title: TDCS increases cortical excitability: direct evidence from TMS-EEG.
abstract: Despite transcranial direct current stimulation (tDCS) is increasingly used in experimental and clinical settings, its precise mechanisms of action remain largely unknown. At a neuronal level, tDCS modulates the resting membrane potential in a polarity-dependent fashion: anodal stimulation increases cortical excitability in the stimulated region, while cathodal decreases it. So far, the neurophysiological underpinnings of the immediate and delayed effects of tDCS, and to what extent the stimulation of a given cerebral region may affect the activity of anatomically connected regions, remain unclear. In the present study, we used a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG) in order to explore local and global cortical excitability modulation during and after active and sham tDCS. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, during, and after 15 min of tDCS over the right PPC, while EEG was recorded from 60 channels. For each session, indexes of global and local cerebral excitability were obtained, computed as global and local mean field power (Global Mean Field Power, GMFP and Local Mean Field Power, LMFP) on mean TMS-evoked potentials (TEPs) for three temporal windows: 0-50, 50-100, and 100-150 msec. The global index was computed on all 60 channels. The local indexes were computed in six clusters of electrodes: left and right in frontal, parietal and temporal regions. GMFP increased, compared to baseline, both during and after active tDCS in the 0-100 msec temporal window. LMFP increased after the end of stimulation in parietal and frontal clusters bilaterally, while no difference was found in the temporal clusters. In sum, a diffuse rise of cortical excitability occurred, both during and after active tDCS. This evidence highlights the spreading of the effects of anodal tDCS over remote cortical regions of stimulated and contralateral hemispheres.
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy. Electronic address: leonor.romero1@unimib.it.
authors.forename: Leonor J
authors.initials: LJ
authors.lastname: Romero Lauro
authors.affiliation: Department of Biomedical and Clinical Sciences "L. Sacco", University of Milano, Via GB Grassi 74, Milano, Italy; Fondazione Europea di Ricerca Biomedica FERB Onlus, Milano, Italy.
authors.forename: Mario
authors.initials: M
authors.lastname: Rosanova
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy.
authors.forename: Giulia
authors.initials: G
authors.lastname: Mattavelli
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Convento
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Pisoni
authors.affiliation: Department of Clinical Neurophysiology, Robert-Koch-Straße 40, Göttingen, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Opitz
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy; Laboratory of Neuropsychology, IRCCS Istituto Auxologico Italiano, Via Mercalli 32, Milano, Italy.
authors.forename: Nadia
authors.initials: N
authors.lastname: Bolognini
authors.affiliation: Department of Psychology, University of Milano-Bicocca, P.za Ateneo Nuovo 1, Milano, Italy; Laboratory of Neuropsychology, IRCCS Istituto Auxologico Italiano, Via Mercalli 32, Milano, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Vallar
mesh_terms: D000328:Adult; D004569:Electroencephalography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D010296:Parietal Lobe / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: Cortical excitability; Posterior parietal cortex; TMS-EEG; tDCS