Methods of Computational Modelling in Studies of Transcranial Direct Current Stimulation (tDCS) in Adults to Inform Protocols for Tinnitus Treatment: A Scoping Review.

Abstract

<b>Background</b>: Transcranial direct current stimulation (tDCS) involves the application of weak electric currents (typically 0.5-2 mA) via scalp electrodes to promote neuroplastic changes that modulate behaviour or cortical activity. Although there have been promising results in eliminating tinnitus or reducing its loudness or severity, there is also a high degree of inter-individual variability. This may be due to anatomical differences and their influence on the resulting electric field. To optimise and personalise tDCS protocols, computational electric field models based on individual clinical imaging may be utilised to give insight into the induced electric field during tDCS and inform more effective protocols for targeted stimulation. To our knowledge, there are currently no standards for current modelling or reviews which detail the optimal parameters for conducting current modelling studies for tDCS. <b>Objectives</b>: The aim of this review is to investigate the methodology of current modelling studies for tDCS so that informed, personalised protocols can be designed by modelling the electric field of the brain during tDCS for tinnitus. By considering the impact of individual anatomical differences on the electric field induced by tDCS, targeted protocols could be developed to reduce tinnitus loudness and severity in a systematic and predictable way. <b>Design</b>: The protocol for this review is based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) Checklist. Using online databases, records were identified based on a keyword search for records relevant to current modelling for tDCS, including peer-reviewed papers, clinical trials, the grey literature, theses, dissertations, and conference abstracts. Four thousand two hundred and fifty-three records were retrieved from thirteen online databases and include 4186 records from the initial search completed between May and July 2024, and 67 records from an updated search completed in August 2025. A further 596 records were retrieved from Google Scholar (501 from the initial search and 95 from the updated search). One hundred and fourteen records met our criteria for inclusion. Each record was charted by two separate reviewers, with attention to the modelling pipeline and predicted values in peak and range of electric field magnitude. <b>Results</b>: There was a consensus that, despite model parameters and pipelines, there was inter-individual variability in the predicted electric fields. The reviewed records highlighted the impact of individual differences, including age, sex, and anatomical variation, on the predicted electric field during tDCS. Increased age was often associated with age-related brain atrophy and high relative cerebrospinal fluid volume, which was a significant influence on the resulting E-field intensity and distribution. <b>Conclusions</b>: When creating personalised tDCS protocols for tinnitus, the model parameters and sources of variability (i.e., morphology, age, and sex) should be carefully considered to achieve the desired stimulation outcomes, particularly in regard to applied current intensity.