Frequency multiplexing in the auditory cortex has been observed during speech processing. High-gamma activity (>100 Hz) follows the fundamental frequency (F0) of the voice in speech, while low-gamma dynamics (~40 Hz) are related to phonetic processing. The amplitude of both is coupled to the phase of theta oscillations (4-8 Hz), which track the syllabic rate of speech. However, the functional role of such phase-amplitude coupling remains debated.
Using intracerebral EEG recordings in epileptic patients, we examined the auditory cortex while they passively listened to a story. We found that both theta and gamma oscillations are modulated by the speech envelope but processed by distinct neural sources. The cross-frequency coupling between these oscillations is driven by the auditory stimulus, reflecting the synchrony between syllables and phonemes present in the speech signal. Our findings  challenge the view that speech perception relies on intrinsic mechanisms within the auditory cortex. Instead, we propose that theta and gamma oscillations are generated during speech production, potentially linked to motor planning and articulation.