Excitability modulation of the motor system induced by transcranial direct current stimulation: A multimodal approach

Abstract

Anodal and cathodal transcranial direct current stimulations (tDCS) are both established techniques to induce cortical excitability changes. Typically, in the human motor system, such cortical modulations are inferred through changes in the amplitude of the motor evoked potentials (MEPs). However, it is now possible to directly evaluate tDCS-induced changes at the cortical level by recording the transcranial magnetic stimulation evoked potentials (TEPs) using electroencephalography (EEG). The present study investigated the modulation induced by the tDCS on the motor system. The study evaluates changes in the MEPs, in the amplitude and distribution of the TEPs, in resting state oscillatory brain activity and in behavioral performance in a simple manual response task. Both the short-and long-term tDCS effects were investigated by evaluating their time course at~0 and 30 min after tDCS. Anodal tDCS over the left primary motor cortex (M1) induced an enhancement of corticospinal excitability, whereas cathodal stimulation produced a reduction. These changes in excitability were indexed by changes in MEP amplitude. More interestingly, tDCS modulated the cortical reactivity, which is the neuronal activity evoked by TMS, in a polarity-dependent and site-specific manner. Cortical reactivity increased after anodal stimulation over the left M1, whereas it decreased with cathodal stimulation. These effects were partially present also at long term evaluation. No polarity-specific effect was found either on behavioral measures or on oscillatory brain activity. The latter showed a general increase in the power density of low frequency oscillations (theta and alpha) at both stimulation polarities. Our results suggest that tDCS is able to modulate motor cortical reactivity in a polarity-specific manner, inducing a complex pattern of direct and indirect cortical activations or inhibitions of the motor system-related network, which might be related to changes in synaptic efficacy of the motor cortex.

Localizing the effects of anodal tDCS at the level of cortical sources: A Reply to Bailey et al., 2015

“…In a recent study (Pellicciari, Brignani, & Miniussi, 2013), an increment of both cortical and corticospinal measures, namely TEPs and MEPs, was found after anodal tDCS over M1, suggesting that tDCS alone might raise cortical reactivity underneath the electrode and in the contralateral homologue area. However, to definitely exclude any potential effect of the TMS-tDCS combination, we ran a control experiment in which no TMS was applied during the tDCS stimulation.…”

TMS and drugs revisited 2014

“…They showed that the motor cortical area ipsilateral to the stimulation responded within 3-7 ms, while contralateral motor areas responded with longer latencies (17-28 ms) (Ilmoniemi et al, 1997;Komssi et al, 2002). More recent studies demonstrated that the amplitude of early components of motor cortex TEPs undergoes significant changes after the application of rTMS or transcranial Direct Current Stimulation (tDCS), two brain stimulation techniques that are thought to modulate synaptic strength and cortical excitability (Esser et al, 2006;Pellicciari et al, 2013;Veniero et al, 2012Veniero et al, , 2010. Finally, it has been shown that the peak-to-peak amplitude of the N15-P30 complex (composed by a negative deflection at 15 ms followed by a positive deflection at 30 ms) correlated with MEP amplitude on a single-trial basis (Mäki and Ilmoniemi, 2010) and was strongly affected by the TMS coil orientation (Bonato et al, 2006).…”

Enhancement of Cortical Excitability and Lower Limb Motor Function in Patients With Stroke by Transcranial Direct Current Stimulation

“…Anodal stimulation increases cortical excitability, whereas cathodal stimulation decreases it. Functional neuroimaging and transcranial magnetic stimulation (TMS) studies report that tDCS can modulate motor cortex excitability in normal patients [16,18,19]. However, little is known about this topic in stroke patients, and knowledge regarding the mechanisms of motor recovery following tDCS remains limited in stroke patients.…”