Theta-gamma coupling in the auditory cortex is purely acoustic, but it is a mechanism for speech perception (starting time 14.15)

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Frequency multiplexing in the auditory cortex occurs during speech processing. High-gamma activity (~120 Hz) follows the fundamental frequency of speech, while low-gamma dynamics (~40 Hz) relate to phonetic processing. Both amplitudes are coupled to the phase of theta oscillations (4-8 Hz), which track speech rhythm. The functional role of this phase-amplitude coupling is still under debate.

Using intracerebral EEG electrodes in epileptic patients, we recorded auditory cortex activity while they listened to a story. Both theta and gamma oscillations were driven by the speech envelope but processed by different neural sources. Cross-frequency coupling between these frequencies was imposed by the auditory stimulus, mirroring the synchrony between syllables and phonemes in the speech signal. However, there is a hierarchical organization with information flowing from gamma to theta sources.

These support the dual timescale view of sensory processing in the auditory cortex but challenge the idea of cross-frequency coupling as a mechanism for speech perception. We propose that the brain processes syllabic and phonemic information in parallel by relatively independent neural sources. Gamma oscillations, being faster, are first entrained by speech, which then adjusts theta oscillations to better parse syllabic information.

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Sandro Meloni

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