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

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 cortical excitability by shifting the resting membrane potential in a polarity-dependent way: anodal stimulation increases the spontaneous firing rate in the stimulated region, while cathodal decreases it. So far, the neurophysiological underpinnings of the immediate and delayed effects of tDCS are still unclear, as well as, it is not fully understood 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 a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG). Results showed a diffuse rise of cortical excitability in a bilateral fronto-parietal networks. In the present study, we used the same technique and methodological settings in order to explore local and global cortical excitability modulation during and after cathodal tDCS. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, during, and after 10 minutes 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 four temporal windows: 0-50, 50-100, 100-150 and 150-200 milliseconds. The global index was computed on all 60 channels. The local indexes were computed in four clusters of electrodes: left and right, in frontal and parietal regions. The preliminary results on fifteen subjects show no differences in both sessions during and after cathodal tDCS compared to pre-stimulation session. 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.