Modulating parameters of excitability during and after transcranial direct current stimulation of the human motor cortex

Abstract

Weak transcranial direct current stimulation (tDCS) of the human motor cortex results in excitability shifts which occur during and after stimulation. These excitability shifts are polarity-specific with anodal tDCS enhancing excitability, and cathodal reducing it. To explore the origin of this excitability modulation in more detail, we measured the input-output curve and motor thresholds as global parameters of cortico-spinal excitability, and determined intracortical inhibition and facilitation, as well as facilitatory indirect wave (I-wave) interactions. Measurements were performed during short-term tDCS, which elicits no after-effects, and during other tDCS protocols which do elicit short-and long-lasting after-effects. Resting and active motor thresholds remained stable during and after tDCS. The slope of the input-output curve was increased by anodal tDCS and decreased by cathodal tDCS. Anodal tDCS of the primary motor cortex reduced intracortical inhibition and enhanced facilitation after tDCS but not during tDCS. Cathodal tDCS reduced facilitation during, and additionally increased inhibition after its administration. During tDCS, I-wave facilitation was not influenced but, for the after-effects, anodal tDCS increased I-wave facilitation, while cathodal tDCS had only minor effects. These results suggest that the effect of tDCS on cortico-spinal excitability during a short period of stimulation (which does not induce after-effects) primarily depends on subthreshold resting membrane potential changes, which are able to modulate the input-output curve, but not motor thresholds. In contrast, the after-effects of tDCS are due to shifts in intracortical inhibition and facilitation, and at least partly also to facilitatory I-wave interaction, which is controlled by synaptic activity.

Beyond the silence: Bilateral somatosensory stimulation enhances skilled movement quality and neural density in intact behaving rats

“…A summary of the measurement procedure for cortical thickness (adapted from [22,23]), using 10 representative histology sections, is shown in Fig. 2A and a. Briefly, four points (medial, central, lateral, and ventrolateral) on ten coronal sections (planes 10,13,17,21,24,27,32,36,41,45) from each brain were selected based on Paxinos and Watson [24]. The most rostral section measured was located at ∼2.20 mm anterior to bregma and the most caudal section at ∼−6.30 mm posterior to bregma.…”

“…A calibration was made by step tablet (http://rsb.info.nih.gov/ij/download.html) for optical density prior to the analysis of density. Two approximate planes (planes 27 and 32, ∼−1.88 mm and 10,13,17,21,24,27,32,36,41,45) from each brain were selected. Panel a shows ten approximate points of the sections, from the most rostral section located at ∼2.20 mm anterior to bregma to the most caudal section at ∼−6.30 mm posterior to bregma.…”

The Use of Transcranial Direct Current Stimulation for Cognitive Enhancement

“…Currently, there are no reports of adverse effects in clinical or healthy adults, children, or elderly populations (Kessler et al, 2013). It also has been consistently shown that stimulation continuously for 20 min (Iyer et al, 2005;Nitsche et al, 2005) and even up to 30 min ( Boggio et al, 2012;Lindenberg et al, 2013;Lindenberg et al, 2010) is safe. Furthermore, it was shown that tDCS-induced behavioral and neurophysiological effects in humans were fully reversible (Nitsche et al, 2003c).…”

Computational modeling of neurostimulation in brain diseases

“…There is still no full agreement on the mechanism of action of TCS, and this might differ for the treatment of different conditions. The general consensus is that TCS is essentially modulating the excitability of the stimulated tissue (Nitsche et al, 2005). We refer the reader to some reviews on this topic for a summary (Bestmann et al, 2015;Brunoni et al, 2012;de Berker et al, 2013;Krause et al, 2013;Liebetanz et al, 2002;Medeiros et al, 2012;Nitsche et al, 2015;Pelletier and Cicchetti, 2014).…”