When Size Matters: Large Electrodes Induce Greater Stimulation-related Cutaneous Discomfort Than Smaller Electrodes at Equivalent Current Density

Abstract

Cutaneous discomfort is typically reported during transcranial direct current stimulation (tDCS), restricting the current intensity and duration at which tDCS can be applied. It is commonly thought that current density is associated with the intensity of perceived cutaneous perception such that larger electrodes with a lower current density results in milder cutaneous sensations. The present study examined the relationship between current density, current intensity and cutaneous sensations perceived during tDCS. Two experiments were performed. In the first control experiment, the cutaneous sensations induced by varying current intensities (0.025, 0.5, 1.0 and 1.5 mA) were examined up to 10 min. These data were used for optimizing inter-stimulation intervals in the second main experiment, where participants rated the intensity, spatial size and location of the cutaneous sensations experienced during tDCS using two electrodes sizes (16 cm2 and 35 cm2). In the equivalent current density condition, the current density was kept constant under both electrodes (0.014, 0.029 and 0.043 mA/cm2), whereas in the equal current intensity condition (0.5, 1.0 and 1.5 mA) the same intensities were used for the two electrode sizes. Large electrodes were associated with greater cutaneous discomfort when compared to smaller electrodes at a given current density. Further, levels of cutaneous perception were similar for small and large electrodes when current intensity was kept constant. Cutaneous sensations during stimulation can be minimized by reducing the electrode size from 35 cm2 to 16 cm2.