Abstract
Cortical entrainment of the auditory cortex to the broad-band temporal envelope of a speech signal is crucial for speech comprehension. This 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, for example masked by irrelevant information such as noise, entrainment to the speech envelope is weaker and speech intelligibility declines.
Besides perceptually evoked cortical entrainment, transcranial alternating current stimulation (tACS) can entrain 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 externally modulate speech intelligibility 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, the time lag between sentence presentation and tACS was manipulated from 0 to 250 ms in 50-ms steps (auditory stimuli were simultaneous to or preceded tACS).
First, we were able to show that envelope-tACS modulated sentence comprehension such that on average sentence comprehension at the time lag of the best performance was significantly better than sentence comprehension of the worst performance. Second, sentence comprehension across time lags was modulated sinusoidally.
In sum, envelope tACS modulates intelligibility of speech in noise presumably by enhancing (time lag with in-phase stimulation) and disrupting (time lag with out-of-phase stimulation) cortical entrainment to the speech envelope in auditory cortex.
