Tracking the effect of cathodal direct current stimulation on cortical excitability and connectivity by means of TMS-EEG

Introduction : Despite tDCS is increasingly used in experimental and clinical settings, its precise mechanisms remain largely unknown. At a neuronal level, tDCS modulates the cortical excitability by shifting the resting membrane potential in a polarity-dependent way: anodal stimulation increases the spontaneous firing rate, while cathodal decreases it. So far, the neurophysiological underpinnings of the immediate and delayed effects of tDCS are still unclear, as well as, how the stimulation of a given cerebral region may affect the activity of anatomically connected regions. In a previous study it has been investigated the tDCS anodal’s effect on the cortical excitability, by the means of TMS-EEG, showing a diffuse rise of cortical excitability in a bilateral fronto-parietal networks. Methods : Fifteen healthy, right-handed volunteers participated in the study. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, during, and after a 10 minutes long session of cathodal tDCS stimulation over the right PPC, while EEG was recorded from 60 channels. For each TMS 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 the mean TMS-evoked potentials (TEPs) for three temporal windows: 0-50, 50-100 and 100-150 ms. The global index was computed on all the 60 channels. While, the local indexes were computed in four different clusters of electrodes: left and right, in frontal and parietal regions. Results : The preliminary results show no differences in both the TMS sessions during and after cathodal tDCS compared to pre-stimulation one, both at global and local level. A further test using bayesian analysis was performed, providing a moderate indication toward null hypothesis. Discussion : These results are consistent with the literature in which the coupling of anodal-excitatory and cathodal-inhibitory effects are well established in the sensory and motor domains, both at physiological and behavioral levels, while the evidence is more controversial for higher-level mental activity. References : Bindman L. et al. Nature. (1962) 196:584e585. Jacobson L.et al. Exp Brain Res. (2012) 216(1):1-10. Romero Lauro L. J. Et al. Cortex, (2014) 58:99e111