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

Mahmoud Keshavarzi,Mikolaj Kegler,Shabnam Kadir,Tobias Reichenbach

doi:10.1016/j.neuroimage.2020.116557

Mahmoud Keshavarzi, Mikolaj Kegler + Show 2 more

Open Access

https://doi.org/10.1016/j.neuroimage.2020.116557

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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.

Highlights

Speech is a complex signal that unfolds over several temporal scales, from phonemes to syllables, words, and phrases
We showed that neurostimulation with the theta-band but not the delta-band portion of the speech envelope impacts comprehension
Our observed lack of modulation of speech comprehension through delta-band stimulation may reflect that, the neural speech tracking in the delta band relates to higher-level linguistic information in speech and to speech comprehension, this relationship originates in only a small portion of the delta-band entrainment (Ding et al, 2016; Broderick et al, 2018; Etard and Reichenbach, 2019)

Summary

Introduction

Speech is a complex signal that unfolds over several temporal scales, from phonemes to syllables, words, and phrases. The neural activity in the auditory cortex entrains to the amplitude modulations in speech, as well as to more specific speech structures such as phonemes, the onset of words, and to higher-level linguistic information such as surprisal of word sequences and syntactic structure (Lakatos et al, 2005; Ding and Simon, 2012; Giraud and Poeppel, 2012; Di Liberto et al, 2015; Ding et al, 2016; Brodbeck et al, 2018; Broderick et al, 2018; Weissbart et al, 2019). It remains unclear which more specific aspects of the cortical speech entrainment underlie the modulation of speech comprehension

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