Immediate neurophysiological effects of transcranial electrical stimulation

Transcranial electrical stimulation (TES) in human motor Optimization: Mechanisms, safety, and emerging applications

Non-invasive brain stimulation (NIBS) has emerged as a rapidly advancing field, offering promising therapeutic interventions for a range of neurological disorders while effectively bridging the gap between laboratory research and clinical applications. Among NIBS technologies, transcranial electrical stimulation (TES) stands out as a notable example, utilizing electrodes of varying sizes to deliver low-intensity electrical currents to specific regions of the cerebral cortex. This technique facilitates the modulation of neuronal excitability, regulation of brainwave activity, promotion of neural remodeling and repair, enhancement of cerebral blood flow, and improvement of brain-muscle connectivity. Despite its potential, current research on the effects of TES on motor function across diverse populations, particularly from a central nervous system perspective, remains limited. This review seeks to establish a theoretical framework for the future advancement of TES technology in sports science, elucidate the neurophysiological mechanisms underlying various TES modalities, and synthesize the most recent experimental findings from the past two decades regarding its impact on physical fitness, motor skill acquisition, and recovery in different populations.

Multimodal personalization of transcranial direct current stimulation for modulation of sensorimotor integration

Transcranial direct current stimulation (tDCS) for the modulation of smooth pursuit eye movements provides an ideal model for investigating sensorimotor integration. Within neural networks subserving smooth pursuit, visual area V5 is a core hub where visual motion information is integrated with oculomotor control. Here, we applied personalized tDCS explicitly targeting individual V5 in healthy human participants using algorithmic optimization informed by functional magnetic resonance imaging and combined electro- and magnetoencephalography. We hypothesized subtle modulation of sensorimotor integration during pursuit and assessed the effects of personalized anodal and cathodal tDCS targeting V5 compared to (a) sham stimulation, (b) personalized tDCS targeting the frontal eye field (FEF), and (c) conventional normative tDCS over V5. We found pursuit initiation specifically delayed during personalized cathodal tDCS targeting right V5 suggesting the involvement of distinct functional subregions of V5 in initial sensorimotor integration of visual motion information during pursuit eye movements. Results were extensively controlled by anodal and sham tDCS, different pursuit tasks, and finite-element simulations of individual electric fields. Importantly, in contrast to the two control experiments (personalized tDCS targeting FEF and normative tDCS over V5) personalized tDCS targeting V5 effectively modulated pursuit by adapting electric fields to individual anatomical and functional V5 properties. Our results provide evidence for the ability of personalized tDCS targeting V5 to introduce targeted subtle modulation of sensorimotor integration, specifically during smooth pursuit initiation. Further, our results indicate the potential of personalized tDCS to alter behavior as the main aspect of interest in human neuromodulation.

Anatomical Characteristics Predict Response to Transcranial Direct Current Stimulation (tDCS): Development of a Computational Pipeline for Optimizing tDCS Protocols

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique promisingly used to treat neurological and psychological disorders. Nevertheless, the inter-subject heterogeneity in its after-effects frequently limits its efficacy. This can be attributed to fixed-dose methods, which do not consider inter-subject anatomical variations. This work attempts to overcome this constraint by examining the effects of age and anatomical features, including the volume of cerebrospinal fluid (CSF), the thickness of the skull, and the composition of brain tissue, on electric field distribution and cortical excitability. A computational approach was used to map the electric field distribution over the brain tissues of realistic head models reconstructed from MRI images of twenty-three subjects, including adults and children of both genders. Significant negative correlations (p < 0.05) were found in the data between the maximum electric field strength and anatomical variable parameters. Furthermore, this study showed that the percentage of brain tissue exposed to an electric field amplitude above a pre-defined threshold (i.e., 0.227 V/m) was the main factor influencing the responsiveness to tDCS. In the end, the research suggests multiple regression models as useful tool to predict subjects' responsiveness and to support a personalized approach that tailors the injected current to the morphology of the patient.

Low-intensity transcranial ultrasound stimulation and its regulatory effect on pain

Transcranial ultrasound stimulation is an emerging non-invasive neuromodulation technology with the advantages of deep tissue penetration, high spatial resolution, and minimal side effects. Low intensity transcranial ultrasound stimulation (LITUS) has been shownto bea promising neuromodulation treatment for psychiatric and neurological disorders. Notably, significant progress has been made recently in both the application of LITUS in pain disorders and the elucidation of its analgesic mechanisms. This review provides an overview of LITUS and its state-of-the-art mechanisms, including cavitation, mechanical, and thermal effects. We summarize studies spanning from animal models to human trials, highlighting the analgesic effects of transcranial ultrasound stimulation on pain-related neural pathways. Furthermore, we explore potential analgesic mechanisms, such as the suppression of neural activity in the ascending pain pathway and other associated processes.Lastly, we discuss the potential of LITUS for future integrative treatments of chronic pain and psychomotor disorders, as well as its broader therapeutic applications.

Transcranial alternating current stimulation for investigating complex oscillatory dynamics and interactions

Neural oscillations play a fundamental role in human cognition and behavior. While electroencephalography (EEG) and related methods provide precise temporal recordings of these oscillations, they are limited in their ability to generate causal conclusions. Transcranial alternating current stimulation (tACS) has emerged as a promising non-invasive neurostimulation technique to modulate neural oscillations, which offers insights into their functional role and relation to human cognition and behavior. Originally, tACS is applied between two or more electrodes at a given frequency. However, recent advances have aimed to apply different current waveforms to target specific oscillatory dynamics. This systematic review evaluates the efficacy of non-standard tACS applications designed to investigate oscillatory patterns beyond simple sinusoidal stimulation. We categorized these approaches into three key domains: (1) phase synchronization techniques, including in-phase, anti-phase, and traveling wave stimulation; (2) non-sinusoidal tACS, which applies alternative waveforms such as composite, broadband or triangular oscillations; and (3) amplitude-modulated tACS and temporal interference stimulation, which allow for concurrent EEG recordings and deeper cortical targeting. While a number of studies provide evidence for the added value of these non-standard tACS procedures, other studies show opposing or null findings. Crucially, the number of studies for most applications is currently low, and as such, the goal of this review is to highlight both the promise and current limitations of these techniques, providing a foundation for future research in neurostimulation.

Electroencephalography-driven brain-network models for personalized interpretation and prediction of neural oscillations

OBJECTIVE

Develop an encephalography (EEG)-driven method that integrates interpretability, predictiveness, and personalization to assess the dynamics of the brain network, with a focus on pathological conditions such as pharmacoresistant epilepsy.

METHODS

We propose a method to identify dominant coherent oscillations from EEG recordings. It relies on the Koopman operator theory to achieve individualized EEG prediction and electrophysiological interpretability. We extend it with concepts from adiabatic theory to address the nonstationary and noisy EEG signals.

RESULTS

By simultaneously capturing the local spectral and connectivity aspects of patient-specific oscillatory dynamics, we are able to clarify the underlying dynamical mechanism. We use it to construct the corresponding generative models of the brain network. We demonstrate the proposed approach on recordings of patients in status epilepticus.

CONCLUSIONS

The proposed EEG-driven method opens new perspectives on integrating interpretability, predictiveness, and personalization within a unified framework. It provides a quantitative approach for assessing EEG recordings, crucial for understanding and modulating pathological brain activity.

SIGNIFICANCE

This work bridges theoretical neuroscience and clinical practice, offering a novel framework for understanding and predicting brain network dynamics. The resulting approach paves the way for data-driven insights into brain network mechanisms and the design of personalized neuromodulation therapies.

Direction of TIS envelope electric field: Perpendicular to the longitudinal axis of the hippocampus

BACKGROUND

Temporal Interference Stimulation (TIS) is a non-invasive approach to deep brain stimulation. However, most research has focused on the intensity of modulation, with limited attention given to the directional properties of the induced electric fields, despite their potential importance for precise stimulation.

NEW METHODS

A novel analytical framework was developed to analyze TIS-induced electric field directions using individual imaging data. For each voxel, the direction corresponding to the maximal modulation depth was calculated. The consistency of these directions within regions of interest (ROIs) and their alignment with the ROI principal axes, derived from principal component analysis (PCA), were assessed.

RESULTS

Simulations revealed complex spatial and temporal trajectories of the electric field at the voxel level. In the left putamen, the maximal modulation depth reached 0.241 ± 0.041 V/m, whereas in the target region, the left hippocampus, it was lower (0.15 ± 0.032 V/m). Notably, in the left hippocampus, the directions of maximal modulation depth were predominantly perpendicular to its longitudinal axis (84.547 ± 8.776°), reflecting structural specificity across its anterior, middle, and posterior regions.

COMPARISON WITH EXISTING METHODS

Unlike previous approaches, this study integrates directional analysis into TIS modeling, providing a foundation for precise stimulation by exploring structural alignment.

CONCLUSION

Our analysis revealed that the orientations of maximal modulation depth in the left hippocampus were perpendicular to its longitudinal axis under the current electrode configuration, but they shifted to parallel alignment when the electrode pairs were swapped. This directional specificity offers insights for optimizing TIS by aligning with structural features, presenting a potential strategy to enhance stimulation precision and broaden its clinical and research applications.

Triangular Wave tACS Improves Working Memory Performance by Enhancing Brain Activity in the Early Stage of Encoding

Working memory is an executive memory process that includes encoding, maintenance, and retrieval. These processes can be modulated by transcranial alternating current stimulation (tACS) with sinusoidal waves. However, little is known about the impact of the rate of current change on working memory. In this study, we aimed to investigate the effects of two types of tACS with different rates of current change on working memory performance and brain activity. We applied a randomized, single-blind design and divided 81 young participants who received triangular wave tACS, sinusoidal wave tACS, or sham stimulation into three groups. Participants performed n-back tasks, and electroencephalograms were recorded before, during, and after active or sham stimulation. Compared to the baseline, working memory performance (accuracy and response time) improved after stimulation under all stimulation conditions. According to drift-diffusion model analysis, triangular wave tACS significantly increased the efficiency of non-target information processing. In addition, compared with sham conditions, triangular wave tACS reduced alpha power oscillations in the occipital lobe throughout the encoding period, while sinusoidal wave tACS increased theta power in the central frontal region only during the later encoding period. The brain network connectivity results showed that triangular wave tACS improved the clustering coefficient, local efficiency, and node degree intensity in the early encoding stage, and these parameters were positively correlated with the non-target drift rate and decision starting point. Our findings on how tACS modulates working memory indicate that triangular wave tACS significantly enhances brain network connectivity during the early encoding stage, demonstrating an improvement in the efficiency of working memory processing. In contrast, sinusoidal wave tACS increased the theta power during the later encoding stage, suggesting its potential critical role in late-stage information processing. These findings provide valuable insights into the potential mechanisms by which tACS modulates working memory.

Organism-wide cellular dynamics and epigenomic remodeling in mammalian aging

Aging leads to functional decline across tissues, often accompanied by profound changes in cellular composition and cell-intrinsic molecular states. However, a comprehensive catalog of how the population of individual cell types change with age and the associated epigenomic dynamics is lacking. Here, we constructed a single-cell chromatin accessibility atlas consisting of ∼7 million cells from 21 tissue types spanning three age groups in both sexes. This dataset revealed 536 main cell types and 1,828 finer-grained subtypes, defined by unique chromatin accessibility landscapes at ∼1.3 million cis-regulatory elements. We observed widespread remodeling of immune lineages, with increases in plasma cells and macrophages, and depletion of T and B cell progenitors. Additionally, non-immune cell populations, including kidney podocytes, ovary granulosa cells, muscle tenocytes and lung aerocytes, showed marked reductions with age. Meanwhile, many subtypes changed synchronously across multiple organs, underscoring the potential influence of systemic inflammatory signals or hormonal cues. At the molecular level, aging was marked by thousands of differentially accessible regions, with the most concordant changes shared across cell types linked to genes related to inflammation or development. Putative upstream factors, such as intrinsic shifts in transcription factor usages and extrinsic cytokine signatures, were identified. Notably, around 40% of aging-associated main cell types and subtypes showed sex-dependent differences, with tens of thousands of chromatin accessibility peaks altered exclusively in one sex. Together, these findings present a comprehensive framework of how aging reshapes the chromatin landscape and cellular composition across diverse tissues, offering a comprehensive resource for understanding the molecular and cellular programs underlying aging and supporting the exploration of targeted therapeutic strategies to address age-related dysfunction.

To enhance or not to enhance: A debate about cognitive enhancement from a psychological and neuroscientific perspective

The enhancement of humans' core cognitive abilities-such as intelligence-is a frequently debated topic in scientific and public discourse. Different enhancement methods such as cognitive trainings, smart drugs, and brain stimulation techniques have been proposed and tested to enhance human's cognition. In this narrative review, we summarize the main psychological and neuroscientific findings regarding those cognitive enhancement methods. We thereby distinguish passive (e.g., smart drugs) and active enhancement methods (e.g., working memory training)-which require different levels of agency. While for both forms of enhancement there is no (or only little) empirical evidence on their effectiveness to improve overall cognitive abilities, passive methods entail severe risks. Thus, we criticize promoting an overly optimistic view on especially passive enhancement. Furthermore, we highlight which individuals might be willing to enhance themselves, related motivational aspects of cognitive enhancement, and ethical considerations thereof. To raise awareness for the (in)effectiveness and risks of passive and active enhancement, we propose a category framework that distinguishes cognitive enhancement from clinical methods to treat disorders or diagnosed deficits and introduces important dimensions thereof. Finally, we present open questions for psychological and neuroscientific research which should become part of enhancement debates. Taken together, our narrative review provides a broad overview and critical assessment of enhancement-related topics such as effectiveness and risks of enhancement methods, motivational aspects to apply enhancement, and societal implications of cognitive enhancement.

Individual alpha frequency tACS reduces static functional connectivity across the default mode network

Introduction

Research on the influence of transcranial alternating current stimulation over alpha functional connectivity (FC) is scarce, even when it poses as a potential treatment for various diseases. This study aimed to investigate the effects of individual alpha frequency tACS (IAF-tACS) on FC within the default mode network (DMN) in healthy individuals, particularly following the triple network model.

Materials and methods

27 healthy participants were recruited, who underwent a 20-min IAF-tACS session over parieto-occipital areas and three magnetoencephalography (MEG) recordings: two pre-stimulation and one post-stimulation. Participants were randomly assigned to either the stimulation or sham group. Both dynamic FC (dFC) and static FC (sFC) were evaluated through the leakage corrected amplitude envelope correlation (AEC-c). Statistical analyses compared both Pre-Post FC ratio between groups through ratio t-tests and intragroup FC changes through repeated measures t-tests, with FDR correction applied to account for multiple comparisons. An additional analysis simulated the influence of the cortical folding on the effect of tACS over FC.

Results

IAF-tACS significantly decreased sFC in intra- and inter-DMN links in the stimulation group compared to the sham group, with a special influence over antero-posterior links between hubs of the DMN. Negative correlations were found between AEC-c sFC changes and power alterations in posterior DMN areas, suggesting a complex interaction between cortical folding and electric field direction. On the other hand, dFC increased in both sham and stimulation groups, and no between-group differences were found.

Conclusion

Against our initial hypothesis, IAF-tACS reduced sFC in the DMN, possibly due to phase disparities introduced by cortical gyrification. These findings suggest that tACS might modulate FC in a more complex manner than previously thought, highlighting the need for further research into the personalized application of neuromodulation techniques, as well as its potential therapeutic implications for conditions like Alzheimer's disease.

Noninvasive Brain Stimulation for Neurodevelopmental Disorders: A Systematic Review

Neurodevelopmental disorders (NDDs) affect brain development, leading to diverse cognitive, social, behavioral, and affective impairments. Noninvasive brain stimulation (NIBS) techniques, such as transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), and transcranial photobiomodulation (tPBM), have been investigated as potential treatments for NDDs. The authors of this systematic review evaluated the literature on NIBS in NDDs, including double-blind, sham-controlled, randomized controlled trials. Following PRISMA guidelines and a registered protocol, the authors conducted a comprehensive search in PubMed, Embase, Cochrane Central Register of Controlled Trials, PsycInfo, and Scopus, identifying 23 studies. TMS showed promise for addressing hyperactivity, inattention, and working memory deficits in attention-deficit hyperactivity disorder (ADHD), with outcomes influenced by coil type (H5 vs. H6) and stimulation site (right vs. left dorsolateral prefrontal cortex). tDCS showed potential for improving inattention and executive function in ADHD, with limited effects observed on reducing symptom severity in autism spectrum disorder (ASD) and dyslexia. tPBM offered specific therapeutic benefits in reducing irritability in ASD. Although NIBS generally showed mild, transient adverse effects, isolated seizure events, such as one during TMS in ADHD, highlight the importance of rigorous safety protocols, especially in NDDs with elevated epilepsy risk. This review identified potential benefits of certain NIBS protocols in NDDs; however, high variability in methodologies, sample size limitations, and bias concerns underscore the need for further research to clarify the therapeutic efficacy and safety of NIBS among patients with NDDs.

The influence of tDCS on the speed-accuracy tradeoff and metacognitive decision making

A fundamental tradeoff exists between speed and accuracy when performing a decision (speed-accuracy tradeoff, SAT). Metacognition allows for the adjustment, monitoring, and evaluation of one's own decisions and strategies. While these aspects of cognition are central to human behavioural performance, their respective causal neural underpinnings are not well understood. Here, we used transcranial direct current stimulation (tDCS) to investigate the causal roles of the prefrontal cortex (PFC), superior medial frontal cortex (SMFC), and posterior parietal cortex (PPC) in the SAT and metacognition. Subjects received active or sham tDCS before completing a perceptual task with explicit SAT cues and reported confidence in their decisions. We fit the linear ballistic accumulator model to behavioural data to extract latent decision variables and used confidence judgments to compute two common indices of metacognition: meta-d' and m-ratio. Stimulation influenced performance on the perceptual task but there was no meaningful evidence for an effect on metacognition. Specifically, PFC stimulation reduced subjects' response caution, especially when accuracy was emphasised; SMFC stimulation decreased response caution and increased the discriminability between choices; and PPC stimulation increased both response caution and discriminability. These results show that the impact of tDCS on the SAT critically depends on the frontoparietal region stimulated. In addition, there was little to no evidence of any effect of tDCS on metacognition, hinting at potential differences in the neural circuitry supporting aspects of object-level computation and meta-level processing. In sum, our findings provide further evidence that tDCS can alter decision making and strategic processes in the human brain.

Predictive role of endogenous phase lags between target brain regions in dual-site transcranial alternating current stimulation

BACKGROUND

Dual-site transcranial alternating current stimulation (tACS) provides a promising tool for modulating interregional brain connectivity by entraining neural oscillations. However, prior studies have reported inconsistent effects on connectivity and behavioral outcomes. They often focused on individualized stimulation-frequency as a key entrainment factor, while typically not focusing on the role of endogenous phase lags. To address this gap, we explored the predictive value of endogenous phase lags in dual-site tACS to modulate interhemispheric connectivity during dichotic listening.

METHODS

Thirty healthy participants (16 females) completed a dichotic listening task while undergoing simultaneous electroencephalography and tACS, including four bitemporal verum conditions with varying phase lags (0°, 45°, 90°, and 180°), and a sham condition across five sessions. Each session involved 20 min of 40-Hz tACS at a 0.5 mA peak-to-baseline amplitude applied to the temporal regions, with phase lags differing across sessions. Endogenous phase lags between the auditory cortices were calculated to explain changes in the laterality index (LI) across stimulation conditions by defining optimal and disruptive stimulation conditions for each participant.

RESULTS

Consistent with our hypothesis, our personalized analysis based on the calculated endogenous phase lags showed a significantly lower LI during the closest (optimal) stimulation condition compared to both the sham and farthest (disruptive) conditions. Conversely, the farthest stimulation condition did not statistically increase the LI compared to sham.

CONCLUSIONS

These findings highlight the importance of incorporating endogenous phase dynamics into dual-site tACS protocols, paving the way for more consistent and individualized neuromodulatory interventions.

Increasing target engagement via customized electrode positioning for personalized transcranial electrical stimulation: A biophysical modeling study

BACKGROUND

Transcranial electric stimulation (TES) is a non-invasive neuromodulation technique with therapeutic potential for diverse neurological disorders including Alzheimer's disease. Conventional TES montages with stimulation electrodes in standardized positions suffer from highly varying electric fields across subjects due to variable anatomy. Biophysical modelling using individual's brain imaging has thus become popular for montage planning but may be limited by fixed scalp electrode locations.

OBJECTIVE

Here, we explore the potential benefits of flexible electrode positioning with 3D-printed neurostimulator caps.

METHODS

We modeled 10 healthy subjects and simulated montages targeting the left angular gyrus, which is relevant for restoring memory functions impaired by Alzheimer's disease. Using quantitative metrics and visual inspection, we benchmark montages with flexible electrode placement against well-established montage selection approaches.

RESULTS

Personalized montages optimized with flexible electrode positioning provided tunable intensity and control over the focality-intensity trade-off, outperforming conventional montages across the range of achievable target intensities. Compared to montages optimized on a reference model, personalized optimization significantly reduced variance of the stimulation intensity in the target. Finally, increasing available electrode positions from 32 to around 86 significantly increased target engagement across a range of target intensities and current limits.

CONCLUSIONS

In summary, we provide an in silico proof-of-concept that digitally designed and 3D-printed TES caps with flexible electrode positioning can increase target engagement with precise and tunable control of applied dose to a cortical target. This is of interest for stimulation of brain networks such as the default mode network with spatially proximate correlated and anti-correlated cortical nodes.

Closed-loop rehabilitation of upper-limb dyskinesia after stroke: from natural motion to neuronal microfluidics

This review proposes an innovative closed-loop rehabilitation strategy that integrates multiple subdomains of stroke science to address the global challenge of upper-limb dyskinesia post-stroke. Despite advancements in neural remodeling and rehabilitation research, the compartmentalization of subdomains has limited the effectiveness of current rehabilitation strategies. Our approach unites key areas-including the post-stroke brain, upper-limb rehabilitation robotics, motion sensing, metrics, neural microfluidics, and neuroelectronics-into a cohesive framework designed to enhance upper-limb motion rehabilitation outcomes. By leveraging cutting-edge technologies such as lightweight rehabilitation robotics, advanced motion sensing, and neural microfluidic models, this strategy enables real-time monitoring, adaptive interventions, and personalized rehabilitation plans. Furthermore, we explore the potential of closed-loop systems to drive neural plasticity and functional recovery, offering a transformative perspective on stroke rehabilitation. Finally, we discuss future directions, emphasizing the integration of emerging technologies and interdisciplinary collaboration to advance the field. This review highlights the promise of closed-loop strategies in achieving unprecedented integration of subdomains and improving post-stroke upper-limb rehabilitation outcomes.

Somatodendritic orientation determines tDCS-induced neuromodulation of Purkinje cell activity in awake mice

Transcranial direct-current stimulation (tDCS) of the cerebellum is a promising non-invasive neuromodulatory technique being proposed for the treatment of neurological and neuropsychiatric disorders. However, there is a lack of knowledge about how externally applied currents affect neuronal spiking activity in cerebellar circuits in vivo. We investigated how Cb-tDCS affects the firing rate of Purkinje cells (PC) and non-PC in the mouse cerebellar cortex to understand the underlying mechanisms behind the polarity-dependent modulation of neuronal activity induced by tDCS. Mice (n=9) were prepared for the chronic recording of local field potentials (LFPs) to assess the actual electric field gradient imposed by Cb-tDCS in our experimental design. Single-neuron extracellular recording of PCs in awake (n=24) and anesthetized (n=27) mice was combined with juxtacellular recordings and subsequent staining of PC with neurobiotin under anesthesia (n=8) to correlate their neuronal orientation with their response to Cb-tDCS. Finally, a high-density Neuropixels recording system was used to demonstrate the relevance of neuronal orientation during the application of Cb-tDCS in awake mice (n=6). In this study, we observe that Cb-tDCS induces a heterogeneous polarity-dependent modulation of the firing rate of PCs and non-PC in the mouse cerebellar cortex. We demonstrate that the apparently heterogeneous effects of tDCS on PC activity can be explained by taking into account the somatodendritic orientation relative to the electric field. Our findings highlight the need to consider neuronal orientation and morphology to improve tDCS computational models, enhance stimulation protocol reliability, and optimize effects in both basic and clinical applications.

Grid-based transcutaneous spinal cord stimulation: probing neuromodulatory effect in spinal flexion reflex circuits

Objective.Non-invasive spinal stimulation has the potential to modulate spinal excitability. This study explored the modulatory capacity of sub-motor grid-based transcutaneous spinal cord stimulation (tSCS) applied to the lumbar spinal cord in neurologically intact participants. Our objective was to examine the effect of grid spinal stimulation on polysynaptic reflex pathways involving motoneurons and interneurons likely activated by Aβ/δfiber-mediated cutaneous afferents.Approach.Stimulation was delivered using two grid electrode montages, generating a net electric field in transverse or diagonal directions. We administered tSCS with the center of the grid aligned with the T10-T11 spinous process. Participants were seated for the 20 min stimulation duration. At 30 min after the cessation of spinal stimulation, we examined neuromodulatory effects on spinal circuit excitability in the tibialis anterior muscle by employing the classical flexion reflex paradigms. Additionally, we evaluated spinal motoneuron excitability using theH-reflex paradigm in the soleus muscle to explore the differential effects of tSCS on the polysynaptic versus monosynaptic reflex pathway and to test the spatial extent of the grid stimulation.Main results.Our findings indicated significant neuromodulatory effects on the flexion reflex, resulting in a net inhibitory effect, regardless of the grid electrode montages. Our data further indicated that the flexion reflex duration was significantly shortened only by the diagonal montage.Significance.Our results suggest that grid-based tSCS may specifically modulate spinal activities associated with polysynaptic flexion reflex pathways, with the potential for grid-specific targeted neuromodulation.

Hybrid Electro-optical Stimulation Improves Ischemic Brain Damage by Augmenting the Glymphatic System

Ischemic brain injury not only results in significant neurological, motor, and cognitive impairment but also contributes to the accumulation of toxic solutes and proinflammatory cytokines in the infarction region, exacerbating ischemic brain damage. The glymphatic system, which is crucial for brain waste clearance and homeostasis, is impaired by ischemic injury, highlighting the importance of developing therapeutic strategies for poststroke complications. Herein, a novel hybrid electro-optical stimulation device is proposed that integrates near-infrared micro-light-emitting diode with transparent microneedles, enabling efficient noninvasive stimulation of the cortical area for ischemic stroke treatment. This study investigates whether this hybrid electro-optical stimulation enhances the glymphatic system function and ameliorates ischemic brain injury in the middle cerebral artery occlusion and reperfusion (MCAO/R) mice model. The results demonstrate that hybrid stimulation improves the neurological, motor, and cognitive functions and reduces brain atrophy following MCAO/R. Moreover, hybrid stimulation restores impaired glymphatic system function by modulation of aquaporin-4 (AQP4) polarization and alleviates the accumulation of proinflammatory cytokines such as IL-1β. Notably, AQP4 inhibition partly reverses the improved functional outcomes of hybrid stimulation. The findings suggest that targeting glymphatic drainage using hybrid electro-optical stimulation is a promising therapeutic approach for treating ischemic brain injury.

Inkjet-printed transparent electrodes: Design, characterization, and initial in vivo evaluation for brain stimulation

Electrical stimulation is a powerful tool for investigating and modulating brain activity, as well as for treating neurological disorders. However, understanding the precise effects of electrical stimulation on neural activity has been hindered by limitations in recording neuronal responses near the stimulating electrode, such as stimulation artifacts in electrophysiology or obstruction of the field of view in imaging. In this study, we introduce a novel stimulation device fabricated from conductive polymers that is transparent and therefore compatible with optical imaging techniques. The device is manufactured using a combination of microfabrication and inkjet printing techniques and is flexible, allowing better adherence to the brain's natural curvature. We characterized the electrical and optical properties of the electrodes, focusing on the trade-off between the maximum current that can be delivered and optical transmittance. We found that a 1 mm diameter, 350 nm thick PEDOT:PSS electrode could be used to apply a maximum current of 130 μA while maintaining 84% transmittance (approximately 50% under 2-photon imaging conditions). We then evaluated the electrode performance in the brain of an anesthetized mouse by measuring the electric field with a nearby recording electrode and found values up to 30 V/m. Finally, we combined experimental data with a finite-element model of the in vivo experimental setup to estimate the distribution of the electric field underneath the electrode in the mouse brain. Our findings indicate that the device can generate an electric field as high as 300 V/m directly beneath the electrode, demonstrating its potential for studying and manipulating neural activity using a range of electrical stimulation techniques relevant to human applications. Overall, this work presents a promising approach for developing versatile new tools to apply and study electrical brain stimulation.

Personalized Theta Transcranial Alternating Current Stimulation and Gamma Transcranial Alternating Current Stimulation Bring Differential Neuromodulatory Effects on the Resting Electroencephalogram: Characterizing the Temporal, Spatial, and Spectral Dimensions of Transcranial Alternating Current Stimulation

OBJECTIVES

The neuromodulatory effects of transcranial alternating current stimulation (tACS) on electroencephalogram (EEG) dynamics are quite heterogenous. The primary objective of the study is to comprehensively characterize the effects of two tACS protocols on resting-state EEG.

MATERIALS AND METHODS

A total of 36 healthy participants were recruited and were randomized into three groups. Two groups received either personalized theta (4-8 Hz) or gamma (40 Hz) stimulation bilaterally in the frontal regions for 20 minutes (4 minutes ON, 1 minute OFF, four cycles). The third group performed relaxed breath watching for 20 minutes. Artifact-free, 1-minute EEG segments from the baseline, during tACS, and after stimulation resting EEG were characterized to see the effects of tACS. Threshold-free cluster enhanced permutation tests (for spectral measures) and two-way mixed analysis of variance (for aperiodic slope) were used for statistical inferences.

RESULTS

Current modeling simulation using ROAST with preset parameters (800 μA, AF3 AF4 locations) showed that induced electric fields can activate frontal cortical regions. During the stimulation period, personalized theta tACS entrained theta band power in the centro-parietal areas. There was a compensatory power decrease in the beta and gamma bands after theta tACS. No entrainment effects were observed for gamma tACS during stimulation, but a significant entrainment was observed in the theta and beta bands in the parieto-occipital regions after stimulation. The delta band power decreased in the central regions. No spectral modulations were seen after breath watching. The spectral slope, which measures aperiodic activity, was not affected by either breath watching or tACS.

CONCLUSIONS

Characterizing the effects of multiple tACS protocols is critical to effectively target specific neural oscillatory patterns and to personalize the protocols. The study can be extended to target specific oscillatory patterns associated with cognitive deficits in neuro-psychiatric conditions.

Is transcranial alternating current stimulation effective for improving working memory? A three-level meta-analysis

Working memory, an essential component of cognitive function, can be improved through specific methods. This meta-analysis evaluates the effectiveness of transcranial alternating current stimulation (tACS), an emerging technique for enhancing working memory, and explores its efficacy, influencing factors, and underlying mechanisms. A PRISMA systematic search was conducted. Hedges's g was used to quantify effect sizes. We constructed a three-level meta-analytic model to account for all effect sizes and performed subgroup analyses to assess moderating factors. Recognizing the distinct neural underpinnings of various working memory processes, we separately assessed the effects on n-back tasks and traditional working memory tasks. A total of 39 studies with 405 effect sizes were included (170 from n-back tasks and 235 from other tasks). The overall analysis indicated a net benefit of g = 0.060 of tACS on working memory. Separate analyses showed that tACS had a small positive effect on n-back tasks (g = 0.102), but almost no effect on traditional working memory tasks (g = 0.045). Further analyses revealed mainly: A moderately positive effect of theta tACS (without anti-phase stimulation) on n-back tasks (g = 0.207); and a small effect of offline stimulation on working memory maintenance (g = 0.127). Overall, tACS has minimal impact on working memory improvement, but it shows potential under certain conditions. Specifically, both online and offline theta tACS can improve n-back task performance, while only offline stimulation enhances working memory maintenance. More research is needed to understand the mechanisms behind these effects to make tACS an effective method.

No aftereffect of transcranial alternating current stimulation (tACS) on theta activity during an inter-sensory selective attention task

BACKGROUND

Selective attention is essential to filter the constant flow of sensory information reaching the brain. The contribution of theta neuronal oscillations to attentional function has been the subject of several electrophysiological studies, yet no causal relationship has been established between theta rhythms and selective attention mechanisms.

OBJECTIVE AND HYPOTHESES

We aimed to clarify the causal role of theta oscillations in inter-sensory selective attention processes by combining transcranial alternating current stimulation (tACS) and electrophysiology (EEG) techniques. We hypothesized that modulation of theta activity by tACS enhances selective attention, with greater behavioral efficiency and theta power over fronto-central regions after theta-tACS compared to control conditions.

METHODS

In a double-blinded within-subject study conducted in young adults (n = 20), three stimulation conditions were applied prior to a cued inter-sensory (auditory and visual) selective attention task. The frequency of theta stimulation was individualized to match the endogenous theta peak of each participant. In addition to a sham condition, stimulation at an off-target frequency (20 Hz) was also applied. We analyzed behavioral efficiency and variability measures and performed spectral and time-frequency power analyses.

RESULTS

No statistically significant differences in task performance or theta EEG activity were found between theta-tACS and control-tACS conditions (ps > 0.05).

CONCLUSIONS

The results of our study suggest that theta-tACS did not modulate performance or offline oscillations in the context of inter-sensory attention. These findings challenge the design of tACS protocols for future studies aiming to understand the contribution of theta oscillations in attentional processes.

Modulation of Local Field Potentials in the Deep Brain of Minipigs Through Transcranial Temporal Interference Stimulation

OBJECTIVES

Transcranial temporal interference stimulation (tTIS) is a novel, noninvasive neuromodulation technique to modulate deep brain neural activity. Despite its potential, direct electrophysiological evidence of tTIS effects remains limited. This study investigates the impact of tTIS on local field potentials (LFPs) in the deep brain using minipigs implanted with deep brain electrodes.

MATERIALS AND METHODS

Three minipigs were implanted with electrodes in the subthalamic nucleus, and tTIS was applied using patch electrode pairs positioned on both sides of the scalp. Stimulation was delivered in sinewave voltage mode with intensities ≤2V. We evaluated the stimulus-response relationship, effects of different carrier frequencies, the range of entrained envelope oscillations, and changes resulting from adjusting the left-right stimulation intensity ratio.

RESULTS

The results indicated that tTIS modulates deep-brain LFPs in an intensity-dependent manner. Carrier frequencies of 1 or 2 kHz were most effective in influencing LFP. Envelope oscillations <200 Hz were effectively entrained into deep-brain LFPs. Adjustments to the stimulation intensity ratio between the left and right sides yielded inconsistent responses, with right-sided stimulation playing a dominant role.

CONCLUSION

These findings indicate that tTIS can regulate LFP changes in the deep brain, highlighting its potential as a promising tool for future noninvasive neuromodulation applications.

Neurostimulation to Improve Cognitive Flexibility

Cognitive flexibility, the capacity to adapt behaviors in response to changing environments, is impaired across mental illnesses, including depression, anxiety, addiction, and obsessive-compulsive disorder. Cortico-striatal-cortical circuits are integral to cognition and goal-directed behavior and disruptions in these circuits are linked to cognitive inflexibility in mental illnesses. We review evidence that neurostimulation of these circuits can improve cognitive flexibility and ameliorate symptoms, and that this may be a mechanism of action of current clinical therapies. Further, we discuss how animal models can offer insights into the mechanisms underlying cognitive flexibility and effects of neurostimulation. We review research from animal studies that may, if translated, yield better approaches to modulating flexibility. Future research should focus on refining definitions of cognitive flexibility, improving detection of impaired flexibility, and developing new methods for optimizing neurostimulation parameters. This could enhance neurostimulation therapies through more personalized treatments that leverage cognitive flexibility to improve patient outcomes.

Application and research progress of different frequency tACS in stroke rehabilitation: A systematic review

After a stroke, abnormal changes in neural oscillations that are related to the severity and prognosis of the disease can occur. Resetting these abnormal neural oscillations is a potential approach for stroke rehabilitation. Transcranial alternating current stimulation (tACS) can modulate intrinsic neural oscillations noninvasively and has attracted attention as a possible technique to improve multiple post-stroke symptoms, including deficits in speech, vision, and motor ability and overall neurological recovery. The clinical effect of tACS varies according to the selected frequency. Therefore, choosing an appropriate frequency to optimize outcomes for specific dysfunctions is essential. This review focuses on the current research status and possibilities of tACS with different frequencies in stroke rehabilitation. We also discuss the possible mechanisms of tACS in stroke to provide a theoretical foundation for the method and highlight the controversial aspects that need further exploration. Although tACS has great potential, few clinical studies have applied it in the treatment of stroke, and no consensus has been reached. We analyze limitations in experimental designs and identify potential tACS approaches worthy of exploration in the future.

Pathogenesis, diagnosis, and treatment of epilepsy: electromagnetic stimulation-mediated neuromodulation therapy and new technologies

Epilepsy is a severe, relapsing, and multifactorial neurological disorder. Studies regarding the accurate diagnosis, prognosis, and in-depth pathogenesis are crucial for the precise and effective treatment of epilepsy. The pathogenesis of epilepsy is complex and involves alterations in variables such as gene expression, protein expression, ion channel activity, energy metabolites, and gut microbiota composition. Satisfactory results are lacking for conventional treatments for epilepsy. Surgical resection of lesions, drug therapy, and non-drug interventions are mainly used in clinical practice to treat pain associated with epilepsy. Non-pharmacological treatments, such as a ketogenic diet, gene therapy for nerve regeneration, and neural regulation, are currently areas of research focus. This review provides a comprehensive overview of the pathogenesis, diagnostic methods, and treatments of epilepsy. It also elaborates on the theoretical basis, treatment modes, and effects of invasive nerve stimulation in neurotherapy, including percutaneous vagus nerve stimulation, deep brain electrical stimulation, repetitive nerve electrical stimulation, in addition to non-invasive transcranial magnetic stimulation and transcranial direct current stimulation. Numerous studies have shown that electromagnetic stimulation-mediated neuromodulation therapy can markedly improve neurological function and reduce the frequency of epileptic seizures. Additionally, many new technologies for the diagnosis and treatment of epilepsy are being explored. However, current research is mainly focused on analyzing patients' clinical manifestations and exploring relevant diagnostic and treatment methods to study the pathogenesis at a molecular level, which has led to a lack of consensus regarding the mechanisms related to the disease.

Repetitive Gamma-tACS Improves the Reaction Times of Healthy Young Adults in a Visuospatial Working Memory Task: A Randomized Study

Objective: The aims of the study were to test the short-term and long-term efficacy of repetitive γ-tACS over the left DLPFC to improve visuospatial working memory performance in the spatial capacity delayed response task (SCDRT). Methods: In a single blind placebo-controlled study, 35 healthy young adults were randomly assigned to three sessions of either active γ-tACS (n = 18) or passive sham γ-tACS (n = 17) The design allowed us to evaluate the influence of the stimulation protocol (active vs. sham), the stimulation session number (day 1 to 3), the session block (before stimulation, during stimulation and after stimulation) and the VSWM retention load (1, 3, 5 or 7 stimuli) on the response speed and accuracy. Results: Active γ-tACS selectively improved VSWM performance on day 2 and 3, and the effect was greater following stimulation rather than during stimulation. Significant effects were seen concerning response speed but not accuracy. The VSWM performance gains of the active γ-tACS were no longer present in the long-term at a follow-up session after two weeks. Conclusions: The present study provides novel evidence for a selective improvement in VSWM performance with three repeated sessions of γ-tACS in young adults through the entrainment of gamma rhythms in the left DLPFC.

Neuroplastic effects of transcranial alternating current stimulation (tACS): from mechanisms to clinical trials

Transcranial alternating current stimulation (tACS) is a promising non-invasive neuromodulation technique with the potential for inducing neuroplasticity and enhancing cognitive and clinical outcomes. A unique feature of tACS, compared to other stimulation modalities, is that it modulates brain activity by entraining neural activity and oscillations to an externally applied alternating current. While many studies have focused on online effects during stimulation, growing evidence suggests that tACS can induce sustained after-effects, which emphasizes the potential to induce long-term neurophysiological changes, essential for therapeutic applications. In the first part of this review, we discuss how tACS after-effects could be mediated by four non-mutually exclusive mechanisms. First, spike-timing-dependent plasticity (STDP), where the timing of pre- and postsynaptic spikes strengthens or weakens synaptic connections. Second, spike-phase coupling and oscillation phase as mediators of plasticity. Third, homeostatic plasticity, emphasizing the importance of neural activity to operate within dynamic physiological ranges. Fourth, state-dependent plasticity, which highlights the importance of the current brain state in modulatory effects of tACS. In the second part of this review, we discuss tACS applications in clinical trials targeting neurological and psychiatric disorders, including major depressive disorder, schizophrenia, Parkinson's disease, and Alzheimer's disease. Evidence suggests that repeated tACS sessions, optimized for individual oscillatory frequencies and combined with behavioral interventions, may result in lasting effects and enhance therapeutic outcomes. However, critical challenges remain, including the need for personalized dosing, improved current modeling, and systematic investigation of long-term effects. In conclusion, this review highlights the mechanisms and translational potential of tACS, emphasizing the importance of bridging basic neuroscience and clinical research to optimize its use as a therapeutic tool.

Modulating Cortico-cortical Networks with Transcranial Alternating Current Stimulation: A Minireview

Advancements in brain imaging and analytical methods have demonstrated that behavior arises from the coordinated activity of multiple brain regions within cortico-cortical networks. Transcranial alternating current stimulation (tACS), a noninvasive brain stimulation (NIBS) technique, applies weak sinusoidal alternating currents to specific brain regions using scalp-mounted electrodes. Traditionally, tACS has been used to target single brain regions to enhance functions such as motor, sensory, and cognitive abilities. However, recent findings indicate its potential for simultaneously stimulating 2 brain regions, thereby modulating cortico-cortical network strength through neural entrainment-where brain oscillations synchronize with external rhythmic stimuli. Despite this potential, tACS applications remain primarily focused on individual brain regions. Given that behavior stems from dynamic interactions within cortico-cortical networks rather than isolated regions, this minireview explores the role of these networks in shaping behavior through functional connectivity as identified by neuroimaging. It also provides an in-depth analysis of tACS as a tool for modifying cortico-cortical networks via neural entrainment, offering promising applications in neurorehabilitation for brain disorders linked to network dysfunction. This highlights tACS as a novel approach for targeted modulation of cortico-cortical networks, distinguishing it from traditional NIBS techniques.

Acupuncture modulates group neural activity in patients with post stroke sensory impairment: An fMRI study based on inter-subject correlation and inter-subject functional connectivity

Sensory impairment after stroke has become an important health problem that affects the health and quality of life of patients. Acupuncture is a widely accepted method for stroke rehabilitation. The development of fMRI provides a good platform for the study of neural activity patterns induced by acupuncture, and many studies have found that acupuncture can induce special activation of the brain in stroke patients. We introduced the inter-subject functional connectivity(ISFC) method into the study of acupuncture treatment for sensory impairment after stroke to explore the group effects of acupuncture treatment and the specific mode of action of acupuncture for sensory impairment. In this study, 24 stroke patients with limb numbness and 23 healthy controls were included, and three functional magnetic resonance scans were designed, including resting state, acupuncture task state, and acupuncture-retention state(LI11 and ST36 were used during the task fMRI). The main observation was the connection changes in 50 regions of interest, including the sensory-motor network, central executive network, thalamus, cingulate gyrus, and other brain regions. The findings showed that acupuncture could cause certain patterns of neural activity in the patients. These patterns included a significant rise in ISFC within the sensory-motor network and between the sensory-motor network and the thalamus and the central executive network. When different types of acupuncture were compared, it was found that the first effect of acupuncture was mostly large-scale activation of the sensory-motor network and the thalamus. The second effect, on the other hand, was low-intensity activation in a limited range. In general, this study explored the group mechanism of acupuncture for sensory function rehabilitation after stroke and provided some help for understanding neural activity patterns from a cross-subject dimension.

μ-Transcranial Alternating Current Stimulation Induces Phasic Entrainment and Plastic Facilitation of Corticospinal Excitability

Transcranial alternating current stimulation (tACS) has been proposed to modulate neural activity through two primary mechanisms: entrainment and neuroplasticity. The current study aimed to probe both of these mechanisms in the context of the sensorimotor μ-rhythm using transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to assess entrainment of corticospinal excitability (CSE) during stimulation (i.e., online) and immediately following stimulation, as well as neuroplastic aftereffects on CSE and μ EEG power. Thirteen participants received three sessions of stimulation. Each session consisted of 90 trials of μ-tACS tailored to each participant's individual μ frequency (IMF), with each trial consisting of 16 s of tACS followed by 8 s of rest (for a total of 24 min of tACS and 12 min of rest per session). Motor-evoked potentials (MEPs) were acquired at the start and end of the session (n = 41), and additional MEPs were acquired across the different phases of tACS at three epochs within each tACS trial (n = 90 for each epoch): early online, late online and offline echo. Resting EEG activity was recorded at the start, end and throughout the tACS session. The data were then pooled across the three sessions for each participant to maximise the MEP sample size per participant. We present preliminary evidence of CSE entrainment persisting immediately beyond tACS and have also replicated the plastic CSE facilitation observed in previous μ-tACS studies, thus supporting both entrainment and neuroplasticity as mechanisms by which tACS can modulate neural activity.

Subthreshold electric fields bidirectionally modulate neurotransmitter release through axon polarization

Subthreshold electric fields modulate brain activity and demonstrate potential in several therapeutic applications. Electric fields are known to generate heterogenous membrane polarization within neurons due to their complex morphologies. While the effects of somatic and dendritic polarization in postsynaptic neurons have been characterized, the functional consequences of axonal polarization on neurotransmitter release from the presynapse are unknown. Here, we combined noninvasive optogenetic indicators of voltage, calcium and neurotransmitter release to study the subcellular response within single neurons to subthreshold electric fields. We first captured the detailed spatiotemporal polarization profile produced by uniform electric fields within individual neurons. Small polarization of presynaptic boutons produces rapid and powerful modulation of neurotransmitter release, with the direction - facilitation or inhibition - depending on the direction of polarization. We determined that subthreshold electric fields drive this effect by rapidly altering the number of synaptic vesicles participating in neurotransmission, producing effects which resemble short-term plasticity akin to presynaptic homeostatic plasticity. These results provide key insights into the mechanisms of subthreshold electric fields at the cellular level.

Abstract Figure

Epileptogenic zone characteristics determine effectiveness of electrical transcranial stimulation in epilepsy treatment

Transcranial direct current stimulation shows promise as a non-invasive therapeutic method for patients with focal drug-resistant epilepsy. However, there is considerable variability in individual responses to transcranial direct current stimulation, and the factors influencing treatment effectiveness in targeted regions are not well understood. We aimed to assess how the extent and depth of the epileptogenic zone and associated networks impact patient responses to transcranial direct current stimulation therapy. We conducted a retrospective analysis of stereoelectroencephalography data from 23 patients participating in a personalized multichannel transcranial direct current stimulation protocol. We evaluated the extent and depth of the epileptogenic zone network, propagation zone network, and the combined network of the entire epileptogenic and propagation zones, correlating these factors with clinical response measured by the reduction in seizure frequency following repeated transcranial direct current stimulation sessions. Among the patients, 10 (43.5%) were classified as responders (R), experiencing a significant (>50%) decrease in seizure frequency, while 13 were non-responders, showing minimal improvement or increased seizure frequency. Importantly, we found a significant positive correlation between the extent of the epileptogenic zone network and changes in seizure frequency. A smaller epileptogenic zone network extent was associated with better transcranial direct current stimulation efficacy, with responders demonstrating a significantly smaller epileptogenic and propagation zones compared with non-responders. Additionally, non-responders tended to have a significantly deeper epileptogenic zone network compared with responders. Our results highlight the significant impact of the extent and depth of the epileptogenic zone network on transcranial direct current stimulation efficacy in patients with refractory focal epilepsy. Responders typically exhibited a smaller and shallower epileptogenic zone network compared with non-responders. These findings suggest that utilizing individualized epileptogenic zone network characteristics could help refine patient selection for personalized transcranial direct current stimulation protocols, potentially improving therapeutic outcomes.

External trigeminal nerve stimulation (eTNS) Exhibits relaxation effects in fatigue states following napping deprivation

BACKGROUND

In the face of inevitable declines in alertness and fatigue resulting from sleep deprivation, effective countermeasures are essential for maintaining performance. External trigeminal nerve stimulation (eTNS) presents a potential avenue for regulating alertness by activating the locus coeruleus and reticular activating system.

METHODS

Here, we conducted a within-subject study with 66 habitual nappers, subjecting them to afternoon nap-deprivation and applying either 20-minute of 120 Hz eTNS or sham stimulation. We compared participants' performance in PVT and N-back tasks, subjective fatigue level and alertness ratings, and changes in heart rate variability, cortisol, and salivary alpha-amylase before and after stimulation.

RESULTS

The results revealed a significant decline in PVT and N-back tasks performance, along with increased subjective fatigue levels in the sham stimulation group. In contrast, the eTNS stimulation group maintained behavioral performance, with lower post-stimulation fatigue levels than sham group. After stimulation, the eTNS group exhibited decreased mean R-R interval and elevated LF/HF ratios, i.e., a shift in autonomic nervous system activity towards sympathetic dominance, and a significant reduction in cortisol levels, indicating a state of relaxation alleviating drowsiness.

CONCLUSION

These findings suggested that 120 Hz eTNS stimulation might induce a relaxing effect, and thereby alleviate fatigue while preserving alertness and cognitive performance.

Cerebellar Neuromodulation in Autism Spectrum Disorders and Social Cognition: Insights from Animal and Human Studies

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental condition characterized by social atypicalities and repetitive behaviors. Growing evidence suggests that alterations in brain networks may contribute to ASD symptoms. The cerebellum, with its widespread connections to the cortex, has emerged as a potential key player in ASD. Non-invasive neuromodulation techniques, such as transcranial direct current stimulation (tDCS) or repetitive transcranial magnetic stimulation (rTMS) offer a promising avenue for modulating brain activity and potentially alleviating ASD symptoms. In addition, preclinical studies in rodents further emphasize the therapeutic effect of cerebellar stimulation to target autism-related symptoms. This article reviews both clinical and preclinical studies aiming to modulate cerebellar circuits to improve symptoms of ASD. We found ten relevant studies assessing the effect of cerebellar neuromodulation in human and preclinical models. Posterior cerebellar tDCS represented the most frequent neuromodulation method and suggested that cerebellar tDCS can lead to improvements in symptoms of ASD and restore cerebellar connectivity in individuals with ASD. In neurotypical participants, there is evidence that cerebellar tDCS can enhance social cognitive abilities. These results are in line with preclinical studies, suggesting that chemogenetic stimulation can modulate cerebellar circuits involved in ASD and improve related behaviors. Further research is needed to establish standardized protocols, assess long-term effects, and investigate the underlying mechanisms of cerebellar stimulation. We examine research questions that need to be addressed before launching large scale randomized clinical trials.

What can neurofeedback and transcranial alternating current stimulation reveal about cross-frequency coupling?

In recent years, the dynamics and function of cross-frequency coupling (CFC) in electroencephalography (EEG) have emerged as a prevalent area of investigation within the research community. One possible approach in studying CFC is to utilize non-invasive neuromodulation methods such as transcranial alternating current stimulation (tACS) and neurofeedback (NFB). In this study, we address (1) the potential applicability of single and multifrequency tACS and NFB protocols in CFC research; (2) the prevalence of CFC types, such as phase-amplitude or amplitude-amplitude CFC, in tACS and NFB studies; and (3) factors that contribute to inter- and intraindividual variability in CFC and ways to address them potentially. Here we analyzed research studies on CFC, tACS, and neurofeedback. Based on current knowledge, CFC types have been reported in tACS and NFB studies. We hypothesize that direct and indirect effects of tACS and neurofeedback can induce CFC. Several variability factors such as health status, age, fatigue, personality traits, and eyes-closed (EC) vs. eyes-open (EO)condition may influence the CFC types. Modifying the duration of the tACS and neurofeedback intervention and selecting a specific demographic experimental group could reduce these sources of CFC variability. Neurofeedback and tACS appear to be promising tools for studying CFC.

Noninvasive Brain Stimulation as Focal Epilepsy Treatment in the Hospital, Clinic, and Home

Introduction

Noninvasive brain stimulation (NIBS) provides a treatment option for patients not eligible for surgical intervention or who seek low-risk approaches and may be used in the hospital, clinic and at home. Our objective is to summarize our single-center experience with multiple NIBS approaches for the treatment of focal epilepsy.

Methods

A retrospective chart review identified drug resistant focal epilepsy patients who received NIBS as an epilepsy treatment at Mayo Clinic in Rochester, MN. Patients were typically treated as follows: (1) for TMS, 1 Hz stimulation was applied for five consecutive days in the neuromodulation clinic, (2) for outpatient tDCS, stimulation was applied for five consecutive days in the clinic, followed by optional treatment at home (3) for inpatient tDCS, stimulation was applied for three consecutive days. We analyzed continuous EEG data for the inpatient tDCS cohort and available HD-EEG data for outpatient cohorts to quantify changes in interictal epileptiform discharges (IEDs) as a result of stimulation. Outcomes were assessed at 1-month for TMS and outpatient tDCS and 1-week for inpatient tDCS.

Results

24 patients were treated with TMS (n=10) and tDCS (n=14, 9 as outpatients). The median age was 40 years (range 15-73). The median seizure reduction following stimulation was 50%. 14 patients (58 %) were responders to treatment (TMS=4/10, tDCS Outpatient =7/9, tDCS Inpatient=3/5). Five outpatient tDCS participants elected to continue treatment at home. 4 TMS and 4 outpatient tDCS underwent high density EEG before and after 5 days of therapy. Following stimulation, IED rate was reduced in 4/5 inpatient tDCS patients, 4/4 outpatient tDCS patients, and 4/4 TMS patients. Two patients experienced an increase in seizure frequency (1 following TMS and 1 following outpatient tDCS), which returned to baseline 4-6 weeks after stimulation treatments were discontinued.

Conclusions

TMS and tDCS are potential treatment approaches for drug resistant focal epilepsy patients in the hospital, clinic, and home. They have a favorable safety profile and can lead to a reduction in IEDs rates and seizures. These results suggest further studies are needed to examine NIBS as treatment for epilepsy.

Somatodendritic orientation determines tDCS-induced neuromodulation of Purkinje cell activity in awake mice

Transcranial direct-current stimulation (tDCS) of the cerebellum is a promising non-invasive neuromodulatory technique being proposed for the treatment of neurological and neuropsychiatric disorders. However, there is a lack of knowledge about how externally applied currents affect neuronal spiking activity in cerebellar circuits in vivo. We investigated how Cb-tDCS affects the firing rate of Purkinje cells (PC) and non-PC in the mouse cerebellar cortex to understand the underlying mechanisms behind the polarity-dependent modulation of neuronal activity induced by tDCS. Mice (n = 9) were prepared for the chronic recording of LFPs to assess the actual electric field gradient imposed by Cb-tDCS in our experimental design. Single-neuron extracellular recording of PCs in awake (n = 24) and anesthetized (n = 27) mice was combined with juxtacellular recordings and subsequent staining of PC with neurobiotin under anesthesia (n = 8) to correlate their neuronal orientation with their response to Cb-tDCS. Finally, a high-density Neuropixels recording system was used to demonstrate the relevance of neuronal orientation during the application of Cb-tDCS in awake mice (n = 6). In this study, we observe that Cb-tDCS induces a heterogeneous polarity-dependent modulation of the firing rate of Purkinje cells (PC) and non-PC in the mouse cerebellar cortex. We demonstrate that the apparently heterogeneous effects of tDCS on PC activity can be explained by taking into account the somatodendritic orientation relative to the electric field. Our findings highlight the need to consider neuronal orientation and morphology to improve tDCS computational models, enhance stimulation protocol reliability, and optimize effects in both basic and clinical applications.

Simultaneous tACS-fMRI reveals state- and frequency-specific modulation of hippocampal-cortical functional connectivity

Non-invasive indirect hippocampal-targeted stimulation is of broad scientific and clinical interest. Transcranial alternating current stimulation (tACS) is appealing because it allows oscillatory stimulation to study hippocampal theta (3-8 Hz) activity. We found that tACS administered during functional magnetic resonance imaging yielded a frequency-, mental state- and topologically-specific effect of theta stimulation (but not other frequencies) enhancing right (but not left) hippocampal-cortical connectivity during resting blocks but not during task blocks. Control analyses showed that this effect was not due to possible stimulation-induced changes in signal quality or head movement. Our findings are promising for targeted network modulations of deep brain structures for research and clinical intervention.

The acute effect of bilateral cathodic transcranial direct current stimulation on respiratory muscle strength and endurance

INTRODUCTION

Transcranial direct current stimulation (tDCS) is a non-invasive technique with therapeutic potential, especially in respiratory muscle training (RMT) in pathological conditions such as chronic obstructive pulmonary disease and heart failure.

OBJECTIVE

To evaluate the effect of bilateral cathodic tDCS on respiratory muscle strength and endurance in healthy young and elderly women.

METHODS

An experimental, randomized study with 80 participants divided into young and old women, subdivided into intervention and sham control groups. The participants were evaluated by spirometry and dynamic muscle strength tests before and after the one session intervention. tDCS was applied with cathode electrodes positioned bilaterally in the motor area.

RESULTS

The elderly women in the intervention group showed significant improvement in dynamic inspiratory muscle strength (S-Index) and dominant hand strength, with moderate to large effect sizes. The young women showed a significant increase only in the strength of the dominant hand, with no improvement in inspiratory muscle strength. There were no significant differences in ventilatory parameters, including Maximal Ventilatory Capacity, in any of the age groups.

CONCLUSION

Bilateral cathodic tDCS was effective in increasing dynamic inspiratory muscle strength and dominant hand strength in elderly women, with more pronounced effects compared to young women. The technique did not produce significant changes in maximal ventilatory capacity in any of the age groups, suggesting that the response to tDCS may vary with age, being more beneficial in elderly women.

Recommendations for the Safe Application of Temporal Interference Stimulation in the Human Brain Part I: Principles of Electrical Neuromodulation and Adverse Effects

Temporal interference stimulation (TIS) is a new form of transcranial electrical stimulation (tES) that has been proposed as a method for targeted, non-invasive stimulation of deep brain structures. While TIS holds promise for a variety of clinical and non-clinical applications, little data is yet available regarding its effects in humans and its mechanisms of action. In order to inform the design and safe conduct of experiments involving TIS, researchers require quantitative guidance regarding safe exposure limits and other safety considerations. To this end, we undertook a two-part effort to determine frequency-dependent thresholds for applied currents below which TIS is unlikely to pose risk to humans in terms of heating or unwanted stimulation. Part I of this effort, described here, comprises a summary of the current knowledge pertaining to the safety of TIS and related techniques. Specifically, we provide: i) a broad overview of the electrophysiological impacts neurostimulation, ii) a review of the (bio-)physical principles underlying the mechanisms of action of transcranial alternating/direct stimulation (tACS/tDCS), deep brain stimulation (DBS), and TIS, and iii) a comprehensive survey of the adverse effects (AEs) associated with each technique as reported in the scientific literature and regulatory and clinical databases. In Part II, we perform an in silico study to determine field exposure metrics for tDCS/tACS and DBS under normal (safe) operating conditions and infer frequency-dependent current thresholds for TIS that result in equivalent levels of exposure.

Comparative efficacy of multiple non-invasive brain stimulation to treat major depressive disorder in older patients: A systematic review and network meta-analysis study based on randomized controlled trials

BACKGROUND

Major depressive disorder (MDD) is prevalent among older patients and is frequently associated with cognitive decline and a reduced quality of life. Non-invasive brain stimulation (NIBS) techniques show promise for treating MDD, but their comparative efficacy and safety older populations remain unclear. This study aimed to compare the efficacy and cognitive effects of various NIBS techniques in treating MDD in older patients.

METHODS

We searched the PubMed, EMBASE, Cochrane Library, and Web of Science core databases from inception to March 2024. Seventeen randomized controlled trials (RCTs) were included.

RESULTS

Surfaces under the cumulative ranking curve (SUCRA) values were used to rank the interventions. The SUCRA rankings for the Hamilton Depression Rating Scale (HDRS) outcomes indicated that repetitive transcranial magnetic stimulation (rTMS) (89.0 %) had the highest efficacy, followed by transcranial direct current stimulation (tDCS) (68.7 %). rTMS demonstrated significantly superior efficacy compared with bilateral electroconvulsive therapy (BL ECT) and right unilateral electroconvulsive therapy (RUL ECT). Theta burst stimulation (TBS) had the highest response rate (69.6 %), followed by rTMS (61.8 %). Based on the Mini-Mental State Examination, rTMS (86.4 %) ranked the highest, with RUL ECT showing significantly better outcomes than BL ECT.

CONCLUSION

NIBS, particularly rTMS and TBS, may offer effective treatment options for older patients with MDD. Further research with larger sample sizes and longer follow-up periods is required to validate these findings and inform clinical practice.

Double-blind, randomized, placebo-controlled pilot clinical trial with gamma-band transcranial alternating current stimulation for the treatment of schizophrenia refractory auditory hallucinations

Gamma oscillations are essential for brain communication. The 40 Hz neural oscillation deficits in schizophrenia impair left frontotemporal connectivity and information communication, causing auditory hallucinations. Transcranial alternating current stimulation is thought to enhance connectivity between different brain regions by modulating brain oscillations. In this work, we applied a frontal-temporal-parietal 40 Hz-tACS stimulation strategy for treating auditory hallucinations and further explored the effect of tACS on functional connectivity of brain networks. 32 schizophrenia patients with refractory auditory hallucinations received 20daily 20-min, 40 Hz, 1 mA sessions of active or sham tACS on weekdays for 4 consecutive weeks, followed by a 2-week follow-up period without stimulation. Auditory hallucination symptom scores and 64-channel electroencephalograms were measured at baseline, week2, week4 and follow-up. For clinical symptom score, we observed a significant interaction between group and time for auditory hallucinations symptoms (F(3,90) = 26.964, p < 0.001), and subsequent analysis showed that the 40Hz-tACS group had a higher symptom reduction rate than the sham group at week4 (p = 0.036) and follow-up (p = 0.047). Multiple comparisons of corrected EEG results showed that the 40Hz-tACS group had higher functional connectivity in the right frontal to parietal (F (1,30) = 7.24, p = 0.012) and right frontal to occipital (F (1,30) = 7.98, p = 0.008) than the sham group at week4. Further, functional brain network controllability outcomes showed that the 40Hz-tACS group had increased average controllability (F (1,30) = 6.26, p = 0.018) and decreased modality controllability (F (1,30) = 6.50, p = 0.016) in the right frontal lobe compared to the sham group. Our polit study indicates that 40Hz-tACS combined with medicine may be an effective treatment for targeting symptoms specific to auditory hallucinations and altering functional connectivity and controllability at the network level.

Neuromodulatory effect of transcranial direct current stimulation on cue reactivity and craving in young adults with internet gaming disorder: an event-related potential study

Objective

This study assessed the effects of transcranial direct current stimulation (tDCS) on cue reactivity and craving for game-related cues using event-related potentials (ERPs) in internet gaming disorder (IGD) patients.

Methods

At baseline, a series of game-related and neutral pictures were shown to both IGD and healthy controls (HCs) while ERPs were recorded. Late positive potentials (LPP) were used to investigate cue reactivity. During intervention, IGD patients received 10 sessions (two sessions/day for 5 consecutive days, 2 mA for 20 min/session) of tDCS to the left (anode stimulation) and right (cathode) dorsolateral prefrontal cortex. Subjectively assessed craving and LPP component was analyzed before stimulation and at the 1-month follow-up after tDCS in IGD.

Results

At baseline, patients with IGD showed higher LPP amplitudes for game-related cues in the centro-parietal and parietal regions than HCs. After 10 sessions of tDCS, increased LPP amplitudes decreased significantly at 1-month follow-up., as well as subjective craving for gaming.

Conclusion

These findings suggest that neurophysiological arousal in response to game-related cues in the IGD group could be modulated by the effects of tDCS. LPP was a significant neurophysiological marker of the neuroplastic response of cue reactivity underlying the therapeutic effect of tDCS on IGD. Based on the present findings, tDCS could be expanded to the treatment of other addictive disorders, including substance use disorder and behavioral addictions.

Modulating delirium through stimulation (MoDeSt): study protocol for a randomized, double-blind, sham-controlled trial assessing the effect of postoperative transcranial electrical stimulation on delirium incidence

BACKGROUND

Postoperative delirium (POD) is the most common neurological adverse event among elderly patients undergoing surgery. POD is associated with an increased risk for postoperative complications, long-term cognitive decline, an increase in morbidity and mortality as well as extended hospital stays. Delirium prevention and treatment options are currently limited. This study will evaluate the effect of transcranial electrical stimulation (tES) on the incidence of POD.

METHODS

We will perform a randomized, double-blind, sham-controlled trial using single-session postoperative application of tES in the recovery room in 225 patients (> 65 years) undergoing elective major surgery. Patients will be randomly allocated (ratio 1:1:1) to one of three study groups: (1) alpha-tACS over posterior parietal cortex [2 mA, 20 min], (2) anodal tDCS over left dorsolateral prefrontal cortex [2 mA, 20 min], (3) sham [2 mA, 30 s]. Delirium will be screened twice daily with the 3-min diagnostic interview Confusion Assessment Method (3D-CAM) in the 5 days following surgery. The primary outcome is the incidence of POD defined as at least one positive screening during the five first postoperative days compared between tACS and sham groups. Secondary outcomes include delirium severity, duration, phenotype, postoperative pain, postoperative nausea and vomiting, electroencephalographic (EEG) markers, and fluid biomarkers.

DISCUSSION

If effective, tES is a novel, easily applicable, non-invasive method to prevent the occurrence of POD. The comprehensive neurophysiological and biofluid assessments for markers of (neuro-)inflammation and neurodegeneration will shed light on the pathomechanisms behind POD and further elucidate the (after-)effects of tES. The potential implications for the postoperative recovery comprise enhanced patient safety, neurocognitive outcome, perioperative manageability but also reduced healthcare costs.

TRIAL REGISTRATION

German Clinical Trial Registry DRKS00033703. Registered on February 23, 2024.

Investigating the Working Mechanism of Transcranial Direct Current Stimulation

BACKGROUND

Transcranial direct current stimulation (tDCS) is used to modulate neuronal activity, but the exact mechanism of action (MOA) is unclear. This study investigates tDCS-induced modulation of the corticospinal excitability and the underlying MOA. By anesthetizing the scalp before applying tDCS and by stimulating the cheeks, we investigated whether stimulation of peripheral and/or cranial nerves contributes to the effects of tDCS on corticospinal excitability.

MATERIALS AND METHODS

In a randomized cross-over study, four experimental conditions with anodal direct current stimulation were compared in 19 healthy volunteers: 1) tDCS over the motor cortex (tDCS-MI), 2) tDCS over the motor cortex with a locally applied topical anesthetic (TA) on the scalp (tDCS-MI + TA), 3) DCS over the cheek region (DCS-C), and 4) sham tDCS over the motor cortex(sham). tDCS was applied for 20 minutes at 1 mA. Motor evoked potentials (MEPs) were measured before tDCS and immediately, 15, 30, 45, and 60 minutes after tDCS. A questionnaire was used to assess the tolerability of tDCS.

RESULTS

A significant MEP amplitude increase compared with baseline was found 30 minutes after tDCS-MI, an effect still observed 60 minutes later; no time∗condition interaction effect was detected. In the other three conditions (tDCS-MI + TA, DCS-C, sham), no significant MEP modulation was found. The questionnaire indicated that side effects are significantly lower when the local anesthetic was applied before stimulation than in the other three conditions.

CONCLUSIONS

The significant MEP amplitude increase observed from 30 minutes on after tDCS-MI supports the modulatory effect of tDCS on corticospinal neurotransmission. This effect lasted one hour after stimulation. The absence of a significant modulation when a local anesthetic was applied suggests that effects of tDCS are not solely established through direct cortical stimulation but that stimulation of peripheral and/or cranial nerves also might contribute to tDCS-induced modulation.

In silico modeling of electric field modulation by transcranial direct current stimulation in stroke patients with skull burr holes: Implications for safe clinical application

BACKGROUND

Transcranial direct current stimulation (tDCS) has emerged as a promising tool for stroke rehabilitation, supported by evidence demonstrating its beneficial effects on post-stroke recovery. However, patients with skull defects, such as burr holes, have been excluded from tDCS due to limited knowledge regarding the effect of skull defects on the electric field.

OBJECTIVE

We investigated the effect of burr holes on the electric field induced by tDCS and identified the electrode location that modulates the electric field.

METHODS

We generated mesh models of the heads of five patients with burr holes and five age-matched control patients who had never undergone brain surgery, based on magnetic resonance imaging. Then we conducted tDCS simulations, with the cathode fixed in one position and the anode in various positions. Regression analysis was employed to investigate the relationship between the electric field at the burr hole and the distance from the burr hole to the anode.

RESULTS

In patients with burr holes, the electric field intensity increased as the anode approached the burr hole, reaching a maximum electric field when the anode covered it, with this pattern remaining consistent across all patient models. Assuming the holes were filled with cerebrospinal fluid, the maximum electric field was 1.20 ± 0.20 V/m (mean ± standard deviation, SD). When the anode was positioned more than 60 mm away from the burr hole, the electric field at the burr hole remained low and constant, with an average value of 0.29 ± 0.04V/m (mean ± SD). In contrast, for all patients without burr holes, the electric field intensity stayed constant regardless of the anode's position, with a maximum amplitude of 0.36 ± 0.04 V/m (mean ± SD). Furthermore, when the burr hole was assumed to be filled with scar tissue, the mean peak electric field was 0.93 ± 0.16 V/m, indicating that the electric field strength varies depending on the conductivity of the tissue filling the burr hole.

CONCLUSION

Based on the simulations, the minimum recommended distance from the burr hole to the anode is 60 mm to prevent unintended stimulation of the brain cortex during tDCS. These findings will contribute to the development of safe and effective tDCS treatments for patients with burr holes.

Enabling electric field model of microscopically realistic brain

BACKGROUND

Modeling brain stimulation at the microscopic scale may reveal new paradigms for various stimulation modalities.

OBJECTIVE

We present the largest map to date of extracellular electric field distributions within a layer L2/L3 mouse primary visual cortex brain sample. This was enabled by the automated analysis of serial section electron microscopy images with improved handling of image defects, covering a volume of 250 × 140 × 90 μm³.

METHODS

The map was obtained by applying a uniform brain stimulation electric field at three different polarizations and accurately computing microscopic field perturbations using the boundary element fast multipole method. We used the map to identify the effect of microscopic field perturbations on the activation thresholds of individual neurons. Previous relevant studies modeled a macroscopically homogeneous cortical volume.

RESULT

Our result shows that the microscopic field perturbations - an 'electric field spatial noise' with a mean value of zero - only modestly influence the macroscopically predicted stimulation field strengths necessary for neuronal activation. The thresholds do not change by more than 10 % on average.

CONCLUSION

Under the stated limitations and assumptions of our method, this result essentially justifies the conventional theory of "invisible" neurons embedded in a macroscopic brain model for transcranial magnetic and transcranial electrical stimulation. However, our result is solely sample-specific and is only relevant to this relatively small sample with 396 neurons. It largely neglects the effect of the microcapillary network. Furthermore, we only considered the uniform impressed field and a single-pulse stimulation time course.