28. May 2017

Prof. Dr. Christoph S. Herrmann und Dr. Anna Wilsch

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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. Participants performed the Oldenburg sentence test with sentences presented in noise in combination with tACS. Critically, tACS was stimulated with time-lags of 0 to 250 ms in 50-ms steps relative to sentence onset (auditory stimuli were simultaneous to or preceded tACS). We were able to show that envelope-tACS modulated sentence comprehension such that sentence comprehension at the time-lag of the best performance was significantly better than at the time-lag of the worst performance. Interestingly, sentence comprehension across time lags was modulated sinusoidally. Altogether, envelope tACS modulates intelligibility of speech in noise presumably by enhancing and disrupting (time-lags resulting in in- or out-of-phase stimulation, respectively) cortical entrainment to the speech envelope in auditory cortex.