title: Physiological processes non-linearly affect electrophysiological recordings during transcranial electric stimulation.
abstract: Transcranial electric stimulation (tES) is a promising tool to non-invasively manipulate neuronal activity in the human brain. Several studies have shown behavioral effects of tES, but stimulation artifacts complicate the simultaneous investigation of neural activity with EEG or MEG. Here, we first show for EEG and MEG, that contrary to previous assumptions, artifacts do not simply reflect stimulation currents, but that heartbeat and respiration non-linearly modulate stimulation artifacts. These modulations occur irrespective of the stimulation frequency, i.e. during both transcranial alternating and direct current stimulations (tACS and tDCS). Second, we show that, although at first sight previously employed artifact rejection methods may seem to remove artifacts, data are still contaminated by non-linear stimulation artifacts. Because of their complex nature and dependence on the subjects' physiological state, these artifacts are prone to be mistaken as neural entrainment. In sum, our results uncover non-linear tES artifacts, show that current techniques fail to fully remove them, and pave the way for new artifact rejection methods.
authors.affiliation: Centre for Integrative Neuroscience & MEG Center, University of Tübingen, Germany; IMPRS for Cognitive and Systems Neuroscience, Tübingen, Germany. Electronic address: nima.noury@cin.uni-tuebingen.de.
authors.forename: Nima
authors.initials: N
authors.lastname: Noury
authors.affiliation: Centre for Integrative Neuroscience & MEG Center, University of Tübingen, Germany; F. Hoffmann-La Roche, Pharma Research and Early Development, Basel, Switzerland.
authors.forename: Joerg F
authors.initials: JF
authors.lastname: Hipp
authors.affiliation: Centre for Integrative Neuroscience & MEG Center, University of Tübingen, Germany. Electronic address: markus.siegel@uni-tuebingen.de.
authors.forename: Markus
authors.initials: M
authors.lastname: Siegel
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001931:Brain Mapping / Q000379:methods*; D002540:Cerebral Cortex / Q000502:physiology*; D004569:Electroencephalography / Q000379:methods*; D005071:Evoked Potentials / Q000502:physiology; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D015225:Magnetoencephalography / Q000379:methods; D008297:Male; D017711:Nonlinear Dynamics; D015203:Reproducibility of Results; D015656:Respiratory Mechanics / Q000502:physiology*; D012680:Sensitivity and Specificity; D055815:Young Adult
keywords: EEG; MEG; Neural entrainment; Stimulation artifacts; Transcranial alternating current stimulation (tACS); Transcranial direct current stimulation (tDCS); Transcranial electric stimulation (tES)