Amplitude modulated transcranial alternating current stimulation (AM-TACS) efficacy evaluation via phosphene induction

Carsten Thiele,Aiden Haghikia,Philipp Ruhnau,Tino Zaehle

doi:10.1038/s41598-021-01482-1

Abstract

Amplitude modulated transcranial alternating current stimulation (AM-tACS) is a novel method of electrostimulation which enables the recording of electrophysiological signals during stimulation, thanks to an easier removable stimulation artefact compared to classical electrostimulation methods. To gauge the neuromodulatory potential of AM-tACS, we tested its capacity to induce phosphenes as an indicator of stimulation efficacy. AM-tACS was applied via a two-electrode setup, attached on FpZ and below the right eye. AM-tACS waveforms comprised of different carrier (50 Hz, 200 Hz, 1000 Hz) and modulation frequencies (8 Hz, 16 Hz, 28 Hz) were administered with at maximum 2 mA peak-to-peak stimulation strength. TACS conditions in the same frequencies were used as a benchmark for phosphene induction. AM-tACS conditions using a 50 Hz carrier frequency were able to induce phosphenes, but with no difference in phosphene thresholds between modulation frequencies. AM-tACS using a 200 Hz or 1000 Hz carrier frequency did not induce phosphenes. TACS conditions induced phosphenes in line with previous studies. Stimulation effects of AM-tACS conditions were independent of amplitude modulation and instead relied solely on the carrier frequency. A possible explanation may be that AM-tACS needs higher stimulation intensities for its amplitude modulation to have a neuromodulatory effect.

Highlights

Neuronal oscillations across a range of frequencies are the basis for communication in the brain and underly many cognitive functions[1]
Other stimulation methods which rely on the combination of a carrier signal and a stimulating component already exist, for instance, in the form of cross-frequency tACS27 where typically a continuous low-frequency carrier is combined with a high-frequency signal aligned to a certain phase of the carrier (e.g., a 6 Hz carrier combined with short gamma-frequency b ursts28)
We examine the efficacy of amplitude modulated Transcranial alternating current stimulation (tACS) (AM-tACS) (see Fig. 1a (Bottom)) stimulation on the central nervous system (CNS)

Summary

Introduction

Neuronal oscillations across a range of frequencies are the basis for communication in the brain and underly many cognitive functions[1]. Most tACS studies are focussing on the behavioural effects of s timulation[14,15,16], while they lack electrophysiological recordings to confirm the modulation of neuronal activity This is due to a significant artefact to any electrophysiological recording caused by the electrostimulation which poses a challenge to analyse online-effects (i.e., effects during stimulation) of tACS17. To be able to study online-effects, it is necessary to analyse (artefact-free) brain activity recorded during stimulation. A different approach to circumvent the artefact problem was proposed by Witkowski et al.[26] who used amplitude modulated tACS (AM-tACS) This method uses a stimulation waveform that consists of two components: a high-frequency (> 150 Hz) sinusoidal carrier and a low-frequency (e.g., 10 Hz) amplitude modulation. Previous studies found stimulation effects of AM-tACS in humans, as it e.g. disrupted performance in a working memory t ask[35] or affected visual perception[36]

Objectives

Methods

Results

Discussion

Conclusion