tDCS induced GABA change is associated with the simulated electric field in M1, an effect mediated by grey matter volume in the MRS voxel

Perspectives on Optimized Transcranial Electrical Stimulation Based on Spatial Electric Field Modeling in Humans

Background: Transcranial electrical stimulation (tES) generates an electric field (or current density) in the brain through surface electrodes attached to the scalp. Clinical significance has been demonstrated, although with moderate and heterogeneous results partly due to a lack of control of the delivered electric currents. In the last decade, computational electric field analysis has allowed the estimation and optimization of the electric field using accurate anatomical head models. This review examines recent tES computational studies, providing a comprehensive background on the technical aspects of adopting computational electric field analysis as a standardized procedure in medical applications. Methods: Specific search strategies were designed to retrieve papers from the Web of Science database. The papers were initially screened based on the soundness of the title and abstract and then on their full contents, resulting in a total of 57 studies. Results: Recent trends were identified in individual- and population-level analysis of the electric field, including head models from non-neurotypical individuals. Advanced optimization techniques that allow a high degree of control with the required focality and direction of the electric field were also summarized. There is also growing evidence of a correlation between the computationally estimated electric field and the observed responses in real experiments. Conclusions: Computational pipelines and optimization algorithms have reached a degree of maturity that provides a rationale to improve tES experimental design and a posteriori analysis of the responses for supporting clinical studies.

Home-Use Transcranial Direct Current Stimulation for the Treatment of a Major Depressive Episode: A Randomized Clinical Trial

Importance

Transcranial direct current stimulation (tDCS) is moderately effective for depression when applied by trained staff. It is not known whether self-applied tDCS, combined or not with a digital psychological intervention, is also effective.

Objective

To determine whether fully unsupervised home-use tDCS, combined with a digital psychological intervention or digital placebo, is effective for a major depressive episode.

Design, Setting, and Participants

This was a double-blinded, sham-controlled, randomized clinical trial with 3 arms: (1) home-use tDCS plus a digital psychological intervention (double active); (2) home-use tDCS plus digital placebo (tDCS only), and (3) sham home-use tDCS plus digital placebo (double sham). The study was conducted between April 2021 and October 2022 at participants' homes and at Instituto de Psiquiatria do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, Brazil. Included participants were aged 18 to 59 years with major depression and a Hamilton Depression Rating Scale, 17-item version (HDRS-17), score above 16, a minimum of 8 years of education, and access to a smartphone and internet at home. Exclusion criteria were other psychiatric disorders, except for anxiety; neurologic or clinical disorders; and tDCS contraindications.

Interventions

tDCS was administered in 2-mA, 30-minute prefrontal sessions for 15 consecutive weekdays (1-mA, 90-second duration for sham) and twice-weekly sessions for 3 weeks. The digital intervention consisted of 46 sessions based on behavioral therapy. Digital placebo was internet browsing.

Main Outcomes and Measures

Change in HDRS-17 score at week 6.

Results

Of 837 volunteers screened, 210 participants were enrolled (180 [86%] female; mean [SD] age, 38.9 [9.3] years) and allocated to double active (n = 64), tDCS only (n = 73), or double sham (n = 73). Of the 210 participants enrolled, 199 finished the trial. Linear mixed-effects models did not reveal statistically significant group differences in treatment by time interactions for HDRS-17 scores, and the estimated effect sizes between groups were as follows: double active vs tDCS only (Cohen d, 0.05; 95% CI, -0.48 to 0.58; P = .86), double active vs double sham (Cohen d, -0.20; 95% CI, -0.73 to 0.34; P = .47), and tDCS only vs double sham (Cohen d, -0.25; 95% CI, -0.76 to 0.27; P = .35). Skin redness and heat or burning sensations were more frequent in the double active and tDCS only groups. One nonfatal suicide attempt occurred in the tDCS only group.

Conclusions and Relevance

Unsupervised home-use tDCS combined with a digital psychological intervention or digital placebo was not found to be superior to sham for treatment of a major depressive episode in this trial.

Trial Registration

ClinicalTrials.gov Identifier: NCT04889976.

The impact of brain lesions on tDCS-induced electric fields

Transcranial direct current stimulation (tDCS) can enhance motor and language rehabilitation after stroke. Though brain lesions distort tDCS-induced electric field (E-field), systematic accounts remain limited. Using electric field modelling, we investigated the effect of 630 synthetic lesions on E-field magnitude in the region of interest (ROI). Models were conducted for two tDCS montages targeting either primary motor cortex (M1) or Broca's area (BA44). Absolute E-field magnitude in the ROI differed by up to 42% compared to the non-lesioned brain depending on lesion size, lesion-ROI distance, and lesion conductivity value. Lesion location determined the sign of this difference: lesions in-line with the predominant direction of current increased E-field magnitude in the ROI, whereas lesions located in the opposite direction decreased E-field magnitude. We further explored how individualised tDCS can control lesion-induced effects on E-field. Lesions affected the individualised electrode configuration needed to maximise E-field magnitude in the ROI, but this effect was negligible when prioritising the maximisation of radial inward current. Lesions distorting tDCS-induced E-field, is likely to exacerbate inter-individual variability in E-field magnitude. Individualising electrode configuration and stimulator output can minimise lesion-induced variability but requires improved estimates of lesion conductivity. Individualised tDCS is critical to overcome E-field variability in lesioned brains.

Outcome measures for electric field modeling in tES and TMS: A systematic review and large-scale modeling study

BACKGROUND

Electric field (E-field) modeling is a potent tool to estimate the amount of transcranial magnetic and electrical stimulation (TMS and tES, respectively) that reaches the cortex and to address the variable behavioral effects observed in the field. However, outcome measures used to quantify E-fields vary considerably and a thorough comparison is missing.

OBJECTIVES

This two-part study aimed to examine the different outcome measures used to report on tES and TMS induced E-fields, including volume- and surface-level gray matter, region of interest (ROI), whole brain, geometrical, structural, and percentile-based approaches. The study aimed to guide future research in informed selection of appropriate outcome measures.

METHODS

Three electronic databases were searched for tES and/or TMS studies quantifying E-fields. The identified outcome measures were compared across volume- and surface-level E-field data in ten tES and TMS modalities targeting two common targets in 100 healthy individuals.

RESULTS

In the systematic review, we extracted 308 outcome measures from 202 studies that adopted either a gray matter volume-level (n = 197) or surface-level (n = 111) approach. Volume-level results focused on E-field magnitude, while surface-level data encompassed E-field magnitude (n = 64) and normal/tangential E-field components (n = 47). E-fields were extracted in ROIs, such as brain structures and shapes (spheres, hexahedra and cylinders), or the whole brain. Percentiles or mean values were mostly used to quantify E-fields. Our modeling study, which involved 1,000 E-field models and > 1,000,000 extracted E-field values, revealed that different outcome measures yielded distinct E-field values, analyzed different brain regions, and did not always exhibit strong correlations in the same within-subject E-field model.

CONCLUSIONS

Outcome measure selection significantly impacts the locations and intensities of extracted E-field data in both tES and TMS E-field models. The suitability of different outcome measures depends on the target region, TMS/tES modality, individual anatomy, the analyzed E-field component and the research question. To enhance the quality, rigor, and reproducibility in the E-field modeling domain, we suggest standard reporting practices across studies and provide four recommendations.

Interindividual differences in posterior fossa morphometry affect cerebellar tDCS-induced electric field strength

OBJECTIVE

Clinical, behavioural, and neurophysiological effects of cerebellar transcranial direct current stimulation (tDCS) are highly variable and difficult to predict. We aimed to examine associations between cerebellar tDCS-induced electric field strength, morphometric posterior fossa parameters, and skin-cerebellum distance. As a secondary objective, field characteristics were compared between cephalic and extracephalic electrode configurations.

METHODS

Electric field simulations of midline cerebellar tDCS (7 × 5 cm electrodes, current intensities of 2 mA) were performed on MRI-based head models from 37 healthy adults using buccinator, frontopolar, and lower neck reference electrodes. Average field strengths were determined in eight regions of interest (ROIs) covering the anterior and posterior vermis and cerebellar hemispheres. Besides skin-cerebellum distance, various angles were measured between posterior fossa structures. Multivariable linear regression models were used to identify predictors of field strength in different ROIs.

RESULTS

Skin-cerebellum distance and "pons angle" were independently associated with field strength in the anterior and posterior vermis. "Cerebellar angle" and skin-cerebellum distance affected field strength in anterior and posterior regions of the right cerebellar hemisphere. Field strengths in all examined cerebellar areas were highest in the frontopolar and lowest in the lower neck montage, while the opposite was found for field focality. The lower neck montage induced considerably less spreading toward anterior cerebellar regions compared with the buccinator and frontopolar montages, which resulted in a more evenly distributed field within the cerebellum.

CONCLUSION

In addition to skin-cerebellum distance, interindividual differences in posterior fossa morphometry, specifically pons and cerebellar angle, explain part of the variability in cerebellar tDCS-induced electric field strength. Furthermore, when targeting the midline cerebellum with tDCS, an extracephalic reference electrode is associated with lower field strengths and higher field focality than cephalic montages.

SIGNIFICANCE

This study identifies two novel subject-specific anatomical factors that partly determine cerebellar tDCS-induced electric field strength and reveals differences in field characteristics between electrode montages.

The Therapeutic Potential of Non-Invasive and Invasive Cerebellar Stimulation Techniques in Hereditary Ataxias

The degenerative ataxias comprise a heterogeneous group of inherited and acquired disorders that are characterized by a progressive cerebellar syndrome, frequently in combination with one or more extracerebellar signs. Specific disease-modifying interventions are currently not available for many of these rare conditions, which underscores the necessity of finding effective symptomatic therapies. During the past five to ten years, an increasing number of randomized controlled trials have been conducted examining the potential of different non-invasive brain stimulation techniques to induce symptomatic improvement. In addition, a few smaller studies have explored deep brain stimulation (DBS) of the dentate nucleus as an invasive means to directly modulate cerebellar output, thereby aiming to alleviate ataxia severity. In this paper, we comprehensively review the clinical and neurophysiological effects of transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), and dentate nucleus DBS in patients with hereditary ataxias, as well as the presumed underlying mechanisms at the cellular and network level and perspectives for future research.

Optimizing the Effect of tDCS on Motor Sequence Learning in the Elderly

One of the most visible effects of aging, even in healthy, normal aging, is a decline in motor performance. The range of strategies applicable to counteract this deterioration has increased. Transcranial direct current stimulation (tDCS), a non-invasive brain stimulation technique that can promote neuroplasticity, has recently gained attention. However, knowledge about optimized tDCS parameters in the elderly is limited. Therefore, in this study, we investigated the effect of different anodal tDCS intensities on motor sequence learning in the elderly. Over the course of four sessions, 25 healthy older adults (over 65 years old) completed the Serial Reaction Time Task (SRTT) while receiving 1, 2, or 3 mA of anodal or sham stimulation over the primary motor cortex (M1). Additionally, 24 h after stimulation, motor memory consolidation was assessed. The results confirmed that motor sequence learning in all tDCS conditions was maintained the following day. While increased anodal stimulation intensity over M1 showed longer lasting excitability enhancement in the elderly in a prior study, the combination of higher intensity stimulation with an implicit motor learning task showed no significant effect. Future research should focus on the reason behind this lack of effect and probe alternative stimulation protocols.

The influence of a single transcranial direct current stimulation session on physical fitness in healthy subjects: a systematic review

Physical fitness is of indisputable importance for both health, and sports. Currently, the brain is being increasingly recognized as a contributor to physical fitness. Hereby, transcranial direct current stimulation (tDCS), as an ergogenic aid, has gained scientific interest. The current PRISMA-adherent review aimed to examine the effect of tDCS on the three core components of physical fitness: muscle strength, -endurance and cardiopulmonary endurance. Randomized controlled- or cross-over trials evaluating the effect of a single tDCS session (vs. sham) in healthy individuals were included. Hereby, a wide array of tDCS-related factors (e.g., tDCS montage and dose) was taken into account. Thirty-five studies (540 participants) were included. Between-study heterogeneity in factors such as age, activity level, tDCS protocol, and outcome measures was large. The capacity of tDCS to improve physical fitness varied substantially across studies. Nevertheless, muscle endurance was most susceptible to improvements following anodal tDCS (AtDCS), with 69% of studies (n = 11) investigating this core component of physical fitness reporting positive effects. The primary motor cortex and dorsolateral prefrontal cortex were targeted the most, with positive results being reported on muscle and cardiopulmonary endurance. Finally, online tDCS seemed most beneficial, and no clear relationship between tDCS and dose-related parameters seemed present. These findings can contribute to optimizing tDCS interventions during the rehabilitation of patients with a variety of (chronic) diseases such as cardiovascular disease. Therefore, future studies should focus on further unraveling the potential of AtDCS on physical fitness and, more specifically, muscle endurance in both healthy subjects and patients suffering from (chronic) diseases. This study was registered in Prospero with the registration number CRD42021258529. "To enable PROSPERO to focus on COVID-19 registrations during the 2020 pandemic, this registration record was automatically published exactly as submitted. The PROSPERO team has not checked eligibility".

Effect of Single Session of Anodal M1 Transcranial Direct Current Stimulation-TDCS-On Cortical Hemodynamic Activity: A Pilot Study in Fibromyalgia

Transcranial direct current stimulation (TDCS) on the primary motor cortex (M1) has been reported to be effective in fibromyalgia (FM). Our previous works have shown hypometabolism of motor networks in FM using Functional Near Infrared Spectroscopy (fNIRS), which could contribute to pain symptoms. To investigate if a single Transcranial Direct Current Stimulation (TDCS) session can restore the reduced metabolism expected in FM patients, we compared metabolic activity in FM patients and controls during a finger-tapping task in basal condition, sham condition, and under anodal TDCS on M1. During the finger tapping task, a continuous wave 20 channel fNIRS system was placed across the bilateral central-frontal areas in 22 healthy controls and 54 FM patients. Subjects were randomly assigned to real TDCS or sham stimulation. The finger-tapping slowness did not change after real and sham stimulation. After real TDCS stimulation, FM patients showed an increased activation of cortical motor regions (t-statistic = -2.5246, p-value = 0.0125 for the stimulated hemisphere and t-statistic = -4.6638, p-value = 0.0001 for the non-stimulated hemisphere). The basal differences between FM and controls reverted after real TDCS, while this effect was not observed for sham stimulation. A single TDCS session of the cortical motor network seemed able to restore basic cortical hypometabolism in FM patients. Further studies could clarify the long-term effect of M1 stimulation on cortical metabolism, and its relevance in pain processing and clinical features.

Addressing transcranial electrical stimulation variability through prospective individualized dosing of electric field strength in 300 participants across two samples: the 2-SPED approach

Objective. Transcranial electrical stimulation (tES) is a promising method for modulating brain activity and excitability with variable results to date. To minimize electric (E-)field strength variability, we introduce the 2-sample prospective E-field dosing (2-SPED) approach, which uses E-field strengths induced by tES in a first population to individualize stimulation intensity in a second population.Approach. We performed E-field modeling of three common tES montages in 300 healthy younger adults. First, permutation analyses identified the sample size required to obtain a stable group average E-field in the primary motor cortex (M1), with stability being defined as the number of participants where all group-average E-field strengths ± standard deviation did not leave the population's 5-95 percentile range. Second, this stable group average was used to individualize tES intensity in a second independent population (n = 100). The impact of individualized versus fixed intensity tES on E-field strength variability was analyzed.Main results. In the first population, stable group average E-field strengths (V/m) in M1 were achieved at 74-85 participants, depending on the tES montage. Individualizing the stimulation intensity (mA) in the second population resulted in uniform M1 E-field strength (all p < 0.001) and significantly diminished peak cortical E-field strength variability (all p < 0.01), across all montages.Significance. 2-SPED is a feasible way to prospectively induce more uniform E-field strengths in a region of interest. Future studies might apply 2-SPED to investigate whether decreased E-field strength variability also results in decreased physiological and behavioral variability in response to tES.

The impact of anodal transcranial direct current stimulation of primary motor cortex on motor learning in older adults with low levels of activity

Background/aims

Older adults with different physical activity levels have often demonstrated individual differences in motor performance and learning. Serial reaction time task training and anodal transcranial direct current stimulation of the primary motor cortex were used in this study to evaluate how these interventions affected motor learning in older adults with low activity levels.

Methods

In this randomised controlled trial, 28 healthy, right-handed, older adults with low activity levels, with a mean age of 69.92 years, were randomly allocated to an anodal transcranial direct current stimulation group (n=14) or sham transcranial direct current stimulation group (n=14), based on a simple non-probability sampling method. The experimental group was exposed to 20 minutes of anodal transcranial direct current stimulation of the motor cortex, using a tDCS device, alongside eight sequenced or randomised blocks of serial reaction time task activities, for 5 consecutive days. In the control group, the tDCS device was automatically switched off after 1 minute. To assess implicit motor learning, the response time and error rate of two sequenced blocks of serial reaction time task activities were collected before, immediately following, 1 day and 1 week after the completion of the intervention.

Results

Immediately following the end of the intervention, at day 5, the mean response time and error rate were 925.09 and 2.55 in the experimental group, and 1016.52 and 4.10 in the control group. At 1 day after the completion of the intervention, the mean response time and error rates were 927.40 and 3.03 in the experimental group and 1021.91 and 4.34 in the control group. At 1 week after the completion of the intervention, the mean response time and error rates were 942.26 and 3.63 in the experimental group and 1050.08 and 5.11 in the control group. These findings indicate that response time and error rate were significantly decreased in both stimulation groups at different time points (P<0.001). At the same time, there were no significant differences in the response time and error rate between the two groups at different time points: immediately (P=0.07, P=0.31), 1 day (P=0.06, P=0.41) and 1 week (P=0.04, P=0.35) after the completion of the intervention respectively.

Conclusions

Serial reaction time task training, with or without applying anodal transcranial direct current stimulation, can improve motor learning in low-activity older adults. Therefore, it appears that anodal transcranial direct current stimulation did not affect or improve motor learning in older adults with low motor activity. Motor learning training can be used alone as a practical and helpful intervention to improve performance and implicit motor skill learning with long-lasting effects in older adults with low levels of activity.