the sentences were preceded by musical sequences that matched their syllabic structure. These results suggest that not only the regularity in music is crucial for influencing cortical tracking of speech, but so is adjusting this regularity to optimally reflect the rhythmic characteristics of listeners' native language(s). Despite finding some language-specific differences across frequencies, we showed that rhythm, inherent in musical signals, guides the adaptation of brain oscillations, by adapting the temporal dynamics of the oscillatory activity to the rhythmic scaffolding of the musical signal.