Abstract
Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique with potential in stroke rehabilitation by modulating cortical excitability. However, the optimal parameters, including electrode placement, current intensity, stimulation duration, and electrode size, remain poorly understood, and the interactions among these factors contribute to mixed results in motor recovery post-stroke. This review explores the various stimulation parameters and their impact on enhancing corticospinal excitability (CSE) and motor function recovery. Different electrode placement (montages), such as anodal, cathodal, and bi-hemispheric stimulation, have demonstrated varying effectiveness in restoring motor function. Bihemispheric stimulation demonstrated a larger effect size compared to other unihemispheric (anodal or cathodal) stimulation; however, its relative superiority remains inconclusive. Inter-individual anatomical variations, such as skull thickness, lesion location, and cortical atrophy, can affect tDCS outcomes, highlighting the need for personalized electrode placement guided by computational modeling based on brain imaging. Furthermore, stimulation intensity, typically 1-2 mA, exhibited nonlinear effects on CSE, contrasting with the dose-response relationships observed in earlier studies. Stimulation duration is also critical, with evidence suggesting that prolonged stimulation may reverse excitability-enhancing effects beyond a certain threshold. While smaller electrodes enhance focality, an appropriately sized electrode is necessary to effectively modulate electrical activity in the target region, with evidence suggesting a dose-response relationship between electrode size and motor recovery. Overall, the interplay among these parameters underscores the need for personalized and optimized tDCS protocols to achieve consistent motor recovery in stroke patients. Future research should focus on refining these parameters to maximize the therapeutic benefits of tDCS.