Coupled rhythms in early auditory cortex mirror speech acoustics

Theta and gamma neural dynamics dominate the human auditory cortex during speech perception and have been proposed to track syllable boundaries and encode phonemic information, respectively. To what extent these rhythms engage intrinsic mechanisms or mirror the phase and frequency of the speech acoustics remains unsolved. Applying signal processing techniques from neuroscience to speech audio corpora from 17 languages, we found that canonical brain features (theta, gamma, and their phase-amplitude coupling) are distinctive and visible in the speech envelope. They represent syllabic rate (between 2 and 6 Hz), vocalic features (between 30 and 50 Hz), and fundamental frequency (between 100 and 150 Hz). Intracerebral (sEEG) recordings from the auditory cortex of 18 epilepsy patients revealed that theta-gamma dynamics and their coupling are absent at rest but emerge during speech perception, linearly driven by the acoustic envelope. These responses originate from distinct neural populations within overlapping auditory regions. Thus, theta-gamma auditory dynamics primarily mirror speech acoustics rather than being generated by endogenous mechanisms.