Temporal Structure of Music Improves the Cortical Encoding of Speech

Abstract

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.

Keywords: cortical tracking; musical exposure; oscillations; rhythm.

FIGURE 1

Acoustic properties of Spanish and Basque musical and speech sequences in Experiment 1. Here, we show the spectral information of speech and musical sequences in normalized power (scaled power of signal). Spectra on the left (A, C, E) correspond to Spanish stimuli, showing peaks at 2 and 4 Hz for the Matching and Mis‐matching Regular condition. Spectra on the right (B, D, F) correspond to Basque stimuli, showing peaks at 1.6 and 4 Hz.

FIGURE 2

Procedure. (1) Graphical example of an experimental block. The block comprises four trials. Each trial comprises one melodic sequence followed by a sentence. Only one type of cue (Matching, Mismatching, or Irregular) was used within a block. After each block, a word appeared on the screen. (2) Example of the musical sequences for each condition. Below each musical stave, an example of the sentence that would follow the musical sequences (English translation: The elephant's trunk is green). Note that only in the Matching Regular musical sequences, the notes structure of the musical sequences and the syllabic structure of the sentences are matched.

FIGURE 3

Cortical tracking (z‐scores indicating stimulus‐brain coherence) of musical and speech sequences in the Spanish blocks. The top left and right panels show the delta and theta frequency bands for music (left) and speech (right). The bottom left and right panels show the topographies of the maximum coherence values extracted for the analysis per frequency and stimulus type.

FIGURE 4

Cortical tracking (z‐scores indicating stimulus‐brain coherence) of musical and speech sequences in the Basque blocks. The top left and right panels show the delta and theta frequency bands for music (left) and speech (right). The bottom left and right panels show the topographies of the maximum coherence values extracted for the analysis per frequency and stimulus type.

FIGURE 5

Acoustic properties of Basque musical and speech sequences in Experiment 2. Here, we show the spectral information of speech and musical sequences in normalized power (scaled power of signal). Spectra A and B correspond to the new Basque musical stimuli, showing peaks at 1.6 and 3.6 Hz for the Matching and Mis‐matching Regular condition. Spectrum C corresponds to the new Basque stimuli for the Irregular condition.

FIGURE 6

Cortical tracking (z‐scores indicating stimulus‐brain coherence) of musical and speech sequences in the Basque blocks of Experiment 2. The top left and right panels show the delta and theta frequency bands for music (left) and speech (right). The bottom left and right panels show the topographies of the maximum coherence values extracted for the analysis per frequency and stimulus type.