Methods to monitor accurate and consistent electrode placements in conventional transcranial electrical stimulation

Background: Inaccurate electrode placement and electrode drift during a transcranial electrical stimulation (tES) session have been shown to alter predicted field distributions in the brain and thus may contribute to a large variation in tES study outcomes. Currently, there is no objective and independent measure to quantify electrode placement accuracy/drift in tES clinical studies.

Objective/hypothesis: We proposed and tested novel methods to quantify accurate and consistent electrode placements in tES using models generated from a 3D scanner.

Methods: Accurate electrode placements were quantified as Discrepancy in eight tES participants by comparing landmark distances of physical electrode locations F3/F4 to their model counterparts. Distances in models were computed using curve and linear based methods. Variability of landmark locations in a single subject was computed for multiple stimulation sessions to determine consistent electrode placements across four experimenters.

Main results: We obtained an average of 0.4 cm in Discrepancy, which was within the placement accuracy/drift threshold (1 cm) for conventional tES electrodes (∼35 cm²) to achieve reliable tES sessions suggested in the literature. Averaged Variability was 5.2%, with F4 electrode location as the least consistent placement.

Conclusions: These methods provide objective feedback for experimenters on their performance in placing tES electrodes. Applications of these methods can be used to monitor electrode locations in tES studies of a larger cohort using F3/F4 montage and other conventional electrode arrangements. Future studies may include co-registering the landmark locations with imaging-derived head models to quantify the effects of electrode accuracy/drift on predicted field distributions in the brain.