Accessibility of Cortical Regions to Focal TES: Dependence on Spatial Position, Safety, and Practical Constraints

TL;DR

This study introduces a novel computational method for optimizing electrode placement and stimulation parameters in transcranial electric stimulation (TES) to improve targeting accuracy, while considering safety and practical constraints, and systematically assesses TES focality across the cortex.

Contribution

The paper presents a new, efficient optimization approach for TES electrode configuration that accounts for safety, electrode count, and field direction, and evaluates its effectiveness across brain regions.

Findings

Focality of TES is limited by biophysical and safety constraints.

Deep brain targeting with TES remains challenging due to physical limitations.

Optimized electrode placement improves targeting precision within safety limits.

Abstract

Transcranial electric stimulation (TES) can modulate intrinsic neural activity in the brain by injecting weak currents through electrodes attached to the scalp. TES has been widely used as a neuroscience tool to investigate how behavioural and physiological variables of brain function are modulated by electric stimulation of specific brain regions. For an unambiguous interpretation of TES experiments, it is important that the electric fields can be steered towards one or several brain regions-of-interest. However, the conductive proprieties of the human head impose inherent physical limitations on how focal the electric fields in the brain produced by multi-electrode TES can be. As a rule of thumb, it is not feasible to target deep brain areas with TES, although focusing the field in some specific deeper locations might be possible due to favourable conductive properties in the surrounding tissue. In the present study, we first propose a novel method for the automatic calculation of electrode placements and stimulation intensities to optimally affect a given target position. We provide a computationally efficient and robust implementation of the optimization procedure that is able to adhere to safety constraints, while explicitly controlling both the number of active electrodes and the angular deviation of the field in the target area relative to the desired field direction. Leveraging the high computational efficiency of our method, we systematically assess the achievable focality of multi-electrode TES for all cortex positions, thereby investigating the dependence on the chosen constraints. Our results provide comprehensive insight into the limitations regarding the achievable TES dose and focality that are imposed by the biophysical constraints and the safety considerations of TES.