Lacking Effects of Envelope Transcranial Alternating Current Stimulation Indicate the Need to Revise Envelope Transcranial Alternating Current Stimulation Methods

Abstract

<jats:p> In recent years, several studies have reported beneficial effects of transcranial alternating current stimulation (tACS) in experiments regarding sound and speech perception. A new development in this field is envelope-tACS: The goal of this method is to improve cortical entrainment to the speech signal by stimulating with a waveform based on the speech envelope. One challenge of this stimulation method is timing; the electrical stimulation needs to be phase-aligned with the naturally occurring cortical entrainment to the auditory stimuli. Due to individual differences in anatomy and processing speed, the optimal time-lag between presentation of sound and applying envelope-tACS varies between participants. To better investigate the effects of envelope-tACS, we performed a speech comprehension task with a larger amount of time-lags than previous experiments, as well as an equal amount of sham conditions. No significant difference between optimal stimulation time-lag condition and best sham condition was found. Further investigation of the data revealed a significant difference between the positive and negative half-cycles of the stimulation conditions but not for sham. However, we also found a significant learning effect over the course of the experiment which was of comparable size to the effects of envelope-tACS found in previous auditory tACS studies. In this article, we discuss possible explanations for why our findings did not match up with those of previous studies and the issues that come with researching and developing envelope-tACS. </jats:p>

Cortical Tracking of a Background Speaker Modulates the Comprehension of a Foreground Speech Signal

“…These studies found that the tACS did modulate speech comprehension, but that the best and worst phase shifts were highly variable between subjects (Riecke et al, 2018;Zoefel, Archer-Boyd and Davis, 2018). Similarly, related work on temporal delays between the sound and the neurostimulation waveform found large variability between subjects, and in one case no significant effect on the population level (Riecke et al, 2018;Wilsch et al, 2018;Erkens et al, 2020;Wang et al, 2020). In our previous work on the role of phase shifts in tACS with the speech envelope we employed, as in this study, natural speech in which syllables did not occur at a single fixed frequency and found a phase dependency of the resulting speech comprehension that was largely consistent across the different subjects.…”

Cortical tracking of a background speaker modulates the comprehension of a foreground speech signal

“…The copyright holder for this preprint this version posted April 12, 2021. ; https://doi.org/10.1101/2021.04.12.439458 doi: bioRxiv preprint worst phase shifts were highly variable between subjects (Riecke et al, 2018;Zoefel, Archer-Boyd and Davis, 2018). Similarly, related work on temporal delays between the sound and the neurostimulation waveform found large variability between subjects, and in one case no significant effect on the population level (Riecke et al, 2018;Wilsch et al, 2018;Erkens et al, 2020;Wang et al, 2020). In our previous work on the role of phase shifts in tACS with the speech envelope we employed, as in this study, natural speech in which syllables did not occur at a single fixed frequency and found a phase dependency of the resulting speech comprehension that was largely consistent across the different subjects.…”

High Gamma and Beta Temporal Interference Stimulation in the Human Motor Cortex Improves Motor Functions

“…The copyright holder for this preprint this version posted March 29, 2021. ; https://doi.org/10.1101/2021.03.26.437107 doi: bioRxiv preprint 14 are still controversial. [52][53][54] Amplitude-modulated tACS (AM-tACS) was proposed as a promising way to allow for effective magnetoencephalography (MEG) or electroencephalography (EEG) signal reconstruction during electrical stimulation. However, similar to TI stimulation, studies on AM-tACS have also mostly focused on simulations, and no systematic experimental test to validate the effectiveness of AM-tACS in humans has been performed.…”