Transcranial Alternating Current Stimulation with Sawtooth Waves: Simultaneous Stimulation and EEG Recording

Abstract

Transcranial alternating current stimulation (tACS) has until now mostly been administered as an alternating sinusoidal wave. Despite modern tACS stimulators being able to deliver alternating current with any arbitrary shape there has been no systematic exploration into the relative benefits of different waveforms. As tACS is a relatively new technique there is a huge parameter space of unexplored possibilities which may prove superior or complimentary to the traditional sinusoidal waveform. Here, we begin to address this with an investigation into the effects of sawtooth wave tACS on individual alpha power. Evidence from animal models suggests that the gradient and direction of an electric current should be important factors for the subsequent neural firing rate; we compared positive and negative ramp sawtooth waves to test this. An additional advantage of sawtooth waves is that the resulting artifact in the electroencephalogram (EEG) recording is significantly simpler to remove than a sine wave; accordingly we were able to observe alpha oscillations both during and after stimulation. We found that positive ramp sawtooth, but not negative ramp sawtooth, significantly enhanced alpha power during stimulation relative to sham (p < 0.01). In addition we tested for an after-effect of both sawtooth and sinusoidal stimulation on alpha power but in this case did not find any significant effect. This preliminary study paves the way for further investigations into the effect of the gradient and direction of the current in tACS which could significantly improve the usefulness of this technique.

Shift in lateralization during illusory self‐motion: EEG responses to visual flicker at 10 Hz and frequency‐specific modulation by tACS

“…In a recent study, we applied tACS using various ‘sawtooth’ waves in addition to the traditional sinusoidal waveform (Dowsett & Herrmann, ); waveforms such as square waves and sawtooth waves, which contain sudden transitions in current, may be more effective at influencing or entraining neuronal oscillations, but they have an additional advantage in that the distinct properties of sawtooth waves, that is, consisting of straight lines with a steep transition, do not occur in nature and are more easily distinguishable from neural activity. This makes removing the artefact from the EEG recording simpler (see below).…”

Electrophysiology equipment for reliable study of kHz electrical stimulation

“…) unless (1) stimulation artifact removal depends on accurate field pick‐up (Helfrich et al . ; Dowsett and Herrmann ); (2) when recording hardware non‐linearities are of concern (Lesperance et al . ; Kasten et al .…”

“…Amplifier and electrode attenuation may not be critical for conventional neurophysiological measures (e.g. action potential threshold or rate, low frequency oscillation power; Grossman et al 2017;Negahbani et al 2018) unless (1) stimulation artifact removal depends on accurate field pick-up (Helfrich et al 2014;Dowsett and Herrmann 2016); (2) when recording hardware non-linearities are of concern (Lesperance et al 2018; Kasten et al 2018); (3) when electrophysiological responses at kHz rate (e.g. kHz fluctuations in membrane potential) are of interest (Pelot et al 2017); and (4) when interactions between equipment and the body introduce complex distortions (Neuling et al 2017;Noury and Siegel, 2018;Gebodh et al 2019).…”

Flexible resonance in prefrontal networks with strong feedback inhibition

“…Transcranial stimulation is often used to enhance neural oscillations [25,26]. [27] showed that transcranial 276 alternating current stimulation (tACS) with sawtooth waves is more effective at enhancing alpha-frequency 277 oscillations than tACS with sinusoidal waves; whether tACS predominantly excites interneurons or principal cells 278 depends on the intensity of stimulation [28]. Our work suggests that, given an excitatory intensity, tACS 279 stimulation with square waves (i.e., periodic pulses) could be even more effective at enhancing neural oscillations.…”