Investigating Conventional Versus Advanced Transcranial Electrical Stimulation Protocols: Neurophysiological Effects on Cortical Excitability

Abstract Transcranial random noise stimulation (tRNS) is a newly designed non-invasive brain stimulation technique demonstrated to increase the motor cortex excitability. Correspondingly, a new electrode placement called high definition transcranial direct current stimulation (HD-tDCS) was proposed to have more focal effect in line with modelling studies and small number of experimental studies. HD-tDCS utilizes one stimulating or active electrode and four reference electrode known as “ring electrode”. We wanted to implement this new electrode placement on tRNS, and have a comparison with conventional tRNS setting to see the effects clearly. Of notice, this is the first study examining the physiological effects of the HD-tRNS. To determine the effects of these different stimulation paradigms transcranial magnetic stimulation (TMS) was utilized with motor evoked potentials (MEPs) measurement. MEPs were recorded from the first dorsal interosseus muscle (FDI). Seven healthy young adult subjects participated in the three experimental sessions with at least three days in between to avoid carry-over effects. Participants received HD-tRNS, conventional tRNS and low frequency tRNS in a randomized fashion. Initially, two separate sets of baselines MEPs with five minutes break were recorded, afterwards tRNS application was conducted for 10 minutes at 0.5 mA intensity. After the electrical stimulation seven separate MEP blocks were recorded starting from the end of tRNS with five minutes breaks. Results showed an excitability increase between 20-60 % for conventional tRNS with respect to low frequency stimulation whereas up to 30 % decrease in the motor cortex excitability was recorded with HD-tRNS electrode settings as compared to low frequency stimulation hinting inhibition. Low frequency stimulation did not affect MEP levels as it is in line with previous studies.