Impact of galvanic vestibular stimulation electrode current density on brain current flow patterns: Does electrode size matter?

Abstract

Background: Galvanic vestibular stimulation (GVS) uses at least one electrode placed on the mastoid process with one or multiple placed over other head areas to stimulate the vestibular system. The exact electrode size used is not given much importance in the literature and has not been reported in several studies. In a previous study, we compared the clinical effects of using different electrode sizes (3 cm2 and 35 cm2) with placebo but with the same injected current, on postural control. We observed significant improvement using the smaller size electrode but not with the bigger size electrode. The goal of this study was to simulate the current flow patterns with the intent to shed light and potentially explain the experimental outcome.

Methods: We used an ultra-high-resolution structural dataset and developed a model to simulate the application of different electrode sizes. We considered current flow in the brain and in the vestibular labyrinth.

Results: Our simulation results verified the focality increase using smaller electrodes that we postulated as the main reason for our clinical effect. The use of smaller size electrodes in combination with the montage employed also result in higher induced electric field (E-field) in the brain.

Conclusions: Electrode size and related current density is a critical parameter to characterize any GVS administration as the choice impacts the induced E-field. It is evident that the higher induced E-field likely contributed to the clinical outcome reported in our prior study.

Fig 1. Electrode placements and model detail.

Two different electrode sizes were simulated to apply GVS: 35 cm² (A-C) and ~3 cm² (D-F). The electrodes were placed on the skin-tissue mask in a Bilateral-Bipolar configuration. (G): The two electrode sizes were overlaid on the same head model geometry to highlight final placement with respect to the anatomy. (H): To highlight the region of interest in the model geometry with respect to other tissues and the stimulation electrode, certain tissue masks were made semi-transparent. (I): The dashed section in (H) is expanded to further highlight model detail.

Fig 2. Comparison of GVS induced current flow using small- and large-size electrodes.

Columns A (A1, A2, A3) and B (B1, B2, B3) correspond to the 35 cm² and 3 cm² electrodes, respectively. The current streamlines from both models are overlaid in C1. C2 and C3 indicate the difference in induced E-field due to the two electrode sizes. The first row (A1, B1 and C1) shows the current streamline plots with the brain, SCC and otolith organs masks visible to provide enhanced visualization of current flow with respect to their anatomy. The second row (A2, B2, and C2) shows the induced brain surface E-field plots. Similarly, the third row (A3, B3, and C3) shows the induced E-field plots on the vestibular labyrinth.