Using TMS-EEG to better understand the effects of cathodal direct current stimulation on cortical excitability and connectivity

Despite transcranial direct current stimulation (tDCS) is a widely used non-invasive brain stimulation techniques, the cathodal tDCS effects on task performance are still controversial and unclear. A deeper understanding of the plastic changes induced by cathodal tDCS would be crucial to improving the refinement of stimulation protocols for clinical and research purposes. In a previous study, we investigated the effect of cathodal tDCS on cortical excitability at a resting state, using a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG). Results showed no significant modulation effects during and after cathodal stimulation in comparison to a pre-tDCS session. In the present study, we used the same paradigm to study the effect of cathodal tDCS during a task. Single TMS pulses were delivered over the left posterior parietal cortex (PPC), before and after 15 minutes of cathodal or sham tDCS over the right PPC, while recording hd-EEG. During the tDCS stimulation, the subjects were involved in a Posner task that requires visual-attentional abilities, likely involving the PPC. We opted for the Posner task in our experimental protocol on the basis of the results of a pilot study in which we found that cathodal tDCS over the right PPC reduced the performance at the same task, increasing reaction times. As in the previous study, we estimated the indexes of global and local cortical excitability, both at sensors and cortical sources level. At sensors, global and local mean field power (GMFP and LMFP) were computed for three temporal windows (0-50, 50-100 and 100-150 ms), in all channels (GMFP), and in four different clusters of electrodes (LMFP, left and right, in frontal and parietal regions). After source reconstruction, Significant Current Density was computed at a global level, and in four Broadmann’s areas (left/right BA 6 and 7). At a behavioural level, we expected to replicate the results of the pilot study; while at a neurophysiological level we predicted a reduction of cortical excitability only in the areas strictly involved in the task performance. Such results would confirm the hypothesis of a causal interaction between the spontaneous ongoing cortical activity and the electric stimulation.