A Future of Current Flow Modelling for Transcranial Electrical Stimulation?

Abstract

Purpose of Review Transcranial electrical stimulation (tES) is used to non-invasively modulate brain activity in health and disease. Current flow modeling (CFM) provides estimates of where and how much electrical current is delivered to in the brain during tES. It therefore holds promise as a method to reduce commonplace variability in tES delivery and, in turn, the outcomes of stimulation. However, the adoption of CFM has not yet been widespread and its impact on tES outcome variability is unclear. Here, we discuss the potential barriers to effective, practical CFM-informed tES use. Recent Findings CFM has progressed from models based on concentric spheres to gyri-precise head models derived from individual MRI scans. Users can now estimate the intensity of electrical fields (E-fields), their spatial extent, and the direction of current flow in a target brain region during tES. Here. we consider the multi-dimensional challenge of implementing CFM to optimise stimulation dose: this requires informed decisions to prioritise E-field characteristics most likely to result in desired stimulation outcomes, though the physiological consequences of the modelled current flow are often unknown. Second, we address the issue of a disconnect between predictions of E-field characteristics provided by CFMs and predictions of the physiological consequences of stimulation which CFMs are not designed to address. Third, we discuss how ongoing development of CFM in conjunction with other modelling approaches could overcome these challenges while maintaining accessibility for widespread use. Summary The increasing complexity and sophistication of CFM is a mandatory step towards dose control and precise, individualised delivery of tES. However, it also risks counteracting the appeal of tES as a straightforward, cost-effective tool for neuromodulation, particularly in clinical settings.

Inter-individual variability in current direction for common tDCS montages

“…Both current direction [19,26,31,32,62,66] and E-field intensity [67,68] correlate with the effects of tDCS. However, common to all CFM approaches is the question how to incorporate this information for targeting or to improve the reliability of tDCS outcomes [15,20,61,64].…”

“…This approach attempts to reduce variability of tDCS outcomes by maximising electric field (E-field) intensity or focality in a target region [13][14][15] and by minimising E-field variability across individuals [16,17]. However, current direction is rarely discussed in the context of variability or dose-control [18][19][20].…”

Transcranial Electrical Stimulation for Associative Memory Enhancement: State-of-the-Art from Basic to Clinical Research

“…However, alternative electrode setups such as multichannel stimulation [ 48 ] or ring electrodes [ 39 , 55 ] also showed memory-modulatory effects. Recent advancement in electrical field modeling allows for optimizing montages to maximize the current density in the desired brain region [ 84 ]. Such an approach has been adopted by several experiments (e.g., [ 39 , 48 ]), however, modeling-informed experiment focusing on PPC has not been conducted yet.…”

Review of individualized current flow modeling studies for transcranial electrical stimulation

“…Importantly, current flow modeling alone cannot predict what modulatory effects will occur, see Figure 1. Current flow models only consider the anatomical structure, not the physiological state of the brain, that is, they do not provide predictions of the physiological impact of the target (Lee et al, 2021). For example, the effects of changes associated with physiological aging are not yet known and thus represent a latent cofactor for the age correlations mentioned above.…”