Polarity-Specific Cortical Effects of Transcranial Direct Current Stimulation in Primary Somatosensory Cortex of Healthy Humans

Abstract

Transcranial direct current stimulation (tDCS) is a non-invasive stimulation method that has been shown to modulate the excitability of the motor and visual cortices in human subjects in a polarity dependent manner in previous studies. The aim of our study was to investigate whether anodal and cathodal tDCS can also be used to modulate the excitability of the human primary somatosensory cortex (S1). We measured paired-pulse suppression (PPS) of somatosensory evoked potentials in 36 right-handed volunteers before and after anodal, cathodal, or sham stimulation over the right non-dominant S1. Paired-pulse stimulation of the median nerve was performed at the dominant and non-dominant hand. After anodal tDCS, PPS was reduced in the ipsilateral S1 compared to sham stimulation, indicating an excitatory effect of anodal tDCS. In contrast, PPS in the stimulated left hemisphere was increased after cathodal tDCS, indicating an inhibitory effect of cathodal tDCS. Sham stimulation induced no pre-post differences. Thus, tDCS can be used to modulate the excitability of S1 in polarity-dependent manner, which can be assessed by PPS. An interesting topic for further studies could be the investigation of direct correlations between sensory changes and excitability changes induced by tDCS.

Keywords: excitability; humans; neuronal plasticity; primary somatosensory cortex; tDCS.

FIGURE 1

Schematic representation of paired-pulse somatosensory evoked potentials from one single subject. Somatosensory evoked potentials are measured over CP3 or CP4 after single (continuous black trace) and paired-pulse stimulation with a stimulus-onset asynchrony of 30 ms (dotted black trace). The continuous grey trace resulted by subtracting the single-pulse trace from the paired-pulse trace. The analyzed amplitudes of the first response (A1) and second response (A2) after paired-pulse stimulation are marked by vertical bars; amplitudes of the second response after subtracting the response to a single pulse are denoted as A2s. Arrowheads mark onsets of the applied electrical stimuli.

FIGURE 2

Schematic representation of current direction and electrode positions for (A) anodal transcranial direct current stimulation (tDCS) and (B) cathodal tDCS of left hemisphere somatosensory cortex.

FIGURE 3

Mean paired-pulse ratios ± SD are plotted for the stimulated (A) and non-stimulated (B) hemisphere. Pre-tDCS and post tDCS ratios are shown for the anodal, cathodal, and sham group. In the stimulated hemisphere, paired-pulse ratios after anodal tDCS are increased compared to pre-tDCS. After cathodal tDCS, paired-pulse ratios are decreased compared to pre-tDCS (student paired t-test, p < 0.05). Post-tDCS ratios are increased in the anodal group compared to the sham group (student unpaired t-test, ^∗p < 0.05). tDCS, transcranial direct current stimulation.