Non-linear transfer characteristics of stimulation and recording hardware account for spurious low-frequency artifacts during amplitude modulated transcranial alternating current stimulation (AM-tACS)

Abstract

Amplitude modulated transcranial alternating current stimulation (AM-tACS) has been recently proposed as a possible solution to overcome the pronounced stimulation artifact encountered when recording brain activity during tACS. In theory, AM-tACS does not entail power at its modulating frequency, thus avoiding the problem of spectral overlap between brain signal of interest and stimulation artifact. However, the current study demonstrates how weak non-linear transfer characteristics inherent to stimulation and recording hardware can reintroduce spurious artifacts at the modulation frequency. The input-output transfer functions (TFs) of different stimulation setups were measured. Setups included recordings of signal-generator and stimulator outputs and M/EEG phantom measurements. 6-degree polynomial regression models were fitted to model the input-output TFs of each setup. The resulting TF models were applied to digitally generated AM-tACS signals to predict the frequency of spurious artifacts in the spectrum. All four setups measured for the study exhibited low-frequency artifacts at the modulation frequency and its harmonics when recording AM-tACS. Fitted TF models showed non-linear contributions significantly different from zero (all p < .05) and successfully predicted the frequency of artifacts observed in AM-signal recordings. Results suggest that even weak non-linearities of stimulation and recording hardware can lead to spurious artifacts at the modulation frequency and its harmonics. These artifacts were substantially larger than alpha-oscillations of a human subject in the MEG. Findings emphasize the need for more linear stimulation devices for AM-tACS and careful analysis procedures, taking into account low-frequency artifacts to avoid confusion with effects of AM-tACS on the brain.

Electrophysiology equipment for reliable study of kHz electrical stimulation

“…; Kasten et al . ). In the case of amplifiers and electrodes, the cost of band‐width limits will reflect the experimental design.…”

“…; Kasten et al . ); (3) when electrophysiological responses at kHz rate (e.g. kHz fluctuations in membrane potential) are of interest (Pelot et al .…”

Transient amplitude modulation of alpha-band oscillations by short-time intermittent closed-loop tACS

“…In principle, the signal to be analyzed and modulated (e.g., a signature of alpha oscillations measured in the EEG) is overwritten by the tACS signal, which is several magnitudes larger but covers the same spatial and temporal space. Thus, substantial efforts in recent studies have been directed to the development of artifact elimination methods using different techniques and experimental protocols (Witkowski et al, 2015;Kohli & Casson, 2019;Kasten et al, 2018;Noury & Siegel, 2017). However, intermittent stimulation allows one to follow another approach while still based on adaptive principles.…”

“…Thirdly, the main technical challenge for analyzing and investigating online effects during stimulation is the induction of stimulation artifacts, which exceed the measured neural signals by several magnitudes. Although there is an increasing number of propositions for overcoming this challenge and some prominent solutions (Witkowski et al, 2015;Kohli & Casson, 2019;Kasten et al, 2018;Noury & Siegel, 2017) directed to elimination of artifacts, such approaches still require sophisticated experimental designs and/or heavy computational procedures. Most human tACS studies have used stimulation protocols with a duration applied in the range of minutes.…”

Temporal interference stimulation targets deep brain regions by modulating neural oscillations

“…With high-intensity electric fields alternative biophysics such as ion accumulation 49 , fiber block 50-52 , asynchronous firing 53 and/or heating 54 may also be considered, but were not required to explain our results. Electrophysiology experiments with kHz electric field stimulation must carefully account for the fidelity of delivered stimulation and recording artifacts 55 . Indeed, we avoided intracellular electrodes because microelectrodes can collect current from kHz stimulation 56 risking either artifactual intracellular stimulation (as the microelectrodes acts as a collector "antenna") or amplifier distortion 57 .…”