Transcranial alternating current stimulation with speech envelopes modulates speech comprehension

Abstract

Cortical entrainment of the auditory cortex to the broadband temporal envelope of a speech signal is crucial for speech comprehension. Entrainment results in phases of high and low neural excitability, which structure and decode the incoming speech signal. Entrainment to speech is strongest in the theta frequency range (4-8 Hz), the average frequency of the speech envelope. If a speech signal is degraded, entrainment to the speech envelope is weaker and speech intelligibility declines. Besides perceptually evoked cortical entrainment, transcranial alternating current stimulation (tACS) entrains neural oscillations by applying an electric signal to the brain. Accordingly, tACS-induced entrainment in auditory cortex has been shown to improve auditory perception. The aim of the current study was to modulate speech intelligibility externally by means of tACS such that the electric current corresponds to the envelope of the presented speech stream (i.e., envelope-tACS). Participants performed the Oldenburg sentence test with sentences presented in noise in combination with envelope-tACS. Critically, tACS was induced at time lags of 0-250 ms in 50-ms steps relative to sentence onset (auditory stimuli were simultaneous to or preceded tACS). We performed single-subject sinusoidal, linear, and quadratic fits to the sentence comprehension performance across the time lags. We could show that the sinusoidal fit described the modulation of sentence comprehension best. Importantly, the average frequency of the sinusoidal fit was 5.12 Hz, corresponding to the peaks of the amplitude spectrum of the stimulated envelopes. This finding was supported by a significant 5-Hz peak in the average power spectrum of individual performance time series. Altogether, envelope-tACS modulates intelligibility of speech in noise, presumably by enhancing and disrupting (time lag with in- or out-of-phase stimulation, respectively) cortical entrainment to the speech envelope in auditory cortex.

Perception of rhythmic speech is modulated by focal bilateral tACS

“…The present study replicates previous findings that the timing of tACS, synchronized with the amplitude envelope of spoken sentences, can change speech processing Wilsch et al, 2018;Zoefel, Archer-Boyd, et al, 2018). In addition, several aspects of the study are novel and go beyond the current state-of-the-art.…”

“…auditory stream segregation) or a modulation of processes that are more central to speech perception (such as the perception of speech sounds and recognition of words). Modulation of any of these processes could lead to the tACS-induced changes in word report shown previously by Wilsch et al (2018). In this study, we demonstrate that the phase relation between tACS and rhythmic speech affects word report accuracy, even for sequences of degraded words presented in silence.…”

“…Combining left-lateralised tACS over auditory brain regions with functional magnetic resonance imaging (fMRI), Zoefel, Archer-Boyd, & Davis (2018) showed that the phase relation between tACS and rhythmic speech impacts the blood-oxygen level dependent (BOLD) response to intelligible but not to matched, unintelligible speech. Two other tACS studies found that bilateral neural modulation also impacts on word report for spoken sentences; Wilsch, Neuling, Obleser, & Herrmann (2018) and Riecke, Formisano, Sorger, Başkent, & Gaudrain (2018) both found that word report accuracy (a measure of speech perception success) depends on the time delay between envelope-shaped tACS stimulation and speech signals. While these findings are striking and provide compelling evidence that tACS modulates neural activity and speech perception in parallel, they still fall short of demonstrating a causal role of oscillatory neural entrainment on speech perception.…”

“…While these findings are striking and provide compelling evidence that tACS modulates neural activity and speech perception in parallel, they still fall short of demonstrating a causal role of oscillatory neural entrainment on speech perception. Primarily, this is because the target sentences in the experiments reported by Wilsch et al (2018) and were masked by background noise or other speech. The success of speech perception in noise depends on listener's ability to segregate speech and background sounds (i.e.…”

“…Previous tACS studies (e.g., Wilsch et al, 2018) focused on the demonstration of a phasic modulation of speech perception outcomes (and therefore a functional consequence of oscillatory brain stimulation). However, for real-world applications of tACS, the most important outcome is clearly to enhance speech perception relative to an unstimulated control condition (Peelle, 2018;Riecke & Zoefel, 2018).…”