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Poster A61, Tuesday, August 20, 2019, 10:15 am – 12:00 pm, Restaurant Hall

Sustained oscillations in MEG and tACS demonstrate true neural entrainment in speech processing

Benedikt Zoefel1, Sander van Bree1, Ediz Sohoglu1, Matthew H Davis1;1MRC Cognition and Brain Sciences Unit, University of Cambridge

A range of existing findings in the literature are consistent with oscillatory neural mechanisms contributing to speech perception and comprehension. Intelligible speech evokes more coherent brain responses at low frequencies (~4 Hz) than unintelligible speech (e.g., Peelle et al., 2013). Furthermore, fMRI responses to intelligible speech (Zoefel et al., 2018) and word report scores (Riecke et al., 2018) depend on the phase lag between transcranial alternating current stimulation (tACS) and the rhythm of heard speech. These (and other) studies suggest changes in the alignment of neural oscillations to speech rhythm, termed “neural entrainment”; this neural entrainment is often argued to underlie successful speech processing and comprehension (e.g., Giraud and Poeppel, 2012; Peelle and Davis, 2012). Nonetheless, putative oscillatory effects on speech processing can also be explained as due to neural activity evoked by speech; neural responses will appear to be rhythmic since they are evoked by a rhythmic stimulus (Zoefel et al., 2018b). To distinguish these views, we performed two experiments in which we tested whether rhythmic sensory (Experiment 1) or electrical (Experiment 2) stimulation produces sustained oscillatory responses which continue after the end of the stimulus. Effects of previous rhythmic stimulation which continue in the absence of stimulation provide stronger evidence for true involvement of oscillatory activity (cf. Kosem et al, 2018). In Experiment 1 (N = 20), we measured brain responses with magnetoencephalography (MEG) during and after presentation of rhythmic intelligible and unintelligible speech sequences, presented at one of two different rates (2 Hz and 3 Hz). We found that auditory MEG responses, specific to the stimulation rate, briefly outlast the offset of the speech stimulus. However, this sustained response did not depend on the intelligibility (16- vs 1 channel vocoded speech) or the length (2s vs 3s) of the preceding speech sequences. These findings suggest brief sustained oscillations produced by rhythmic speech, though these do not appear to be affected by cognitive factors (such as intelligibility and prediction) that we might expect to influence neural oscillations. In Experiment 2 (N = 19), many of the same participants were asked to report spoken words embedded in noise. These words were presented at six different phase delays, during or immediately after bilateral tACS at 3 Hz focussed on the STG. We found that word report accuracy fluctuates rhythmically for spoken words presented after the offset of tACS, at a frequency corresponding to the stimulation. This sustained, phasic modulation of performance was numerically larger than, but not significantly different from, that observed for tACS that continued at the time of speech presentation. These findings show that modulation of neural oscillations produces a perceptual effect that is sustained beyond the end of tACS.

Themes: Speech Perception, Perception: Auditory
Method: Electrophysiology (MEG/EEG/ECOG)

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