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

21Background: Amplitude modulated transcranial alternating current stimulation (AM-tACS) has 22 been recently proposed as a possible solution to overcome the pronounced stimulation artifact 23 encountered when recording brain activity during tACS. In theory, AM-tACS does not entail 24 power at its modulating frequency, thus avoiding the problem of spectral overlap between brain 25 signal of interest and stimulation artifact. However, the current study demonstrates how weak 26 non-linear transfer characteristics inherent in stimulation and recording hardware can reintro-27 duce spurious artifacts at the modulation frequency. 28 Method:The input-output transfer functions (TFs) of different stimulation setups were meas-29 ured. The setups included basic recordings of signal-generator and stimulator outputs as well 30 as M/EEG phantom measurements. 6 th -degree polynomial regression models were fitted to 31 model the input-output TFs of each setup. The resulting TF models were applied to digitally 32 generated AM-tACS signals to predict the location of spurious artifacts in the spectrum. 33 not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/246710 doi: bioRxiv preprint first posted online Jan. 16, 2018; Kasten et al., 2018 Spurious low-frequency artifacts of AM-tACS 2 Results: All four setups measured for the study exhibited low-frequency artifacts at the mod-34 ulation frequency and its harmonics when recording AM-tACS. Fitted TF models showed non-35 linear contributions significantly different from zero (all p < .05) and successfully predicted the 36 frequency of artifacts observed in AM-signal recordings. 37 Conclusions:Our results suggest that even weak non-linearities of stimulation and recording 38 hardware can lead to spurious artifacts at the modulation frequency and its harmonics. Thus, 39 findings emphasize the need for more linear stimulation devices for AM-tACS and careful anal-40 ysis procedures taking into account these low-frequency artifacts.

Integrating electric field modelling and neuroimaging to explain inter-individual variability of tACS effects

“…1a). Although the recording was continued during stimulation, signals acquired during this period were discarded from the analysis due to the massive electromagnetic stimulation artifact, that can currently not be reliably removed from resting state recordings [51][52][53][54] .…”

Electrophysiology equipment for reliable study of kHz electrical stimulation

“…presence of higher frequency components, total charge) influence neurophysiological response (Song et al 2015;Patel & Butera, 2018); or if isolators distortion is not-linear (e.g. low frequency content is introduced) the experiment may be fundamentally compromised (Franke et al 2014;Kasten et al 2018). In the case of amplifiers and electrodes, the cost of band-width limits will reflect the experimental design.…”

“…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).…”

Oscillating neural networks: perspectives from rhythmic brain stimulation

“…In principle, this would effectively avoid any spectral overlap between the brain signals of interest (here near 10 Hz) and the artefact of tACS (here 220 Hz ± 10 Hz). However, preliminary work suggests the M/EEG spectrum remains corrupted near the modulation frequency (Minami and Amano, 2017;Kasten et al, 2018b), though to a much lesser extent than observed during monosinusoidal tACS.…”

“…There is growing evidence that AM-tACS can entrain oscillations at the frequency of the envelope Witkowski et al, 2016;Minami and Amano, 2017), though may require greater stimulation intensities (Negahbani et al, 2018) than mono-sinusoidal tACS. Unfortunately, some artefact still remains at the modulation frequency, likely arising from additional nonlinearities at the level of the stimulator or recording hardware (Kasten et al, 2018b) or from the analog/digital conversion (Minami and Amano, 2017). Without these hardware nonlinearities, AM-tACS would well tolerate heartbeat-and respiration-related modulations (Figure 5.3), with the physiologically derived symmetry also occurring around the 220 Hz carrier-clever use of the type of AM-tACS can minimise this symmetry, even delivered with the same envelope frequency (see Appendix D).…”