Transcranial direct current stimulation (tDCS) is increasingly used in both research and therapeutic settings, but its precise mechanisms remain largely unknown. At a neuronal level, tDCS modulates cortical excitability by shifting the resting membrane potential in a polarity-dependent way: anodal stimulation increases the spontaneous firing rate, while cathodal decreases it. However, the neurophysiological underpinnings of anodal/cathodal tDCS seem to be different, as well as their behavioral effect, in particular when high order areas are involved, compared to when motor or sensory brain areas are targeted. Previously, we investigated the effect of anodal tDCS on cortical excitability, by means of a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG). Results showed a diffuse rise of cortical excitability in a bilateral fronto-parietal network. In the present study, we tested, with the same paradigm, the effect of cathodal tDCS. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, during, and after 10 min of cathodal or sham tDCS over the right PPC, while recording HD-EEG. Indexes of global and local cortical excitability were obtained 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), on 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 the global level, and for four Broadmann's areas (left/right BA 6 and 7). Both sensors and cortical sources results converge in showing no differences during and after cathodal tDCS compared to pre-stimulation sessions, both at global and local level. The same holds for sham tDCS. These data highlight an asymmetric impact of anodal and cathodal stimulation on cortical excitability, with a diffuse effect of anodal and no effect of cathodal tDCS over the parietal cortex. These results are consistent with the current literature: while anodal-excitatory and cathodal-inhibitory effects are well-established in the sensory and motor domains, both at physiological and behavioral levels, results for cathodal stimulation are more controversial for modulation of exitability of higher order areas.
34Monophasic and biphasic TMS pulses and coil orientations produce different responses in terms of 35 motor output and sensory perception. Those differences have been attributed to the activation of 36 specific neural populations. However, up to date, direct evidence supporting this hypothesis is still 37 missing since studies were mostly based on indirect measures of cortical activation, i.e., motor evoked 38 potentials or phosphenes. Here, we investigated for the first time the impact of different coil 39 orientations and waveforms on a non-primary cortical area, namely the premotor cortex, by measuring 40 TMS evoked EEG potentials (TEPs). We aimed at determining whether TEPs produced by differently 41 oriented biphasic and monophasic TMS pulses diverge and whether these differences are underpinned 42 by the activation of specific neural populations. To do so, we applied TMS over the right premotor 43 cortex with monophasic or biphasic waveforms oriented perpendicularly (in the anterior-posterior 44 direction and vice-versa) or parallel (latero-medial or medio-laterally) to the target gyrus. EEG was 45 concurrently recorded from 60 electrodes. We analyzed TEPs at the level of EEG sensors and cortical 46 sources both in time and time-frequency domain. Biphasic pulses evoked larger early TEP 47 components, which reflect cortical excitability properties of the underlying cortex, in both parallel 48 directions when compared to the perpendicular conditions. Conversely, monophasic pulses, when 49 oriented perpendicularly to the stimulated gyrus, elicited a greater N100, which is a reliable TEP 50 component linked to GABAb-mediated inhibitory processes, than when parallel to the gyrus. Our 51 results provide direct evidence supporting the hypothesis that TMS pulse waveform and TMS coil 52 orientations affect which neural population is engaged. 53 54 55 56 57 58 59 60
The extensive use of transcranial direct current stimulation (tDCS) in experimental and clinical settings does not correspond to an in-depth understanding of its underlying neurophysiological mechanisms. In previous studies, we employed an integrated system of Transcranial Magnetic Stimulation and Electroencephalography (TMS-EEG) to track the effect of tDCS on cortical excitability. At rest, anodal tDCS (a-tDCS) over the right Posterior Parietal Cortex (rPPC) elicits a widespread increase in cortical excitability. In contrast, cathodal tDCS (c-tDCS) fails to modulate cortical excitability, being indistinguishable from sham stimulation. Here we investigated whether an endogenous task-induced activation during stimulation might change this pattern, improving c-tDCS effectiveness in modulating cortical excitability. In Study 1, we tested whether performance in a Visuospatial Working Memory Task (VWMT) and a modified Posner Cueing Task (mPCT), involving rPPC, could be modulated by c-tDCS. Thirty-eight participants were involved in a two-session experiment receiving either c-tDCS or sham during tasks execution. In Study 2, we recruited sixteen novel participants who performed the same paradigm but underwent TMS-EEG recordings pre- and 10 minutes post-sham and c-tDCS. Behavioral results showed that c-tDCS significantly modulated mPCT performance compared to sham. At a neurophysiological level, c-tDCS significantly reduced cortical excitability in a frontoparietal network involved in task execution. Taken together, our results provide evidence of the state dependence of c-tDCS in modulating cortical excitability effectively. The conceptual and applicative implications are discussed.
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