Transcranial Alternating Current Stimulation With the Theta-Band Portion of the Temporally-Aligned Speech Envelope Improves Speech-in-Noise Comprehension.

Mahmoud Keshavarzi,Tobias Reichenbach

doi:10.3389/fnhum.2020.00187

Abstract

Transcranial alternating current stimulation with the speech envelope can modulate the comprehension of speech in noise. The modulation stems from the theta- but not the delta-band portion of the speech envelope, and likely reflects the entrainment of neural activity in the theta frequency band, which may aid the parsing of the speech stream. The influence of the current stimulation on speech comprehension can vary with the time delay between the current waveform and the audio signal. While this effect has been investigated for current stimulation based on the entire speech envelope, it has not yet been measured when the current waveform follows the theta-band portion of the speech envelope. Here, we show that transcranial current stimulation with the speech envelope filtered in the theta frequency band improves speech comprehension as compared to a sham stimulus. The improvement occurs when there is no time delay between the current and the speech stimulus, as well as when the temporal delay is comparatively short, 90 ms. In contrast, longer delays, as well as negative delays, do not impact speech-in-noise comprehension. Moreover, we find that the improvement of speech comprehension at no or small delays of the current stimulation is consistent across participants. Our findings suggest that cortical entrainment to speech is most influenced through current stimulation that follows the speech envelope with at most a small delay. They also open a path to enhancing the perception of speech in noise, an issue that is particularly important for people with hearing impairment.

Highlights

Understanding speech in noisy backgrounds such as in a loud pub or restaurant is a challenging task at which humans excel (Cherry, 1953; Bregman et al, 1990)
Negative delays implied that the current waveform preceded the speech signal, whereas the current waveform lagged the audio for positive delays
To investigate the effect of the current stimulation at the various delays on speech comprehension, we computed the difference of the corresponding speech comprehension scores and the score that was obtained during sham stimulation (Figure 2B)

Summary

INTRODUCTION

Understanding speech in noisy backgrounds such as in a loud pub or restaurant is a challenging task at which humans excel (Cherry, 1953; Bregman et al, 1990) It requires the segregation of a target speech stream from other sound sources as well as the further parsing and processing of the speech signal. We found that only the theta-band current waveform, but not the delta-band one, modulated the comprehension of speech in background noise We obtained these results by considering current waveforms that were temporally aligned to the speech signal but had different phase shifts. We did not further investigate the role of temporal delays between the current waveform and the audio signal We address this issue by considering how transcranial alternating current stimulation, in which the current waveform is obtained from the theta-band portion of the speech envelope and shifted by different lags, impacts speech comprehension

Participants

Experimental Setup

Experimental Procedure

RESULTS

DISCUSSION AND CONCLUSION

ETHICS STATEMENT