title: Faith and oscillations recovered: On analyzing EEG/MEG signals during tACS.
abstract: Despite recent success in analyzing brain oscillations recorded during transcranial alternating current stimulation (tACS), the field still requires further research to establish standards in artifact removal methods. This includes taking a step back from the removal of the tACS artifact and thoroughly characterizing the to-be-removed artifact. A recent study by Noury et al. (2016) contributed importantly to this endeavour by showing the existence of nonlinear artefacts in the tACS signal as seen by MEG and EEG. Unfortunately however this paper conveys the message that current artifact removal attempts have failed altogether and that-based on these available tools-brain oscillations recorded during tACS cannot be analyzed using MEG and EEG. Here we want to balance this overly pessimistic conclusion: In-depth reanalyses of our own data and phantom-head measurements indicate that nonlinearities can occur, but only when technical limits of the stimulator are reached. As such they are part of the "real" stimulation and not a specific MEG analysis problem. Future tACS studies should consider these technical limits to avoid any nonlinear modulations of the tACS artifact. We conclude that even with current approaches, brain oscillations recorded during tACS can be meaningfully studied in many practical cases.
authors.affiliation: Center for Mind/Brain Sciences, University of Trento, Via Delle Regole 101, 38123 Mattarello, Trento, Italy; Centre for Cognitive Neuroscience and Division of Physiological Psychology, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria. Electronic address: toralf.neuling@sbg.ac.at.
authors.forename: Toralf
authors.initials: T
authors.lastname: Neuling
authors.affiliation: Department of Neurology, Section of Neuropsychology, University of Magdeburg, Leipziger Str. 44, 39120 Magdeburg, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Ruhnau
authors.affiliation: Center for Mind/Brain Sciences, University of Trento, Via Delle Regole 101, 38123 Mattarello, Trento, Italy; Centre for Cognitive Neuroscience and Division of Physiological Psychology, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria.
authors.forename: Nathan
authors.initials: N
authors.lastname: Weisz
authors.affiliation: Experimental Psychology Lab, Cluster for Excellence "Hearing4all", European Medical School, University of Oldenburg, 26111 Oldenburg, Germany; Research Center Neurosensory Science, University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany.
authors.forename: Christoph S
authors.initials: CS
authors.lastname: Herrmann
authors.affiliation: Center for Mind/Brain Sciences, University of Trento, Via Delle Regole 101, 38123 Mattarello, Trento, Italy; Centre for Cognitive Neuroscience and Division of Physiological Psychology, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria.
authors.forename: Gianpaolo
authors.initials: G
authors.lastname: Demarchi
mesh_terms: D016477:Artifacts; D001921:Brain / Q000502:physiology*; D004569:Electroencephalography / Q000379:methods / Q000592:standards*; D006801:Humans; D015225:Magnetoencephalography / Q000379:methods / Q000592:standards*; D065908:Transcranial Direct Current Stimulation / Q000379:methods / Q000592:standards*
keywords: EEG; MEG; artifact removal; beamforming; brain oscillations; tACS