Electrical field modelling of transcranial direct current stimulation

Abstract. Transcranial Direct Current Stimulation (tDCS) is a technology for neurotransmission that provides direct current to relatively intensive cortical areas in order to modulate internal brain activity. There have been inaccuracies in tDCS findings identified by recent studies. In this study, we present a modeling pipeline for computer-based tDCS analysis for studying different electrode montages and different sizes (1 x 6 cm²) and (5 x 7 cm²) of electrodes to find out the outcomes. The assumption is that a given brain region’s stimulation would be stable in different montages. The total simulation current flow and electric field distribution within the brain were determined for the four most widely used tDCS montages: F3-F4, F3-FP2, FP1-FP2, and C4-FP2, using the COMETS2 software tool. The effect of the size of the electrodes is simulated for tDCS in F3-FP2 mounts in small (1 x 6 cm²) and wide (5 x 7 cm²) rectangular electrodes. The current flow is calculated in order to examine the impact of the mounting setup for current density and electric field. Regional as well as localized current densities in the electrode sites have been considered for each of the various mounting setups

Transcranial Direct Current Stimulation (tDCS): A Beginner's Guide for Design and Implementation

Transcranial direct current stimulation (tDCS) is a popular brain stimulation method that is used to modulate cortical excitability, producing facilitatory or inhibitory effects upon a variety of behaviors. There is, however, a current lack of consensus between studies, with many results suggesting that polarity-specific effects are difficult to obtain. This article explores some of these differences and highlights the experimental parameters that may underlie their occurrence. We provide a general, practical snapshot of tDCS methodology, including what it is used for, how to use it, and considerations for designing an effective and safe experiment. Our aim is to equip researchers who are new to tDCS with the essential knowledge so that they can make informed and well-rounded decisions when designing and running successful experiments. By summarizing the varied approaches, stimulation parameters, and outcomes, this article should help inform future tDCS research in a variety of fields

Inter-individual variability in current direction for common tDCS montages

The direction of applied electric current relative to the cortical surface is a key determinant of transcranial direct current stimulation (tDCS) effects. Inter-individual differences in anatomy affect the consistency of current direction at a cortical target, likely leading to inter-individual variability in current direction. However, the degree of this variability remains undetermined. Using current flow modelling (CFM), we quantified the inter-individual variability in tDCS current direction at a cortical target (left primary motor cortex, M1). Three montages targeting M1 using circular electrodes were compared: PA-tDCS directed current perpendicular to the central sulcus in a posterior-anterior direction relative to M1, ML-tDCS directed current parallel to the central sulcus in a medio-lateral direction, and conventional-tDCS applied electrodes over M1 and the contralateral forehead. In 50 healthy brain scans from the Human Connectome Project, we extracted current direction and intensity from the gray matter surface in the sulcal bank (M1BANK) and gyral crown (M1CROWN), and neighbouring primary somatosensory cortex (S1BANK and S1CROWN). Results confirmed substantial inter-individual variability in current direction (50%-150%) across all montages. Radial inward current produced by PA-tDCS was predominantly located in M1BANK, whereas for conventional-tDCS it was clustered in M1CROWN. The predominantly radial inward current in functionally distinct subregions of M1 raises the testable hypothesis that PA-tDCS and conventional-tDCS modulate cortical excitability through different mechanisms. We show that electrode locations can be used to closely approximate current direction in M1 and precentral gyrus, providing a landmark-based method for tDCS application to address the hypothesis without the need for MRI. By contrast, ML-tDCS current was more tangentially oriented, which is associated with little somatic polarization. Substantial inter-individual variability in current direction likely contributes to variable neuromodulation effects reported for these protocols, emphasising the need for individualised electrode montages, including the control of current direction

Development of an Electrical Impedance Tomography Algorithm to Estimate the Scalp and Skull Conductivity

Tese de mestrado, Engenharia Biomédica e Biofísica , 2022, Universidade de Lisboa, Faculdade de CiênciasElectrical impedance tomography (EIT) is a technique used to estimate the conductivity value of biological tissues. EIT comprises two parts: forward and inverse solver. The forward solver aims measuring the electric potential in the brain after applying constant current directly on the scalp by at least two electrodes. Then, the inverse solver uses those electric potential values to predict the conductivity of the brain tissues. The project described in this report aimed to evaluate the possibility of using an EIT algorithm to predict scalp and skull conductivities to generate more realist head models. These heads models are used to generate transcranial direct current stimulation (tDCS) electrode montages for patients with mental disorders. This project took place in Neuroelectrics, a company already with a pipeline able to generate these montages based on the anatomy of the patient but using standard conductivity values for brain tissues. After testing the EIT algorithm, the project also aimed to evaluate the impact of using standard head models or personalized ones not only with the anatomy of the patient but also with the correct values for conductivity brain tissues on the generation of tDCS electrode montages. In fact, the results of this study showed that changes in the conductivity values can have a huge impact in the electric field applied in the brain, which means that it is important to generate a montage that takes the correct values into account instead of standard ones. In more detail, after comparing the error obtained with montages generated by template head model used in Neuroelectrics and montages generated by EIT template developed in this project, it was possible to state that there was a reduction around 21% in the error of the average of the electric field applied in the brain and a reduction around 10% in the error of the focality of the stimulation

A Future of Current Flow Modelling for Transcranial Electrical Stimulation?

Purpose of Review: Transcranialelectrical stimulation (tES) is used to non-invasively modulate brain activityin health and disease. Current flow modeling (CFM) provides estimates of whereand how much electrical current is delivered to in the brain during tES. Ittherefore holds promise as a method to reduce commonplace variability in tESdelivery and, in turn, the outcomes of stimulation. However, the adoption ofCFM has not yet been widespread and its impact on tES outcome variability isunclear. Here, we discuss the potential barriers to effective, practicalCFM-informed tES use. Recent Findings: CFMhas progressed from models based on concentric spheres to gyri-precise headmodels derived from individual MRI scans. Users can now estimate the intensityof electrical fields (E-fields), their spatial extent, and the direction ofcurrent flow in a target brain region during tES. Here. we consider the multi-dimensionalchallenge of implementing CFM to optimise stimulation dose: this requiresinformed decisions to prioritise E-field characteristics most likely to resultin desired stimulation outcomes, though the physiological consequences of themodelled current flow are often unknown. Second, we address the issue of adisconnect between predictions of E-field characteristics provided by CFMs andpredictions of the physiological consequences of stimulation which CFMs are notdesigned to address. Third, we discuss how ongoing development of CFM inconjunction with other modelling approaches could overcome these challengeswhile maintaining accessibility for widespread use. Summary: Theincreasing complexity and sophistication of CFM is a mandatory step towards dosecontrol and precise, individualised delivery of tES. However, it also riskscounteracting the appeal of tES as a straightforward, cost-effective tool forneuromodulation, particularly in clinical settings

Optimising the Application of Transcranial Direct Current Stimulation

The ability of transcranial direct current stimulation (tDCS) to modulate brain activity has vast scientific and therapeutic potential, however, its effects are often variable which limit its utility. Both current flow direction and variance in electric field intensities reaching a cortical target may be vital sources of the variable tDCS effects on neuroplastic change. Controlling for these and exploring the subsequent effects on corticospinal excitability is the aim of this thesis. I here attempted to optimise the delivery of tDCS application by investigating the controlled application of current flow direction and whether through the use of current flow models, we can deliver comparable electric fields with reduced variability across differential montages. To assess whether current flow models are useful, I further investigated if dose-control translates to more consistent physiological outcomes. I demonstrate, firstly, that different current flow directions did not differentially affect the two banks of the central sulcus. Secondly, with the use of dose-control, high-definition tDCS (HD-tDCS) remains focally more advantageous, even with the delivery of comparable electric field intensity and variability as posterior-anterior tDCS (PA-tDCS) to a cortical region. Thirdly, dose-controlled tDCS does not translate to reduced physiological variability. Together, the work presented here suggests that current flow models are useful for informing dose-controlled protocols and montage comparisons for improved tDCS delivery, however, controlling for anatomical differences in the delivery of electric fields to a target is not sufficient to reduce the variability of tDCS effects in physiology. Thus, the methodology for optimised tDCS delivery remains a subject for further improvement and investigation. Advancements in this field may lead to a trusted methodology assisting stroke survivors with a more effective and efficient motor recovery journey

Interindividual variability of electric fields during transcranial temporal interference stimulation (tTIS)

Abstract Transcranial temporal interference stimulation (tTIS) is a novel non-invasive brain stimulation technique for electrical stimulation of neurons at depth. Deep brain regions are generally small in size, making precise targeting a necessity. The variability of electric fields across individual subjects resulting from the same tTIS montages is unknown so far and may be of major concern for precise tTIS targeting. Therefore, the aim of the current study is to investigate the variability of the electric fields due to tTIS across 25 subjects. To this end, the electric fields of different electrode montages consisting of two electrode pairs with different center frequencies were simulated in order to target selected regions-of-interest (ROIs) with tTIS. Moreover, we set out to compare the electric fields of tTIS with the electric fields of conventional tACS. The latter were also based on two electrode pairs, which, however, were driven in phase at a common frequency. Our results showed that the electric field strengths inside the ROIs (left hippocampus, left motor area and thalamus) during tTIS are variable on single subject level. In addition, tTIS stimulates more focally as compared to tACS with much weaker co-stimulation of cortical areas close to the stimulation electrodes. Electric fields inside the ROI were, however, comparable for both methods. Overall, our results emphasize the potential benefits of tTIS for the stimulation of deep targets, over conventional tACS. However, they also indicate a need for individualized stimulation montages to leverage the method to its fullest potential