Abstract
The use of indirect cortical stimulation techniques to affect human corticomotor plasticity is a burgeoning field, not least because of the non-invasive nature of such protocols. Relatively new as a form of transcranial electrical stimulation (tES) is transcranial random noise stimulation (tRNS). While initial studies involving its use have shown promise, there is some concern that the effects of tRNS may share the between- and within-participants variability seen with indirect stimulation procedures (Such as Theta Burst Stimulation (TBS) protocols of Transcranial Magnetic Stimulation (TMS)), and its sibling techniques (Most notably tDCS). A review study consisting of the use of both tRNS and a sham procedure over 46 healthy participants was conducted, in order to test this variability of outcome. A literature review was conducted to examine the history of tES protocols and the ideal parameters for assessing tRNS effect variation. TMS was used to assess changes in motor evoked potential (MEP) amplitude both before and after the application of a standard facilitatory tRNS protocol. Baseline latency measurements with antero-posterior (AP) and latero-medial (LM) coil orientations and MEPs were recorded from the target muscle - the abductor pollis brevis - prior to the application of 10 min of 2 mA tRNS. Twenty MEPs were measured every 5 min for approximately 30 mins after the intervention to assess after-effects on corticospinal excitability. The experiment showed that tRNS at 2 mA was linked to a significant net facilitation of MEPs in the post-stimulation period, compared to sham stimulation. Furthermore, a two-step cluster analysis suggested alongside the presence of an intervention effect (ie. An effect caused by the experimental proceedings, instead of any effect from the tRNS protocol) as a result of both the tRNS and sham protocols, that tRNS had a facilitatory effect which mitigated inhibition or enhanced facilitation occuring as a result of that intervention effect. There was no significant correlation between the AP-LM latency difference of MEPs and the response to tRNS, or with any subject-specific variable beyond time of day. This study indicates that tRNS has a facilitatory effect in general upon corticomotor excitability, albeit rather too variable between subjects to allow tRNS to be considered a robust effector. Ideally, future studies should attempt to define what external or internal factors determine 'ideal' (i.e. significantly facilitatory participant responsiveness to tRNS.)