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

ABSTRACT Background High Definition transcranial Direct Current Stimulation (HD-tDCS) is a method meant to explore the causal structure-function relationship of brain areas, developed to improve the spatial resolution of tDCS, but the validity of tDCS results is currently under intense debate Objective The goal of this study is to validate a new HD-tDCS protocol for bilateral modulation of the Somatosensory Cortex (SI). The new montage is meant to increase the focus of the stimulation while limiting the area of the scalp covered by electrodes. We aim to characterize the effect of the stimulation in terms of directionality, consistency and reproducibility. Methods We aim to leverage a 1 × 1 montage to most focally stimulate the primary somatosensory cortex (SI) and measure modulation via Somatosensory Evoked Potentials (SEP) triggered by median nerve stimulation. Results The results of Experiment1 suggest that our montage increases the amplitude of the SEP component N30. In Experiment2, we aim to replicate our finding and to assess the duration of the modulatory effect on N30 over time. Data from Experiment2 fails to replicate N30 modulation. A sequential Bayesian analysis performed on N30 data from both experiments indicates that the effect fluctuates across participants, without a clear homogenous directionality. Conclusion This study sets boundaries on the effect size that can be expected for this montage and illustrates the need to include replication samples or larger sample sizes to avoid overestimating effect sizes. We conclude that our montage has insufficient effect size for use in moderately sample-sized experimental studies and clinical applications.