Transcranial direct current stimulation (tDCS) causes a complex spatial distribution of the electric current flow in the head which hampers the accurate localization of the stimulated brain areas. In ...this study we show how various anatomical features systematically shape the electric field distribution in the brain during tDCS. We constructed anatomically realistic finite element (FEM) models of two individual heads including conductivity anisotropy and different skull layers. We simulated a widely employed electrode montage to induce motor cortex plasticity and moved the stimulating electrode over the motor cortex in small steps to examine the resulting changes of the electric field distribution in the underlying cortex. We examined the effect of skull thickness and composition on the passing currents showing that thinner skull regions lead to higher electric field strengths. This effect is counteracted by a larger proportion of higher conducting spongy bone in thicker regions leading to a more homogenous current over the skull. Using a multiple regression model we could identify key factors that determine the field distribution to a significant extent, namely the thicknesses of the cerebrospinal fluid and the skull, the gyral depth and the distance to the anode and cathode. These factors account for up to 50% of the spatial variation of the electric field strength. Further, we demonstrate that individual anatomical factors can lead to stimulation "hotspots" which are partly resistant to electrode positioning. Our results give valuable novel insights in the biophysical foundation of tDCS and highlight the importance to account for individual anatomical factors when choosing an electrode montage.
Transcranial direct current stimulation (tDCS) uses electrode pads placed on the head to deliver weak direct current to the brain and modulate neuronal excitability. The effects depend on the ...intensity and spatial distribution of the electric field. This in turn depends on the geometry and electric properties of the head tissues and electrode pads. Previous numerical studies focused on providing a reasonable level of detail of the head anatomy, often using simplified electrode models. Here, we explore via finite element method (FEM) simulations based on a high-resolution head model how detailed electrode modeling influences the calculated electric field in the brain. We take into account electrode shape, size, connector position and conductivities of different electrode materials (including saline solutions and electrode gels). These factors are systematically characterized to demonstrate their impact on the field distribution in the brain. The goals are to assess the effect of simplified electrode models; and to develop practical rules-of-thumb to achieve a stronger stimulation of the targeted brain regions underneath the electrode pads. We show that for standard rectangular electrode pads, lower saline and gel conductivities result in more homogeneous fields in the region of interest (ROI). Placing the connector at the center of the electrode pad or farthest from the second electrode substantially increases the field strength in the ROI. Our results highlight the importance of detailed electrode modeling and of having an adequate selection of electrode pads/gels in experiments. We also advise for a more detailed reporting of the electrode montages when conducting tDCS experiments, as different configurations significantly affect the results.
•Realistic modeling of the electrode pads used in tDCS•The electrode properties have a strong impact on the electric field in the brain.•The generated fields differ markedly from those predicted by simplified models.•Electrode and gel properties should be carefully selected and reported.
Despite the widespread use of transcranial magnetic stimulation (TMS), the precise cortical locations underlying the resulting physiological and behavioral effects are still only coarsely known. To ...date, mapping strategies have relied on projection approaches (often termed “center of gravity” approaches) or maximum electric field value evaluation, and therefore localize the stimulated cortical site only approximately and indirectly. Focusing on the motor cortex, we present and validate a novel method to reliably determine the effectively stimulated cortical site at the individual subject level. The approach combines measurements of motor evoked potentials (MEPs) at different coil positions and orientations with numerical modeling of induced electric fields. We identify sharply bounded cortical areas, around the gyral crowns and rims of the motor hand area, as the origin of MEPs and show that the magnitude of the tangential component and the overall magnitude of the electric field are most relevant for the observed effect. To validate our approach, we identified the coil location and orientation that produces the maximal electric field at the predicted stimulation site, and then experimentally show that this location produces MEPs more efficiently than other tested locations/orientations. Moreover, we used extensive uncertainty and sensitivity analyses to verify the robustness of the method and identify the most critical model parameters. Our generic approach improves the localization of the cortical area effectively stimulated by TMS and may be transferred to other modalities such as language mapping.
The spatial extent of the effects of transcranial magnetic stimulation (TMS) on neural tissue is only coarsely understood. One key problem is the realistic calculation of the electric field induced ...in the brain, which proves difficult due to the complex gyral folding pattern that results in an inhomogeneous conductivity distribution within the skull. We used the finite element method (FEM) together with a high-resolution volume mesh of the human head to better characterize the field induced in cortical gray matter (GM). The volume mesh was constructed from T1-weighted structural magnetic resonance images to allow for an anatomically accurate modeling of the gyrification pattern. Five tissue types were taken into account, corresponding to skin, skull, cerebrospinal fluid (CSF) including the ventricles as well as cortical gray and white matter. We characterized the effect of the current direction on the electric field distribution in GM. Importantly, the field strength in GM was increased by up to 51% when the induced currents were perpendicular to the local gyrus orientation. This effect was mainly restricted to the gyral crowns and lips, but did not extend into the sulcal walls. As a result, the focality of the fields induced in GM was increased. This enhancement effect might in part underlie the dependency of stimulation thresholds on coil orientation, as commonly observed in TMS motor cortex studies. In contrast to the clear-cut effects of the gyrification pattern on the induced field strength, current directions were predominantly influenced by the CSF–skull boundary.
► The Finite Element Method and a high-resolution head mesh was used to calculate the electric field induced by TMS. ► The gyrification had marked effects on the field distribution. ► The field in GM was strongly increased when the currents were perpendicular to the local gyrus orientation. ► This effect was mainly restricted to the gyral crowns and lips.
Much of our knowledge on the physiological mechanisms of transcranial magnetic stimulation (TMS) stems from studies which targeted the human motor cortex. However, it is still unclear which part of ...the motor cortex is predominantly affected by TMS. Considering that the motor cortex consists of functionally and histologically distinct subareas, this also renders the hypotheses on the physiological TMS effects uncertain. We use the finite element method (FEM) and magnetic resonance image-based individual head models to get realistic estimates of the electric field induced by TMS. The field changes in different subparts of the motor cortex are compared with electrophysiological threshold changes of 2 hand muscles when systematically varying the coil orientation in measurements. We demonstrate that TMS stimulates the region around the gyral crown and that the maximal electric field strength in this region is significantly related to the electrophysiological response. Our study is one of the most extensive comparisons between FEM-based field calculations and physiological TMS effects so far, being based on data for 2 hand muscles in 9 subjects. The results help to improve our understanding of the basic mechanisms of TMS. They also pave the way for a systematic exploration of realistic field estimates for dosage control in TMS.
Transcranial Magnetic Stimulation (TMS) excites populations of neurons in the stimulated cortex, and the resulting activation may spread to connected brain regions. The distributed cortical response ...can be recorded with electroencephalography (EEG). Since TMS also stimulates peripheral sensory and motor axons and generates a loud “click” sound, the TMS-evoked EEG potentials (TEPs) reflect not only neural activity induced by transcranial neuronal excitation but also neural activity due to somatosensory and auditory processing. In 17 healthy young individuals, we systematically assessed the contribution of multisensory peripheral stimulation to TEPs using a TMS-compatible EEG system. Real TMS was delivered with a figure-of-eight coil over the left para-median posterior parietal cortex or superior frontal gyrus with the coil being oriented perpendicularly or in parallel to the target gyrus. We also recorded the EEG responses evoked by realistic sham stimulation over the posterior parietal and superior frontal cortex, mimicking the auditory and somatosensory sensations evoked by real TMS. We applied state-of-the-art procedures to attenuate somatosensory and auditory confounds during real TMS, including the placement of a foam layer underneath the coil and auditory noise masking. Despite these precautions, the temporal and spatial features of the cortical potentials evoked by real TMS at the prefrontal and parietal site closely resembled the cortical potentials evoked by realistic sham TMS, both for early and late TEP components. Our findings stress the need to include a peripheral multisensory control stimulation in the design of TMS-EEG studies to enable a dissociation between truly transcranial and non-transcranial components of TEPs.
•Individually adjusted somato-auditory sham stimulation can mimic real TMS.•A realistic sham stimulation is capable to evoke complex EEG potentials.•Real TMS and sham evoked responses strongly correlated at early and late latencies.•Future TMS-EEG studies need to control for the effect of multisensory stimulation.
Variations in head and brain anatomy determine the strength and distribution of electrical fields in humans and may account for inconsistent behavioral and neurophysiological results in transcranial ...electrical stimulation (tES) studies. However, it is insufficiently understood which anatomical features contribute to the variability of the modelled electric fields, and if their impact varies across age groups. In the present study, we tested the associations of global head anatomy, indexed by extra- and intra-cranial volumes, with electric field measures, comparing young and older adults. We modelled six “conventional” electrode montages typically used in tES studies using SimNIBS software in 40 individuals (20 young, 20 older adults; 20-35, 64-79 years). We extracted individual electric field strengths and focality values for each montage to identify tissue volumes that account for variability of the induced electric fields in both groups. Linear mixed models explained most of the inter-individual variability of the overall induced field strength in the brain, but not of field focality. Higher absolute head volume and relative volume of skin, skull and cerebrospinal fluid (CSF) were associated with lower overall electric field strengths. Additionally, we found interactions of age group with head volume and CSF, indicating that this relationship was mitigated in the older group. Our results demonstrate the importance to adjust brain stimulation not only according to brain atrophy, but also to additional parameters of head anatomy. Future studies need to elucidate the mechanisms underlying individual variability of tES effects in young and older adults, and verify the usefulness of the proposed models in terms of neurophysiology and behavior in empirical studies.
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 selectively 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 computationally efficient method for the automatic determination of electrode placements and stimulation intensities to optimally affect a given target position. We provide a 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.
•New formulation of TES multi-electrode optimization with practically meaningful input parameters.•New highly efficient way to limit the number of active TES electrodes for optimization.•Exploration the effects of safety, practical and biophysical constraints, for targets across the whole cortex.•Optimization with multiple, distant targets.
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