Abstract
Speech-evoked envelope-following response (EFR) reflects brain encoding of speech periodicity that serves as a biomarker for pitch and speech perception and various auditory and language disorders. Although EFR is thought to originate from the subcortex, recent research illustrated a right-hemispheric cortical contribution to EFR. However, it is unclear whether this contribution is causal. This study aimed to establish this causality by combining transcranial direct current stimulation (tDCS) and measurement of EFR (pre- and post-tDCS) via scalp-recorded electroencephalography. We applied tDCS over the left and right auditory cortices in right-handed normal-hearing participants and examined whether altering cortical excitability via tDCS causes changes in EFR during monaural listening to speech syllables. We showed significant changes in EFR magnitude when tDCS was applied over the right auditory cortex compared with sham stimulation for the listening ear contralateral to the stimulation site. No such effect was found when tDCS was applied over the left auditory cortex. Crucially, we further observed a hemispheric laterality where aftereffect was significantly greater for tDCS applied over the right than the left auditory cortex in the contralateral ear condition. Our finding thus provides the first evidence that validates the causal relationship between the right auditory cortex and EFR.
Highlights
The speech-evoked frequency-following responses (FFRs) are phase-locked neural activities that reflect early processing of periodic features of input speech signals in the human brain (Aiken and Picton 2008; Coffey et al 2019)
We showed a significant (Stimulated Hemisphere × Contralateral) interaction and a hemispheric laterality in which the decrease in envelope-following response (EFR) magnitude in the contralateral ear was significantly greater when transcranial direct current stimulation (tDCS) was applied over the right than the left auditory cortex
Our results showed that tDCS over the right auditory cortex resulted in significant decrease in EFR magnitude compared with Sham as well as tDCS over the left auditory cortex when the listening ear was contralateral to the stimulated site
Summary
The speech-evoked frequency-following responses (FFRs) are phase-locked neural activities that reflect early processing of periodic features of input speech signals in the human brain (Aiken and Picton 2008; Coffey et al 2019). One of the most important FFR components in the central auditory systems is the envelope-following response (EFR) that encodes the periodicity envelopes at fundamental frequency (F0) that represents the vocal pitch information (Aiken and Picton 2008; Coffey et al 2019). The EFR is associated with various human auditory and language processing It reflects the neural encoding of linguistic pitch and is stronger in tonal language than nontonal language speakers (Krishnan et al 2004, 2005, 2009). EFR reflects neural plasticity related to fundamental cognitive and physiological processes such as auditory learning (Skoe et al 2014), changes in arousal (Mai et al 2019), and attention (Lehmann and Schönwiesner 2014; Hartmann and Weisz 2019)
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