How does the electric field induced by tDCS influence motor-related connectivity? Model-guided perspectives

Abstract

Over the last decade, Transcranial Direct Current Stimulation (tDCS) has been applied not only to modulate local cortical activation, but also to address communication between functionally-related brain areas. Stimulation protocols based on simple two-electrode placements are being replaced by multi-electrode montages to target intra- and inter-hemispheric neural networks using multichannel/high definition paradigms. 
Objective: This study aims to investigate the characteristics of electric field (EF) patterns originated by tDCS experiments addressing changes in functional brain connectivity. 
Methods: A previous selection of tDCS experimental studies aiming to modulate motor-related connectivity in health and disease was conducted. Simulations of the EF induced in the cortex were then performed for each protocol selected. The EF magnitude and orientation are determined and analysed in motor-related cortical regions for five different head models to account for inter-subject variability. Functional connectivity outcomes obtained are qualitatively analysed at the light of the simulated EF and protocol characteristics, such as electrode position, number and stimulation dosing.
Main findings: The EF magnitude and orientation predicted by computational models can be related with the ability of tDCS to modulate brain functional connectivity. Regional differences in EF distributions across subjects can inform electrode placements more susceptible to inter-subject variability in terms of brain connectivity-related outcomes. 
Significance: Neuronal facilitation/inhibition induced by tDCS fields may indirectly influence intra and inter-hemispheric connectivity by modulating neural components of motor-related networks. Optimization of tDCS using computational models is essential for adequate dosing delivery in specific networks related to clinically relevant connectivity outcomes.

Addressing the sources of inter-subject variability in E-field parameters in anodal tDCS stimulation over motor cortical network

“…Following the procedure reported in (Fernandes et al 2024), we assumed E-field values larger than 0.15 V m −1 as thresholds values for neuromodulatory effects over the hand knob, based on previous modeling works that reproduced clinical studies where these effects were observed (Yousry et al 1997, Simone et al 2021. As such, we calculated the individual stimulation intensities which led to E-field values larger than 0.15 V m −1 in the hand-knob region for each of the two montages selected.…”

“…On the other hand, the E-field threshold considered (0.15 V m −1 ) has been selected according to previous experimental Paulus 2000, Lang et al 2004) and modeling studies (Miranda et al 2013, Fernandes et al 2024. However, there is no consensus on a definitive threshold for neuromodulatory effects, which may vary across subjects exhibiting a variety of source dependencies (Ishii et al 2021).…”