P230 Localizing the effects of anodal tDCS: A TMS-EEG study

Abstract

Introduction In the last decade, transcranial direct current stimulation (tDCS) has been increasingly used in experimental and clinical settings. Nevertheless, its neurophysiological underpinnings still remain unclear. Moreover, under debate is how and to what extent these effect spread through brain pathways. Objectives In this study, we explored local and global cortical excitability and time–frequency modulation after active and sham tDCS by means of an integrated Transcranial Magnetic Stimulation and Electroencephalography (TMS-EEG) system. Materials and methods Fourteen healthy subjects took part in the experiment. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, and after 15 min of anodal tDCS over the right PPC. 180 trials was delivered in each session, while EEG was concurrently recorded from 60 channels. Six subjects performed an additional session receiving sham tDCS. For each session, indexes of global and local cerebral excitability were obtained, computed as global and local mean field power (GMFP and LMFP) on mean TMS-evoked potentials (TEPs). LMFP were computed in six clusters of electrodes placed over the left and right frontal, parietal and temporal regions. Additionally, source analysis was performed computing Significant Current Density both globally and locally on Brodmann areas 6, 7 and 21. Moreover, we computed and index for Current Spread, to estimate where the current peak occurred in the pre and post stimulation conditions. Finally, time–frequency analyses were run to investigate how the natural frequencies of the stimulated area were affected by tDCS. Results Concerning results recorded at the sensor level, GMFP increased, compared to baseline, after active tDCS, while no difference was found between pre and post sham tDCS. LMFP increased after the end of stimulation in parietal and frontal clusters bilaterally, while no difference was found in the temporal clusters. Source modeling confirmed GMFP and LMFP results. Frontal and parietal regions (namely BA 6 and 7) increased in cortical current density after real tDCS. Conclusion Anodal tDCS induces a widespread increase of cortical excitability, both during and after the end of the stimulation, modulating a fronto-parietal network, likely following structural connections.