Bayesian statistics show a lack of change in excitability following bi-hemispheric HD-TDCS over the primary somatosensory cortices

Disrupting brain activity focally is important to pinpoint which brain regions are involved in a cognitive process. Using small ring electrodes, High Definition transcranial Current Stimulation (HDtDCS) should increases focality up to 80% compared with standard tDCS [1]. Likewise, simulations using the Finite Element Method (FEM) suggest that using only one cathode and one anode should maximize focality, albeit with reduced current penetration [2]. Here, we share the utility of Bayesian statistics in an attempt to leverage a 1x1montage using 5.5mm diameter electrodes to focally stimulate the primary somatosensory cortex (SI). The role of SI in cognitive functions has received increasing interest, motivating our desire to interrogate its function [3]. SI is necessary for understanding others people’s pain, but whether the hand vs. the face regions of SI support this function remains difficult to investigate without increased focality [4]. Since SI has bilateral receptive fields [5] we were interested in bi-hemispherically stimulating SI. For cognitive neuroscience experiments with moderate sample sizes (n 1⁄4 20), a power analysis shows, a montage would need to have an effect size d 0.6 to be useful. We used an intensity of stimulation similar to previously HDtDCS studies (1.5 mA), and divided it across hemispheres/anodes (0.75 mA each). We placed one anode on the left between P3 and CP3 and one on the right between P4 and CP4, and the respective cathodes along the central sulcus between CZ and CP1 and between CZ and CP2 (Fig. 1A). Computational models suggest this montage should induce currents over the hand region of SI (Fig. 1A). We used a sham and an active tDCS session per participant, one week apart, in counterbalanced order. In the active stimulation, current was ramped up over 30s, held at 0.75 mA in both anodes for 18 min, and ramped down over 30s. In the sham condition, the current was ramped up and down at the beginning and at the end of the 18 min and kept around 0mA in-between [6]. We measured the effect via Somatosensory Evoked Potentials (SEP; N20, N30, P24 and P45, in particular; Fig. 1B) triggered by stimulation of the right median