P 63 Investigating the effects of tRNS variants and task dependency on cortical excitability

Abstract

Background Transcranial random noise stimulation (tRNS) has specific benefits and is fundamentally different as compared to more established methods such as transcranial direct current stimulation (tDCS) in modulating brain function (Terney et al., 2008; Ambrus et al., 2010). For example, previous research in the motor system has shown corticospinal excitability (CSE) to be task-dependent (Terney et al., 2008). It remains unresolved and largely unaddressed what the critical stimulation parameters of tRNS are, e.g. with regard to noise type (i.e. Gaussian, uniform) or current intensity (i.e. peak-to-peak amplitude, I max; effective current, I eff ). To study the effects of different tRNS parameters and task-dependency, Experiment 1 compared conventional, Gaussian tRNS (g-tRNS, 1 mA I max ) with uniform tRNS (u-tRNS, 1 mA I max ). Experiment 2 compared tRNS to anodal tDCS at higher, but equivalent effective current intensities (0.8 mA I eff ). The hypotheses were that (i) effects would be independent of noise distribution type and dependent on current intensity and (ii) that anodal tDCS and tRNS would induce similar effects if applied with comparable current intensities. The primary outcome measure was corticospinal excitability (CSE). Methods Twenty-nine healthy, right-handed individuals received stimulation over the left M1 in a double-blind, sham-controlled, within-subjects study design. Experiment 1 investigated g-tRNS (1 mA I max , 0.22 mA I eff ) and u-tRNS (1 mA I max ; 0.52 mA I eff ). Experiment 2 investigated tRNS (0.8 mA I eff ; 1.51 mA I max ) and anodal tDCS (0.8 mA I eff ; 0.8 mA I max ). Participants were randomly assigned one of two motor tasks to perform during tRNS/tDCS or sham stimulation. Half of the participants performed a simple motor task associated with cortical excitation (finger tapping-task, FT), the others performed a complex motor task associated with response inhibition (go/no-go-task, GNG). CSE was evaluated with 20 MEPs from the FDI prior to- and after 10 min of stimulation with task-execution. Results In Experiment 1, both g-tRNS and u-tRNS did not affect CSE. At higher current intensities, Experiment 2 found diverging task dependent effects comparing tRNS to anodal tDCS. TRNS facilitated CSE in the FT-group (331 μ V, +/−133 μ V) while inhibiting CSE in the GNG-group (−113 μ V, +/−59 μ V) with a significant difference between groups (pairwise comparisons, p = 0.004). TDCS in the FT group, significantly inhibited CSE (−80 μ V +/−106 μ V; pairwise comparisons, p = 0.044) and showed no effect in the GNG group. Conclusions Low tRNS current intensities do not influence CSE. Higher tRNS intensities (0.8 mA I eff ), but not anodal tDCS (0.8 mA), modulate CSE in a task-dependent manner. Future studies may benefit from the higher effective current intensities possible with uniform but not Gaussian noise and may take advantage of task dependency to potentiate the effect of tRNS.