Abstract
Rhythmic sensory or electrical stimulation will produce rhythmic brain responses. These rhythmic responses are often interpreted as endogenous neural oscillations aligned (or "entrained") to the stimulus rhythm. However, stimulus-aligned brain responses can also be explained as a sequence of evoked responses, which only appear regular due to the rhythmicity of the stimulus, without necessarily involving underlying neural oscillations. To distinguish evoked responses from true oscillatory activity, we tested whether rhythmic stimulation produces oscillatory responses which continue after the end of the stimulus. Such sustained effects provide evidence for true involvement of neural oscillations. In Experiment 1, we found that rhythmic intelligible, but not unintelligible speech produces oscillatory responses in magnetoencephalography (MEG) which outlast the stimulus at parietal sensors. In Experiment 2, we found that transcranial alternating current stimulation (tACS) leads to rhythmic fluctuations in speech perception outcomes after the end of electrical stimulation. We further report that the phase relation between electroencephalography (EEG) responses and rhythmic intelligible speech can predict the tACS phase that leads to most accurate speech perception. Together, we provide fundamental results for several lines of research-including neural entrainment and tACS-and reveal endogenous neural oscillations as a key underlying principle for speech perception.
Highlights
The alignment of oscillatory neural activity to a rhythmic stimulus, often termed “neural entrainment,” is an integral part of many current theories of speech processing [1,2,3,4]
We provide 2 distinct sources of evidence for sustained oscillatory effects: (1) oscillatory MEG responses that continue after rhythmic intelligible speech and (2) oscillatory effects of transcranial alternating current stimulation (tACS) on speech perception that continue after the termination of electrical stimulation
More than 70 years later, it remains an ongoing debate whether “neural entrainment,” brain responses aligned to rhythmic input, is due to the operation of endogenous neural oscillations or reflects a regular repetition of stimulus-evoked responses [16,36,37,38,39]
Summary
The alignment of oscillatory neural activity to a rhythmic stimulus, often termed “neural entrainment,” is an integral part of many current theories of speech processing [1,2,3,4]. Brain responses seem to align more reliably to intelligible than to unintelligible speech [5,6]. Rhythmic electrical stimulation applied to the scalp (transcranial alternating current stimulation (tACS)) is assumed to “entrain” brain oscillations and has been shown to modulate speech processing and perception [7,8,9,10,11]. Despite the prominence of entrainment theories in speech research and elsewhere [1,12,13,14], it has been surprisingly difficult to demonstrate that.
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