Abstract
Author SummaryContinuous speech is organized into a nested hierarchy of quasi-rhythmic components (prosody, syllables, phonemes) with different time scales. Interestingly, neural activity in the human auditory cortex shows rhythmic modulations with frequencies that match these speech rhythms. Here, we use magnetoencephalography and information theory to study brain oscillations in participants as they process continuous speech. We show that auditory brain oscillations at different frequencies align with the rhythmic structure of speech. This alignment is more precise when participants listen to intelligible rather than unintelligible speech. The onset of speech resets brain oscillations and improves their alignment to speech rhythms; it also improves the alignment between the different frequencies of nested brain oscillations in the auditory cortex. Since these brain oscillations reflect rhythmic changes in neural excitability, they are strong candidates for mediating the segmentation of continuous speech at different time scales corresponding to key speech components such as syllables and phonemes.
Highlights
A large number of invasive and non-invasive neurophysiological studies provide converging evidence that cortical oscillations play an important role in gating information flow in the human brain, thereby supporting a variety of cognitive processes including attention, working memory, and decision-making [1,2,3]
Oscillatory Speech Tracking Relies on Two Mechanisms We first asked whether there is phase-locking between rhythmic changes in the speech envelope and corresponding oscillatory brain activity
Whereas most previous studies quantify phaselocking to stimulus onset across repeated presentations of the same stimulus, here we studied phase-locking over time directly between speech envelope and brain oscillations
Summary
A large number of invasive and non-invasive neurophysiological studies provide converging evidence that cortical oscillations play an important role in gating information flow in the human brain, thereby supporting a variety of cognitive processes including attention, working memory, and decision-making [1,2,3]. Speech comprises a remarkably similar hierarchy of rhythmic components representing prosody (delta band), syllables (theta band), and phonemes (gamma band) [9,10,11,12]. The hierarchical coupling of oscillations (with fast oscillations nested in slow oscillations) could be used to multiplex complementary information over multiple time scales [18] (see [19]) for example by separately encoding fast (e.g., phonemic) and slower (e.g., syllabic) information and their temporal relationships
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