Phase−amplitude coupling between theta and gamma oscillations adapts to speech rate

First published: 24 April 2019Low- and high-frequency cortical oscillations play an important role in speech processing. Low-frequency neural oscillations in the delta (<4Hz) and theta (4–8 Hz) bands entrain to the prosodic and syllabic rates of speech, respectively. Theta band neural oscillations modulate high-frequency neural oscillations in the gamma band (28−40Hz), which have been hypothesized to be crucial for processing phonemes in natural speech. Since speech rate is known to vary considerably, both between and within talkers, it has yet to be determined whether this nested gamma response reflects an externally induced rhythm sensitive to the rate of the fine-grained structure of the input or a speech rate−independent endogenous response. Here, we recorded magnetoencephalography responses from participants listening to a speech delivered at different rates: decelerated, normal, and accelerated. We found that the phase of theta band oscillations in left and right auditory regions adjusts to speech rate variations. Importantly, we showed that the peak of the gamma response—coupled to the phase of theta—follows the speech rate. This indicates that gamma activity in auditory regions synchronizes with the fine-grain properties of speech, possibly reflecting detailed acoustic analysis of the input.This research was supported in part by the Agence Nationale de la Recherche (ANR-10-LABX-0087 IEC and ANR- 10-IDEX-0001-02 PSL), the EUR Frontiers (ANR- 17-EURE-0017), the European Research Council (ERC-2011-ADG-295362), and the MINECO (PSI2015-67353-R). This work was also partially supported by the Agencia Estatal de Investigación (AEI), the Fondo Europeo de Desarrollo Regional (FEDER) (grant PSI2015-65694-P), the “Severo Ochoa” programme (SEV-2015-490) for Centres of Excellence in R&D, and the Basque government (grant PI_2016_1_0014). The authors would like to acknowledge all the participants taking part in this study