Electrode-specific dosimetric robustness of group-level optimized transcranial alternating current and temporal interference stimulation montages under placement variability.

The objective of this study was to characterize, for selected group-level optimized montages, electrode-specific dosimetric sensitivity to placement variability in transcranial alternating current stimulation (tACS) and transcranial temporal interference stimulation (tTIS).
Approach: Electric fields (EFs) were computed using the scalar-potential finite-difference (SPFD) method in 60 paired model instances generated from MRI data of 30 male participants (30 original models and their left-right mirrored counterparts). Three anatomically defined targets spanning cortical and subcortical structures were examined: the primary somatosensory hand area (S1 Hand), the insula, and the hippocampus. Every electrode constituting each selected montage was independently displaced from -10 mm to +10 mm in 5 mm increments on a 5 × 5 tangential grid. Three descriptive outcomes were evaluated: intensity sensitivity, defined as the absolute percentage change in the region-of-interest (ROI) mean; hotspot displacement; and position-dependent intensity variability, defined as the standard deviation (SD) of the percentage change across the full ±10 mm grid.
Main results: Across all 16 displaced electrodes, intensity sensitivity ranged from 0.41% to 14.88%, and hotspot displacement ranged from 0.63 mm to 6.86 mm. Marked within-pair asymmetries were observed in several montages, and position-dependent intensity variability differed across electrodes and model instances. These patterns were descriptively associated with montage geometry and current allocation.
Significance: The independent all-electrode perturbation framework introduced here enables electrode-specific robustness assessment of group-level optimized tACS and tTIS montages. The results identify montage-specific electrodes for which placement control is particularly important within the studied configurations, informing the practical implementation of these montages.