Perception of comfort during transcranial DC stimulation: Effect of NaCl solution concentration applied to sponge electrodes

Objective To investigate the relationship between perception of comfort and electrolyte concentration and applied voltage during transcranial direct current stimulation (tDCS). Methods NaCl solutions (15, 140 and 220 mM NaCl) or deionised water were used as electrolytes to dampen tDCS sponge electrodes. Subjects (14, 7M, 20–60 years of age) rated comfort on an 11-point scale during 2 min of tDCS (1 mA). Results Overall participants rated tDCS as comfortable. Perception of comfort was negatively correlated with NaCl concentration (Spearman’s ρ = −0.88; p < 0.05), and a logarithmic relationship was found between applied voltage and ionic strength of electrolytes (Pearson’s r = −0.635; p < 0.01). There was no relationship between applied voltage and perception of comfort. Conclusions The application of NaCl solutions between 15 and 140 mM to sponge electrodes is more likely to be perceived as comfortable during tDCS. Significance The reporting of solution concentration and ratings of perception would be useful adjuncts to tDCS studies.