Elsevier

NeuroImage

Volume 210, 15 April 2020, 116557
NeuroImage

Transcranial alternating current stimulation in the theta band but not in the delta band modulates the comprehension of naturalistic speech in noise

https://doi.org/10.1016/j.neuroimage.2020.116557Get rights and content
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Highlights

  • Neurostimulation with the theta-band but not with the delta-band part of the speech envelope modulates comprehension.

  • The modulation of speech comprehension with the theta-band current is consistent across subjects.

  • Theta-band current stimulation can enhance speech comprehension compared to sham stimulation.

Abstract

Auditory cortical activity entrains to speech rhythms and has been proposed as a mechanism for online speech processing. In particular, neural activity in the theta frequency band (4–8 ​Hz) tracks the onset of syllables which may aid the parsing of a speech stream. Similarly, cortical activity in the delta band (1–4 ​Hz) entrains to the onset of words in natural speech and has been found to encode both syntactic as well as semantic information. Such neural entrainment to speech rhythms is not merely an epiphenomenon of other neural processes, but plays a functional role in speech processing: modulating the neural entrainment through transcranial alternating current stimulation influences the speech-related neural activity and modulates the comprehension of degraded speech. However, the distinct functional contributions of the delta- and of the theta-band entrainment to the modulation of speech comprehension have not yet been investigated. Here we use transcranial alternating current stimulation with waveforms derived from the speech envelope and filtered in the delta and theta frequency bands to alter cortical entrainment in both bands separately. We find that transcranial alternating current stimulation in the theta band but not in the delta band impacts speech comprehension. Moreover, we find that transcranial alternating current stimulation with the theta-band portion of the speech envelope can improve speech-in-noise comprehension beyond sham stimulation. Our results show a distinct contribution of the theta- but not of the delta-band stimulation to the modulation of speech comprehension. In addition, our findings open up a potential avenue of enhancing the comprehension of speech in noise.

Keywords

Neural entrainment
Delta and theta frequency bands
Transcranial alternating current stimulation
Speech envelope
Speech comprehension
Speech-shaped-noise
Normal hearing

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