Transcranial direct current stimulation: estimation of the electric field and of the current density in an anatomical human head model

Small effects of electric field on motor cortical excitability following anodal tDCS

The dose-response characteristics of transcranial direct current stimulation (tDCS) remain uncertain but may be related to variability in brain electric fields due to individual anatomical factors. Here, we investigated whether the electric fields influence the responses to motor cortical tDCS. In a randomized cross-over design, 21 participants underwent 10 min of anodal tDCS with 0.5, 1.0, 1.5, or 2.0 mA or sham. Compared to sham, all active conditions increased the size of motor evoked potentials (MEP) normalized to the pre-tDCS baseline, irrespective of anterior or posterior magnetic test stimuli. The electric field calculated in the motor cortex of each participant had a nonlinear effect on the normalized MEP size, but its effects were small compared to those of other participant-specific factors. The findings support the efficacy of anodal tDCS in enhancing the MEP size but do not demonstrate any benefits of personalized electric field modeling in explaining tDCS response variability.

Monopolar tDCS might affect brainstem reflexes: A computational and neurophysiological study

OBJECTIVE

To assess whether monopolar multi-electrode transcranial direct current stimulation (tDCS) montages might selectively affect deep brain structures through computational predictions and neurophysiological assessment.

METHODS

Electric field distribution in deep brain structures (i.e., thalamus and midbrain) were estimated through computational models simulating tDCS with two monopolar and two monopolar multi-electrode montages. Monopolar multi-electrode tDCS was then applied to healthy subject, and effects on pontine and medullary circuitries was evaluated studying changes in blink reflex (BR) and masseter inhibitory reflex (MIR).

RESULTS

Computational results suggest that tDCS with monopolar multi-electrode montages might induce electric field intensities in deep brain structure comparable to those in grey matter, while neurophysiological results disclosed that BR and MIR were selectively modulated by tDCS only when cathode was placed over the right deltoid.

CONCLUSIONS

Multi-electrode tDCS (anodes over motor cortices, cathode over right deltoid) could induce significant electric fields in the thalamus and midbrain, and selectively affect brainstem neural circuits.

SIGNIFICANCE

Multi-electrode tDCS (anodes over motor cortices, cathode over right deltoid) might be further explored to affect brainstem activity, also in the context of non-invasive deep brain stimulation.

Direct current stimulation increases blood flow and permeability of cortical microvasculature in vivo

BACKGROUND AND PURPOSE

Transcranial direct current stimulation (DCS) structurally and functionally modulates neuronal networks and microglia dynamics. Neurovascular coupling adapts regional cerebral blood flow to neuronal activity and metabolic demands.

METHODS

In this study, we examined effects of anodal DCS on vessel morphology, blood flow parameters, permeability of cortical microvasculature, and perivascular microglia motility by time-lapse two-photon microscopy in anaesthetized mice.

RESULTS

Low-intensity DCS significantly increased vessel diameter and blood flow parameters. These effects were transient and dependent on the spontaneous vasomotion characteristics of the individual vessel. Vessel leakage increased significantly after DCS at 1.1 and was more pronounced at 2.2 A/m² , indicating a dose-dependent increase in vascular permeability. Perivascular microglia exhibited increased soma motility post-DCS at both intensities, potentially triggered by the extravasation of intravascular substrates.

CONCLUSIONS

Our findings demonstrate that DCS affected only vessels with spontaneous vasomotion. This rapid vascular response may occur as an adaptation of regional blood supply to neuronal excitability altered by DCS or as a direct effect on the vessel wall. In contrast to these immediate effects during stimulation, increases in cortical vessel permeability and perivascular microglia motility appeared after the stimulation had ended.

Computational modeling of electric fields for prefrontal tDCS across patients with schizophrenia and mood disorders

This cross-diagnostic study aims to computationally model electric field (efield) for prefrontal transcranial direct current stimulation in mood disorders and schizophrenia. Enrolled were patients with major depressive disorder (n = 23), bipolar disorder (n = 24), schizophrenia (n = 23), and healthy controls (n = 23). The efield was simulated using SimNIBS software (ver.2.1.1). Electrodes were placed at the left and right prefrontal areas and the current intensity was set to 2 mA intensity. Schizophrenia and major depressive disorder groups showed significantly lower 99.5th percentile efield strength than healthy controls. In voxel-wise analysis, patients with schizophrenia showed a significant reduction of simulated efield strength in the bilateral frontal lobe, cerebellum and brain stem compared with healthy controls. Among the patients with schizophrenia, reduction of simulated efield strength was not significantly correlated with psychiatric symptoms or global functioning. The patients with bipolar disorder showed no significant difference in simulated efield strength compared with healthy controls, and there was no significant difference between the clinical groups. Our results suggest attenuated electrophysiological response to transcranial direct current stimulation to the prefrontal cortex in patients with schizophrenia, and to some extent in patients with major depressive disorder.

Computational Evaluation of Combined Cerebellar and Frontal Transcranial Direct Current Stimulation for Treatment-Resistant Depression

This work aimed to estimate the distribution of the electric field generated by a combined cerebellar and frontal transcranial direct current stimulation (tDCS) for treatment-resistant depression using electromagnetics computational techniques applied to a realistic head human model. Results showed that the stronger electric fields occur mainly in the cerebellum and in DLPFC areas, where the two pairs of electrodes were applied. Furthermore, the study demonstrated that the simultaneous use of the two pairs of electrodes did not imply a lower effectiveness of the tDCS technique, in fact the electric field distributions in the primarily targets of the anatomical regions (i.e., cerebellum and DLPFC) were very similar to when the pairs of electrodes were applied separately.

Subcortical Cognition: The Fruit Below the Rind

Cognitive neuroscience has highlighted the cerebral cortex while often overlooking subcortical structures. This cortical proclivity is found in basic and translational research on many aspects of cognition, especially higher cognitive domains such as language, reading, music, and math. We suggest that, for both anatomical and evolutionary reasons, multiple subcortical structures play substantial roles across higher and lower cognition. We present a comprehensive review of existing evidence, which indeed reveals extensive subcortical contributions in multiple cognitive domains. We argue that the findings are overall both real and important. Next, we advance a theoretical framework to capture the nature of (sub)cortical contributions to cognition. Finally, we propose how new subcortical cognitive roles can be identified by leveraging anatomical and evolutionary principles, and we describe specific methods that can be used to reveal subcortical cognition. Altogether, this review aims to advance cognitive neuroscience by highlighting subcortical cognition and facilitating its future investigation. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Cognitive Telerehabilitation with Transcranial Direct Current Stimulation Improves Cognitive and Emotional Functioning Following a Traumatic Brain Injury: A Case Study

OBJECTIVE

Cognitive deficits following a traumatic brain injury (TBI) are a leading cause of disability in young adults and there is a critical need for novel approaches to improve cognitive outcomes in TBI survivors. Transcranial direct current stimulation (tDCS) paired with cognitive remediation has emerged as a viable, cost-effective, noninvasive approach for treating cognitive impairments in a wide variety of neurological conditions. Here, we report the first case study utilizing remotely supervised tDCS (RS-tDCS) protocol paired with cognitive remediation in a 29-year-old man with persisting cognitive and emotional sequelae following TBI.

METHOD

Neuropsychological measures were administered before and after the patient completed 20 daily sessions of RS-tDCS (2.0 mA × 20 minutes, left anodal dorsolateral prefrontal cortex montage). During the daily stimulation period, he completed adaptive cognitive training. All treatment procedures were delivered at home and monitored in real time via videoconference with a study technician.

RESULTS

Following 20 RS-tDCS and cognitive training sessions, he had significant improvements (>1 SD) on tests of attention and working memory, semantic fluency, and information processing speed. Mood was also improved.

CONCLUSIONS

This is the first demonstration of at-home telerehabilitation with RS-tDCS and cognitive training to improve cognitive outcomes following TBI.

Effect of triangular electrode schemes on Broca's cortical stimulation: conventional and HD-tDCS study

Transcranial direct current stimulation (tDCS) is a therapeutic and complementary treatment in several cognitive diseases, psychiatric disorders, and disabilities that occur due to an accident or stroke. In the current research, we aimed to boost some parts of the stimulation structure and proposed a new electrode scheme in the mentioned approach. After segmenting magnetic resonance imaging (MRI) scans and using a tissue correction routine algorithm, we attempted to create an appropriate head model and electrode placement according to electric stimulation, whereby we completed tDCS processing. The considered electrodes are divided into two general categories. All the considered electrodes consist of rectangular, circular, triangular, and empty triangular patches with specific dimensions. We investigated common electrode schemes and introduced better electrode schemes for more effective cortical stimulation. We observed that the triangular electrodes in the conventional and anodal arrangement in the triangular 4 × 1 HD-tDCS create more electric field than others. Also, we calculated the current density and attempted to substantially improve it. Therefore, we recommended the empty triangular schemes. We investigated the designed model thoroughly and observed that it increased the current density not only in the conventional but also in the HD-tDCS. Graphical abstract.

TDCS effects on pointing task learning in young and old adults

Skill increase in motor performance can be defined as explicitly measuring task success but also via more implicit measures of movement kinematics. Even though these measures are often related, there is evidence that they represent distinct concepts of learning. In the present study, the effect of multiple tDCS-sessions on both explicit and implicit measures of learning are investigated in a pointing task in 30 young adults (YA) between 27.07 ± 3.8 years and 30 old adults (OA) between 67.97 years ± 5.3 years. We hypothesized, that OA would show slower explicit skill learning indicated by higher movement times/lower accuracy and slower implicit learning indicated by higher spatial variability but profit more from anodal tDCS compared with YA. We found age-related differences in movement time but not in accuracy or spatial variability. TDCS did not skill learning facilitate learning neither in explicit nor implicit parameters. However, contrary to our hypotheses, we found tDCS-associated higher accuracy only in YA but not in spatial variability. Taken together, our data shows limited overlapping of tDCS effects in explicit and implicit skill parameters. Furthermore, it supports the assumption that tDCS is capable of producing a performance-enhancing brain state at least for explicit skill acquisition.

Effect of skin conductivity on the electric field induced by transcranial stimulation techniques in different head models

This study aims at quantifying the effect that using different skin conductivity values has on the estimation of the electric (E)-field distribution induced by transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in the brain of two anatomical models. The induced E-field was calculated with numerical simulations inside MIDA and Duke models, assigning to the skin a conductivity value estimated from a multi-layered skin model and three values taken from literature. The effect of skin conductivity variations on the local E-field induced by tDCS in the brain was up to 70%. In TMS, minor local differences, in the order of 20%, were obtained in regions of interest for the onset of possible side effects. Results suggested that an accurate model of the skin is necessary in all numerical studies that aim at precisely estimating the E-field induced during TMS and tDCS applications. This also highlights the importance of further experimental studies on human skin characterization, especially at low frequencies.

Noninvasive Brain Stimulation & Space Exploration: Opportunities and Challenges

As NASA prepares for longer space missions aiming for the Moon and Mars, astronauts' health and performance are becoming a central concern due to the threats associated with galactic cosmic radiation, unnatural gravity fields, and life in extreme environments. In space, the human brain undergoes functional and structural changes related to fluid shift and changes in intracranial pressure. Behavioral abnormalities, such as cognitive deficits, sleep disruption, and visuomotor difficulties, as well as psychological effects, are also an issue. We discuss opportunities and challenges of noninvasive brain stimulation (NiBS) methods - including transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES) - to support space exploration in several ways. NiBS includes safe and portable techniques already applied in a wide range of cognitive and motor domains, as well as therapeutically. NiBS could be used to enhance in-flight performance, supporting astronauts during pre-flight Earth-based training, as well as to identify biomarkers of post-flight brain changes for optimization of rehabilitation/compensatory strategies. We review these NiBS techniques and their effects on brain physiology, psychology, and cognition.

Transcranial direct current stimulation: A review of electrode characteristics and materials

Electrode characteristics are crucial in transcranial direct current stimulation (tDCS) since electrode design and placement determine the cortical area being modulated, current density and spatial resolution of stimulation. Early research on tDCS sought to determine optimal parameters for stimulation by specifying maximum current, duration and sizes of electrodes. Further research focused on determining efficient ways to deliver stimulation to targeted regions on the cortex with minimal discomfort to the user by altering electrode size, placement, shape and material. This review aims to give an insight on the main characteristics of electrodes used in tDCS and on the variability found in electrode parameters and placements from tDCS to high definition tDCS (HD-tDCS) applications and beyond.

Cerebellar transcranial direct current stimulation for learning a novel split-belt treadmill task: a randomised controlled trial

This study aimed to examine the effect of repeated anodal cerebellar transcranial direct current stimulation (ctDCS) on learning a split-belt treadmill task. Thirty healthy individuals randomly received three consecutive sessions of active or sham anodal ctDCS during split-belt treadmill training. Motor performance and strides to steady-state performance were evaluated before (baseline), during (adaptation), and after (de-adaptation) the intervention. The outcomes were measured one week later to assess absolute learning and during the intervention to evaluate cumulative, consecutive, and session-specific effects. Data were analysed using linear mixed-effects regression models. During adaptation, there was no significant difference in absolute learning between the groups (p > 0.05). During de-adaptation, a significant difference in absolute learning between the groups (p = 0.03) indicated slower de-adaptation with anodal ctDCS. Pre-planned secondary analysis revealed that anodal ctDCS significantly reduced the cumulative (p = 0.01) and consecutive-session effect (p = 0.01) on immediate adaptation. There were significant cumulative (p = 0.02) and session-specific effects (p = 0.003) on immediate de-adaptation. Repeated anodal ctDCS does not enhance motor learning measured during adaptation to a split-belt treadmill task. However, it influences the maintenance of learnt walking patterns, suggesting that it may be beneficial in maintaining therapeutic effects.

Impact of brain atrophy on tDCS and HD-tDCS current flow: a modeling study in three variants of primary progressive aphasia

BACKGROUND

During transcranial direct current stimulation (tDCS), the amount and distribution of current that reaches the brain depends on individual anatomy. Many progressive neurodegenerative diseases are associated with cortical atrophy, but the importance of individual brain atrophy during tDCS in patients with progressive atrophy, including primary progressive aphasia (PPA), remains unclear.

OBJECTIVE

In the present study, we addressed the question whether brain anatomy in patients with distinct cortical atrophy patterns would impact brain current intensity and distribution during tDCS over the left IFG.

METHOD

We developed state-of-the-art, gyri-precise models of three subjects, each representing a variant of primary progressive aphasia: non-fluent variant PPA (nfvPPA), semantic variant PPA (svPPA), and logopenic variant PPA (lvPPA). We considered two exemplary montages over the left inferior frontal gyrus (IFG): a conventional pad montage (anode over F7, cathode over the right cheek) and a 4 × 1 high-definition tDCS montage. We further considered whether local anatomical features, specifically distance of the cortex to skull, can directly predict local electric field intensity.

RESULTS

We found that the differences in brain current flow across the three PPA variants fall within the distribution of anatomically typical adults. While clustering of electric fields was often around individual gyri or sulci, the minimal distance from the gyri/sulci to skull was not correlated with electric field intensity.

CONCLUSION

Limited to the conditions and assumptions considered here, this argues against a specific need to adjust the tDCS montage for these patients any more than might be considered useful in anatomically typical adults. Therefore, local atrophy does not, in isolation, reliably predict local electric field. Rather, our results are consistent with holistic head anatomy influencing brain current flow, with tDCS producing diffuse and individualized brain current flow patterns and HD-tDCS producing targeted brain current flow across individuals.

Effects of 1 mA and 2 mA transcranial direct current stimulation on working memory performance in healthy participants

Anodal transcranial current stimulation (tDCS) to the left dorsolateral prefrontal cortex (DLPFC) has been shown to enhance working memory (WM) in neuropsychiatric patients. In healthy populations, however, tDCS obtains inconclusive results, mostly due to heterogeneous study and stimulation protocols. Here, we approached these issues by investigating effects of tDCS intensity on simultaneous WM performance with three cognitive loads by directly comparing findings of two double-blind, cross-over, sham-controlled experiments. TDCS was administrated to the left DLPFC at intensity of 1 mA (Experiment 1) or 2 mA (Experiment 2), while participants completed a verbal n-back paradigm (1-, 2-, 3-back). Analysis showed no overall effects of tDCS on WM, but a significant interaction with cognitive load. The present study suggests that cognitive load rather than tDCS intensity could be a decisive factor for effects on WM. Moreover, it emphasizes the need of thorough investigation on study parameters to develop more efficient stimulation protocols.

Enhanced tES and tDCS computational models by meninges emulation

OBJECTIVE

Understanding how current reaches the brain during transcranial electrical stimulation (tES) underpins efforts to rationalize outcomes and optimize interventions. To this end, computational models of current flow relate applied dose to brain electric field. Conventional tES modeling considers distinct tissues like scalp, skull, cerebrospinal fluid (CSF), gray matter and white matter. The properties of highly conductive CSF are especially important. However, modeling the space between skull and brain as entirely CSF is not an accurate representation of anatomy. The space conventionally modeled as CSF is approximately half meninges (dura, arachnoid, and pia) with lower conductivity. However, the resolution required to describe individual meningeal layers is computationally restrictive in an MRI-derived head model. Emulating the effect of meninges through CSF conductivity modification could improve accuracy with minimal cost.

APPROACH

Models with meningeal layers were developed in a concentric sphere head model. Then, in a model with only CSF between skull and brain, CSF conductivity was optimized to emulate the effect of meningeal layers on cortical electric field for multiple electrode positions. This emulated conductivity was applied to MRI-derived models.

MAIN RESULTS

Compared to a model with conventional CSF conductivity (1.65 S m-1), emulated CSF conductivity (0.85 S m-1) produced voltage fields better correlated with intracranial recordings from epilepsy patients.

SIGNIFICANCE

Conventional tES models have been validated using intracranial recording. Residual errors may nonetheless impact model utility. Because CSF is so conductive to current flow, misrepresentation of the skull-brain interface as entirely CSF is not realistic for tES modeling. Updating the conventional model with a CSF conductivity emulating the effect of the meninges enhances modeling accuracy without increasing model complexity. This allows existing modeling pipelines to be leveraged with a simple conductivity change. Using 0.85 S m-1 emulated CSF conductivity is recommended as the new standard in non-invasive brain stimulation modeling.

Realistic volumetric-approach to simulate transcranial electric stimulation-ROAST-a fully automated open-source pipeline

OBJECTIVE

Research in the area of transcranial electrical stimulation (TES) often relies on computational models of current flow in the brain. Models are built based on magnetic resonance images (MRI) of the human head to capture detailed individual anatomy. To simulate current flow on an individual, the subject's MRI is segmented, virtual electrodes are placed on this anatomical model, the volume is tessellated into a mesh, and a finite element model (FEM) is solved numerically to estimate the current flow. Various software tools are available for each of these steps, as well as processing pipelines that connect these tools for automated or semi-automated processing. The goal of the present tool-realistic volumetric-approach to simulate transcranial electric simulation (ROAST)-is to provide an end-to-end pipeline that can automatically process individual heads with realistic volumetric anatomy leveraging open-source software and custom scripts to improve segmentation and execute electrode placement.

APPROACH

ROAST combines the segmentation algorithm of SPM12, a Matlab script for touch-up and automatic electrode placement, the finite element mesher iso2mesh and the solver getDP. We compared its performance with commercial FEM software, and SimNIBS, a well-established open-source modeling pipeline.

MAIN RESULTS

The electric fields estimated with ROAST differ little from the results obtained with commercial meshing and FEM solving software. We also do not find large differences between the various automated segmentation methods used by ROAST and SimNIBS. We do find bigger differences when volumetric segmentation are converted into surfaces in SimNIBS. However, evaluation on intracranial recordings from human subjects suggests that ROAST and SimNIBS are not significantly different in predicting field distribution, provided that users have detailed knowledge of SimNIBS.

SIGNIFICANCE

We hope that the detailed comparisons presented here of various choices in this modeling pipeline can provide guidance for future tool development. We released ROAST as an open-source, easy-to-install and fully-automated pipeline for individualized TES modeling.

Neural basis of induced phantom limb pain relief

OBJECTIVE

Phantom limb pain (PLP) is notoriously difficult to treat, partly due to an incomplete understanding of PLP-related disease mechanisms. Noninvasive brain stimulation (NIBS) is used to modulate plasticity in various neuropathological diseases, including chronic pain. Although NIBS can alleviate neuropathic pain (including PLP), both disease and treatment mechanisms remain tenuous. Insight into the mechanisms underlying both PLP and NIBS-induced PLP relief is needed for future implementation of such treatment and generalization to related conditions.

METHODS

We used a within-participants, double-blind, and sham-controlled design to alleviate PLP via task-concurrent NIBS over the primary sensorimotor missing hand cortex (S1/M1). To specifically influence missing hand signal processing, amputees performed phantom hand movements during anodal transcranial direct current stimulation. Brain activity was monitored using neuroimaging during and after NIBS. PLP ratings were obtained throughout the week after stimulation.

RESULTS

A single session of intervention NIBS significantly relieved PLP, with effects lasting at least 1 week. PLP relief associated with reduced activity in the S1/M1 missing hand cortex after stimulation. Critically, PLP relief and reduced S1/M1 activity correlated with preceding activity changes during stimulation in the mid- and posterior insula and secondary somatosensory cortex (S2).

INTERPRETATION

The observed correlation between PLP relief and decreased S1/M1 activity confirms our previous findings linking PLP with increased S1/M1 activity. Our results further highlight the driving role of the mid- and posterior insula, as well as S2, in modulating PLP. Lastly, our novel PLP intervention using task-concurrent NIBS opens new avenues for developing treatment for PLP and related pain conditions. ANN NEUROL 2019;85:59-73.

Benchmarking transcranial electrical stimulation finite element models: a comparison study

OBJECTIVE

To compare field measure differences in simulations of transcranial electrical stimulation (tES) generated by variations in finite element (FE) models due to boundary condition specification, use of tissue compartment smoothing filters, and use of free or structured tetrahedral meshes based on magnetic resonance imaging (MRI) data.

APPROACH

A structural MRI head volume was acquired at 1 mm³ resolution and segmented into ten tissue compartments. Predicted current densities and electric fields were computed in segmented models using modeling pipelines involving either an in-house (block) or a commercial platform commonly used in previous FE tES studies involving smoothed compartments and free meshing procedures (smooth). The same boundary conditions were used for both block and smooth pipelines. Differences caused by varying boundary conditions were examined using a simple geometry. Percentage differences of median current density values in five cortical structures were compared between the two pipelines for three electrode montages (F3-right supraorbital, T7-T8 and Cz-Oz).

MAIN RESULTS

Use of boundary conditions commonly used in previous tES FE studies produced asymmetric current density profiles in the simple geometry. In head models, median current density differences produced by the two pipelines, using the same boundary conditions, were up to 6% (isotropic) and 18% (anisotropic) in structures targeted by each montage. Tangential electric field measures calculated via either pipeline were within the range of values reported in the literature, when averaged over cortical surface patches.

SIGNIFICANCE

Apparently equivalent boundary settings may affect predicted current density outcomes and care must be taken in their specification. Smoothing FE model compartments may not be necessary, and directly translated, voxellated tissue boundaries at 1 mm³ resolution may be sufficient for use in tES FE studies, greatly reducing processing times. The findings here may be used to inform future current density modeling studies.

MRI-Guided Regional Personalized Electrical Stimulation in Multisession and Home Treatments

The shape and position of the electrodes is a key factor for the efficacy of transcranial electrical stimulations (tES). We have recently introduced the Regional Personalized Electrode (RePE), a tES electrode fitting the personal cortical folding, that has been able to differentiate the stimulation of close by regions, in particular the primary sensory (S1) and motor (M1) cortices, and to personalize tES onto such an extended cortical district. However, neuronavigation on individual brain was compulsory for the correct montage. Here, we aimed at developing and testing a neuronavigation-free procedure for easy and quick positioning RePE, enabling multisession RePE-tES at home. We used off-line individual MRI to shape RePE via an ad-hoc computerized procedure, while an ad-hoc developed Adjustable Helmet Frame (AHF) was used to properly position it in multisession treatments, even at home. We used neuronavigation to test the RePE shape and position obtained by the new computerized procedure and the re-positioning obtained via the AHF. Using Finite Element Method (FEM) model, we also estimated the intra-cerebral current distribution induced by transcranial direct current stimulation (tDCS) comparing RePE vs. non-RePE with fixed reference. Additionally, we tested, using FEM, various shapes, and positions of the reference electrode taking into account possible small displacements of RePE, to test feasibility of RePE-tES sessions at home. The new RePE neuronavigation-free positioning relies on brain MRI space distances, and produced a mean displacement of 3.5 ± 0.8 mm, and the re-positioning of 4.8 ± 1.1 mm. Higher electric field in S1 than in M1 was best obtained with the occipital reference electrode, a montage that proved to feature low sensitivity to typical RePE millimetric displacements. Additionally, a new tES accessory was developed to enable repositioning the electrodes over the scalp also at home, with a precision which is acceptable according to the modeling-estimated intracerebral currents. Altogether, we provide here a procedure to simplify and make easily applicable RePE-tDCS, which enables efficacious personalized treatments.

Redesigning existing transcranial magnetic stimulation coils to reduce energy: application to low field magnetic stimulation

OBJECTIVE

To present a systematic framework and exemplar for the development of a compact and energy-efficient coil that replicates the electric field (E-field) distribution induced by an existing transcranial magnetic stimulation coil.

APPROACH

The E-field generated by a conventional low field magnetic stimulation (LFMS) coil was measured for a spherical head model and simulated in both spherical and realistic head models. Then, using a spherical head model and spatial harmonic decomposition, a spherical-shaped cap coil was synthesized such that its windings conformed to a spherical surface and replicated the E-field on the cortical surface while requiring less energy. A prototype coil was built and electrically characterized. The effect of constraining the windings to the upper half of the head was also explored via an alternative coil design.

MAIN RESULTS

The LFMS E-field distribution resembled that of a large double-cone coil, with a peak field strength around 350 mV m^-1 in the cortex. The E-field distributions of the cap coil designs were validated against the original coil, with mean errors of 1%-3%. The cap coil required as little as 2% of the original coil energy and was significantly smaller in size.

SIGNIFICANCE

The redesigned LFMS coil is substantially smaller and more energy-efficient than the original, improving cost, power consumption, and portability. These improvements could facilitate deployment of LFMS in the clinic and potentially at home. This coil redesign approach can also be applied to other magnetic stimulation paradigms. Finally, the anatomically-accurate E-field simulation of LFMS can be used to interpret clinical LFMS data.

Significance of Biological Membranes for Accurate Computational Dosimetry of Low Frequency Electric Fields

Computational dosimetry has become the main tool for estimating induced electric fields within brain tissues in transcranial direct current stimulation (tDCS) which is recently attracting the attention of researches for motor function disturbances such as Parkinson's disease. This paper investigates the effect of including or excluding the very thin meninges in computing tDCS electric fields using CST software. For this purpose, two models of the brain with and without meninges were used to induce electric field with two DC current electrodes (2 mA) in regions of the model referring to M1 and Prefrontal Cortex (FP2) similar to tDCS. Considering meninges, the results have shown differences in the induced field showing that there might be problems with conventional models in which meninges are not taken into account.

Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Spinal control of motor outputs by intrinsic and externally induced electric field potentials

Despite numerous studies on spinal neuronal systems, several issues regarding their role in motor behavior remain unresolved. One of these issues is how electric fields associated with the activity of spinal neurons influence the operation of spinal neuronal networks and how effects of these field potentials are combined with other means of modulating neuronal activity. Another closely related issue is how external electric field potentials affect spinal neurons and how they can be used for therapeutic purposes such as pain relief or recovery of motor functions by transspinal direct current stimulation. Nevertheless, progress in our understanding of the spinal effects of electric fields and their mechanisms has been made over the last years, and the aim of the present review is to summarize the recent findings in this field.

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

Transcranial electric stimulation aims to stimulate the brain by applying weak electrical currents at the scalp. However, the magnitude and spatial distribution of electric fields in the human brain are unknown. We measured electric potentials intracranially in ten epilepsy patients and estimated electric fields across the entire brain by leveraging calibrated current-flow models. When stimulating at 2 mA, cortical electric fields reach 0.8 V/m, the lower limit of effectiveness in animal studies. When individual whole-head anatomy is considered, the predicted electric field magnitudes correlate with the recorded values in cortical (r = 0.86) and depth (r = 0.88) electrodes. Accurate models require adjustment of tissue conductivity values reported in the literature, but accuracy is not improved when incorporating white matter anisotropy or different skull compartments. This is the first study to validate and calibrate current-flow models with in vivo intracranial recordings in humans, providing a solid foundation to target stimulation and interpret clinical trials.

DOI:http://dx.doi.org/10.7554/eLife.18834.001

A simple method for EEG guided transcranial electrical stimulation without models

OBJECTIVE

There is longstanding interest in using EEG measurements to inform transcranial Electrical Stimulation (tES) but adoption is lacking because users need a simple and adaptable recipe. The conventional approach is to use anatomical head-models for both source localization (the EEG inverse problem) and current flow modeling (the tES forward model), but this approach is computationally demanding, requires an anatomical MRI, and strict assumptions about the target brain regions. We evaluate techniques whereby tES dose is derived from EEG without the need for an anatomical head model, target assumptions, difficult case-by-case conjecture, or many stimulation electrodes.

APPROACH

We developed a simple two-step approach to EEG-guided tES that based on the topography of the EEG: (1) selects locations to be used for stimulation; (2) determines current applied to each electrode. Each step is performed based solely on the EEG with no need for head models or source localization. Cortical dipoles represent idealized brain targets. EEG-guided tES strategies are verified using a finite element method simulation of the EEG generated by a dipole, oriented either tangential or radial to the scalp surface, and then simulating the tES-generated electric field produced by each model-free technique. These model-free approaches are compared to a 'gold standard' numerically optimized dose of tES that assumes perfect understanding of the dipole location and head anatomy. We vary the number of electrodes from a few to over three hundred, with focality or intensity as optimization criterion.

MAIN RESULTS

Model-free approaches evaluated include (1) voltage-to-voltage, (2) voltage-to-current; (3) Laplacian; and two Ad-Hoc techniques (4) dipole sink-to-sink; and (5) sink to concentric. Our results demonstrate that simple ad hoc approaches can achieve reasonable targeting for the case of a cortical dipole, remarkably with only 2-8 electrodes and no need for a model of the head.

SIGNIFICANCE

Our approach is verified directly only for a theoretically localized source, but may be potentially applied to an arbitrary EEG topography. For its simplicity and linearity, our recipe for model-free EEG guided tES lends itself to broad adoption and can be applied to static (tDCS), time-variant (e.g., tACS, tRNS, tPCS), or closed-loop tES.

Using reciprocity for relating the simulation of transcranial current stimulation to the EEG forward problem

To explore the relationship between transcranial current stimulation (tCS) and the electroencephalography (EEG) forward problem, we investigate and compare accuracy and efficiency of a reciprocal and a direct EEG forward approach for dipolar primary current sources both based on the finite element method (FEM), namely the adjoint approach (AA) and the partial integration approach in conjunction with a transfer matrix concept (PI). By analyzing numerical results, comparing to analytically derived EEG forward potentials and estimating computational complexity in spherical shell models, AA turns out to be essentially identical to PI. It is then proven that AA and PI are also algebraically identical even for general head models. This relation offers a direct link between the EEG forward problem and tCS. We then demonstrate how the quasi-analytical EEG forward solutions in sphere models can be used to validate the numerical accuracies of FEM-based tCS simulation approaches. These approaches differ with respect to the ease with which they can be employed for realistic head modeling based on MRI-derived segmentations. We show that while the accuracy of the most easy to realize approach based on regular hexahedral elements is already quite high, it can be significantly improved if a geometry-adaptation of the elements is employed in conjunction with an isoparametric FEM approach. While the latter approach does not involve any additional difficulties for the user, it reaches the high accuracies of surface-segmentation based tetrahedral FEM, which is considerably more difficult to implement and topologically less flexible in practice. Finally, in a highly realistic head volume conductor model and when compared to the regular alternative, the geometry-adapted hexahedral FEM is shown to result in significant changes in tCS current flow orientation and magnitude up to 45° and a factor of 1.66, respectively.

Technical aspects of neurostimulation: Focus on equipment, electric field modeling, and stimulation protocols

Neuromodulation is a field of science, medicine, and bioengineering that encompasses implantable and non-implantable technologies for the purpose of improving quality of life and functioning of humans. Brain neuromodulation involves different neurostimulation techniques: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS), which are being used both to study their effects on cognitive brain functions and to treat neuropsychiatric disorders. The mechanisms of action of neurostimulation remain incompletely understood. Insight into the technical basis of neurostimulation might be a first step towards a more profound understanding of these mechanisms, which might lead to improved clinical outcome and therapeutic potential. This review provides an overview of the technical basis of neurostimulation focusing on the equipment, the present understanding of induced electric fields, and the stimulation protocols. The review is written from a technical perspective aimed at supporting the use of neurostimulation in clinical practice.

The application of tDCS in psychiatric disorders: a brain imaging view

Background

Transcranial direct current stimulation (tDCS) is a non-invasive, non-convulsive technique for modulating brain function. In contrast to other non-invasive brain stimulation techniques, where costs, clinical applicability, and availability limit their large-scale use in clinical practices, the low-cost, portable, and easy-to-use tDCS devices may overcome these restrictions.

Objective

Despite numerous clinical applications in large numbers of patients suffering from psychiatric disorders, it is not quite clear how tDCS influences the mentally affected human brain. In order to decipher potential neural mechanisms of action of tDCS in patients with psychiatric conditions, we focused on the combination of tDCS with neuroimaging techniques.

Design

We propose a contemporary overview on the currently available neurophysiological and neuroimaging data where tDCS has been used as a research or treatment tool in patients with psychiatric disorders.

Results

Over a reasonably short period of time, tDCS has been broadly used as a research tool to examine neuronal processes in the healthy brain. tDCS has also commonly been applied as a treatment application in a variety of mental disorders, with to date no straightforward clinical outcome and not always accompanied by brain imaging techniques.

Conclusion

tDCS, as do other neuromodulation devices, clearly affects the underlying neuronal processes. However, research on these mechanisms in psychiatric patients is rather limited. A better comprehension of how tDCS modulates brain function will help us to define optimal parameters of stimulation in each indication and may result in the detection of biomarkers in favor of clinical response.

A technical guide to tDCS, and related non-invasive brain stimulation tools

Transcranial electrical stimulation (tES), including transcranial direct and alternating current stimulation (tDCS, tACS) are non-invasive brain stimulation techniques increasingly used for modulation of central nervous system excitability in humans. Here we address methodological issues required for tES application. This review covers technical aspects of tES, as well as applications like exploration of brain physiology, modelling approaches, tES in cognitive neurosciences, and interventional approaches. It aims to help the reader to appropriately design and conduct studies involving these brain stimulation techniques, understand limitations and avoid shortcomings, which might hamper the scientific rigor and potential applications in the clinical domain.

The Effect of Transcranial Direct Current Stimulation (tDCS) Electrode Size and Current Intensity on Motor Cortical Excitability: Evidence From Single and Repeated Sessions

BACKGROUND

Current density is considered an important factor in determining the outcomes of tDCS, and is determined by the current intensity and electrode size. Previous studies examining the effect of these parameters on motor cortical excitability with small sample sizes reported mixed results.

OBJECTIVE/HYPOTHESIS

This study examined the effect of current intensity (1 mA, 2 mA) and electrode size (16 cm(2), 35 cm(2)) on motor cortical excitability over single and repeated tDCS sessions.

METHODS

Data from seven studies in 89 healthy participants were pooled for analysis. Single-session data were analyzed using mixed effects models and repeated-session data were analyzed using mixed design analyses of variance. Computational modeling was used to examine the electric field generated.

RESULTS

The magnitude of increases in excitability after anodal tDCS was modest. For single-session tDCS, the 35 cm(2) electrodes produced greater increases in cortical excitability compared to the 16 cm(2) electrodes. There were no differences in the magnitude of cortical excitation produced by 1 mA and 2 mA tDCS. The repeated-sessions data also showed that there were greater increases in excitability with the 35 cm(2) electrodes. Further, repeated sessions of tDCS with the 35 cm(2) electrodes resulted in a cumulative increase in cortical excitability. Computational modeling predicted higher electric field at the motor hotspot for the 35 cm(2) electrodes.

CONCLUSIONS

2 mA tDCS does not necessarily produce larger effects than 1 mA tDCS in healthy participants. Careful consideration should be given to the exact positioning, size and orientation of tDCS electrodes relative to cortical regions.

Computational modeling of neurostimulation in brain diseases

Neurostimulation as a therapeutic tool has been developed and used for a range of different diseases such as Parkinson's disease, epilepsy, and migraine. However, it is not known why the efficacy of the stimulation varies dramatically across patients or why some patients suffer from severe side effects. This is largely due to the lack of mechanistic understanding of neurostimulation. Hence, theoretical computational approaches to address this issue are in demand. This chapter provides a review of mechanistic computational modeling of brain stimulation. In particular, we will focus on brain diseases, where mechanistic models (e.g., neural population models or detailed neuronal models) have been used to bridge the gap between cellular-level processes of affected neural circuits and the symptomatic expression of disease dynamics. We show how such models have been, and can be, used to investigate the effects of neurostimulation in the diseased brain. We argue that these models are crucial for the mechanistic understanding of the effect of stimulation, allowing for a rational design of stimulation protocols. Based on mechanistic models, we argue that the development of closed-loop stimulation is essential in order to avoid inference with healthy ongoing brain activity. Furthermore, patient-specific data, such as neuroanatomic information and connectivity profiles obtainable from neuroimaging, can be readily incorporated to address the clinical issue of variability in efficacy between subjects. We conclude that mechanistic computational models can and should play a key role in the rational design of effective, fully integrated, patient-specific therapeutic brain stimulation.

Computational model of cerebellar transcranial direct current stimulation

This work aimed to estimate the distribution of the electric field and current density generated by cerebellar tDCS using electromagnetics computational techniques applied to a realistic human models of different ages and gender. Results show that the stronger electric field and current density occur mainly in the cerebellar cortex, with a spread toward the occipital region of the cortex, while the current spread to other structures is negligible. Moreover, changes of about 1 cm in the position of the scalp electrode delivering tDCS did not influence the E and J distribution in the cerebellum.

Study of the influence of the orientation of a 50-Hz magnetic field on fetal exposure using polynomial chaos decomposition

Human exposure modelling is a complex topic, because in a realistic exposure scenario, several parameters (e.g., the source, the orientation of incident fields, the morphology of subjects) vary and influence the dose. Deterministic dosimetry, so far used to analyze human exposure to electromagnetic fields (EMF), is highly time consuming if the previously-mentioned variations are considered. Stochastic dosimetry is an alternative method to build analytical approximations of exposure at a lower computational cost. In this study, it was used to assess the influence of magnetic flux density (B) orientation on fetal exposure at 50 Hz by polynomial chaos (PC). A PC expansion of induced electric field (E) in each fetal tissue at different gestational ages (GA) was built as a function of B orientation. Maximum E in each fetal tissue and at each GA was estimated for different exposure configurations and compared with the limits of the International Commission of Non-Ionising Radiation Protection (ICNIRP) Guidelines 2010. PC theory resulted in an efficient tool to build accurate approximations of E in each fetal tissue. B orientation strongly influenced E, with a variability across tissues from 10% to 43% with respect to the mean value. However, varying B orientation, maximum E in each fetal tissue was below the limits of ICNIRP 2010 at all GAs.

Inter-subject Variability in Electric Fields of Motor Cortical tDCS

BACKGROUND

The sources of inter-subject variability in the efficacy of transcranial direct current stimulation (tDCS) remain unknown. One potential source of variations is the brain's electric field, which varies according to each individual's anatomical features.

OBJECTIVE

We employed an approach that combines imaging and computational modeling to quantitatively study the extent and primary causes of inter-subject variation in tDCS electric fields.

METHODS

Anatomically-accurate models of the head and brain of 24 males (age: 38.63 ± 11.24 years) were constructed from structural MRI. Finite-element method was used to computationally estimate the electric fields for tDCS of the motor cortex. Surface-based inter-subject registration of the electric field and functional MRI data was used for group level statistical analysis.

RESULTS

We observed large differences in each individual's electric field patterns. However, group level analysis revealed that the average electric fields concentrated in the vicinity of the primary motor cortex. The variations in the electric fields in the hand motor area could be characterized by a normal distribution with a standard deviation of approximately 20% of the mean. The cerebrospinal fluid (CSF) thickness was the primary factor influencing an individual's electric field, thereby explaining 50% of the inter-individual variability, a thicker layer of CSF decreasing the electric field strength.

CONCLUSIONS

The variability in the electric fields is related to each individual's anatomical features and can only be controlled using detailed image processing. Age was found to have a slight negative effect on the electric field, which might have implications on tDCS studies on aging brains.

Electric Field Model of Transcranial Electric Stimulation in Nonhuman Primates: Correspondence to Individual Motor Threshold

OBJECTIVE

To develop a pipeline for realistic head models of nonhuman primates (NHPs) for simulations of noninvasive brain stimulation, and use these models together with empirical threshold measurements to demonstrate that the models capture individual anatomical variability.

METHODS

Based on structural MRI data, we created models of the electric field (E-field) induced by right unilateral (RUL) electroconvulsive therapy (ECT) in four rhesus macaques. Individual motor threshold (MT) was measured with transcranial electric stimulation (TES) administered through the RUL electrodes in the same subjects.

RESULTS

The interindividual anatomical differences resulted in 57% variation in median E-field strength in the brain at fixed stimulus current amplitude. Individualization of the stimulus current by MT reduced the E-field variation in the target motor area by 27%. There was significant correlation between the measured MT and the ratio of simulated electrode current and E-field strength (r(2) = 0.95, p = 0.026). Exploratory analysis revealed significant correlations of this ratio with anatomical parameters including of the superior electrode-to-cortex distance, vertex-to-cortex distance, and brain volume (r(2) > 0.96, p < 0.02). The neural activation threshold was estimated to be 0.45 ±0.07 V/cm for 0.2-ms stimulus pulse width.

CONCLUSION

These results suggest that our individual-specific NHP E-field models appropriately capture individual anatomical variability relevant to the dosing of TES/ECT. These findings are exploratory due to the small number of subjects.

SIGNIFICANCE

This study can contribute insight in NHP studies of ECT and other brain stimulation interventions, help link the results to clinical studies, and ultimately lead to more rational brain stimulation dosing paradigms.

The value and cost of complexity in predictive modelling: role of tissue anisotropic conductivity and fibre tracts in neuromodulation

OBJECTIVES

Computational methods are increasingly used to optimize transcranial direct current stimulation (tDCS) dose strategies and yet complexities of existing approaches limit their clinical access. Since predictive modelling indicates the relevance of subject/pathology based data and hence the need for subject specific modelling, the incremental clinical value of increasingly complex modelling methods must be balanced against the computational and clinical time and costs. For example, the incorporation of multiple tissue layers and measured diffusion tensor (DTI) based conductivity estimates increase model precision but at the cost of clinical and computational resources. Costs related to such complexities aggregate when considering individual optimization and the myriad of potential montages. Here, rather than considering if additional details change current-flow prediction, we consider when added complexities influence clinical decisions.

APPROACH

Towards developing quantitative and qualitative metrics of value/cost associated with computational model complexity, we considered field distributions generated by two 4 × 1 high-definition montages (m1 = 4 × 1 HD montage with anode at C3 and m2 = 4 × 1 HD montage with anode at C1) and a single conventional (m3 = C3-Fp2) tDCS electrode montage. We evaluated statistical methods, including residual error (RE) and relative difference measure (RDM), to consider the clinical impact and utility of increased complexities, namely the influence of skull, muscle and brain anisotropic conductivities in a volume conductor model.

MAIN RESULTS

Anisotropy modulated current-flow in a montage and region dependent manner. However, significant statistical changes, produced within montage by anisotropy, did not change qualitative peak and topographic comparisons across montages. Thus for the examples analysed, clinical decision on which dose to select would not be altered by the omission of anisotropic brain conductivity.

SIGNIFICANCE

Results illustrate the need to rationally balance the role of model complexity, such as anisotropy in detailed current flow analysis versus value in clinical dose design. However, when extending our analysis to include axonal polarization, the results provide presumably clinically meaningful information. Hence the importance of model complexity may be more relevant with cellular level predictions of neuromodulation.

Modulating human procedural learning by cerebellar transcranial direct current stimulation

Neuroimaging studies suggest that the cerebellum contributes to human cognitive processing, particularly procedural learning. This type of learning is often described as implicit learning and involves automatic, associative, and unintentional learning processes. Our aim was to investigate whether cerebellar transcranial direct current stimulation (tDCS) influences procedural learning as measured by the serial reaction time task (SRTT), in which subjects make speeded key press responses to visual cues. A preliminary modeling study demonstrated that our electrode montage (active electrode over the cerebellum with an extra-cephalic reference) generated the maximum electric field amplitude in the cerebellum. We enrolled 21 healthy subjects (aged 20-49 years). Participants did the SRTT, a visual analogue scale and a visual attention task, before and 35 min after receiving 20-min anodal and sham cerebellar tDCS in a randomized order. To avoid carry-over effects, experimental sessions were held at least 1 week apart. For our primary outcome measure (difference in RTs for random and repeated blocks) anodal versus sham tDCS, RTs were significantly slower for sham tDCS than for anodal cerebellar tDCS (p = 0.04), demonstrating that anodal tDCS influenced implicit learning processes. When we assessed RTs for procedural learning across the one to eight blocks, we found that RTs changed significantly after anodal stimulation (interaction "time" × "blocks 1/8": anodal, p = 0.006), but after sham tDCS, they remained unchanged (p = 0.094). No significant changes were found in the other variables assessed. Our finding that anodal cerebellar tDCS improves an implicit learning type essential to the development of several motor skills or cognitive activity suggests that the cerebellum has a critical role in procedural learning. tDCS could be a new tool for improving procedural learning in daily life in healthy subjects and for correcting abnormal learning in neuropsychiatric disorders.