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

Theta-gamma tACS modulates attention network synchronization, not isolated network performance

As the brain ages, oscillatory changes disrupt neuronal communication, contributing to cognitive decline in key areas such as parts of the attention network system. This study explores the effects of multi-session low-intensity transcranial Alternating Current Stimulation (tACS) on the efficiency of the alerting, orienting, and executive control networks in older adults. Using a 16-session theta-gamma tACS protocol targeting the prefrontal cortex, we examined its impact on Attention Network Task (ANT) performance of 76 participants aged 55 to 84 in a randomized, double-blind, sham-controlled design. To account for the influence of brain state, both active and sham tACS groups underwent cognitive n-back training during stimulation. Despite no significant modulations in attention network efficiencies, generalized linear mixed-effect modeling revealed that active tACS negatively influenced overall reaction time (RT) improvements, resulting in poorer ANT performance compared to the sham group. Additionally, active tACS disrupted network correlations post-intervention, particularly affecting the alerting network's interactions with the orienting and executive networks. These findings provide further evidence for the involvement of theta-gamma coupling in attention processes, though without network-specific effects. The results underscore the potential of frequency-specific neurostimulation to modulate cognitive functions but also emphasize the need for caution, as such interventions may inadvertently impair brain network dynamics.

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.

Transcranial Alternating Current Stimulation and Cognitive Training Enhanced Performance and Theta Activity in Adults With Cognitive Impairment

BACKGROUND

Age-related cognitive decline is rising due to longer life expectancy, necessitating new treatments as current drugs are ineffective and costly. Transcranial alternating current stimulation at the theta frequency (theta-tACS) has shown promise in enhancing cognitive function in both young and elderly adults, but its effectiveness in those with cognitive decline is not well-studied.

METHOD

This study involved 27 participants with subjective cognitive decline (SCD), mild cognitive impairment (MCI), and dementia, who underwent multiple sessions combining computerized cognitive training with theta-tACS to assess its efficacy. Participants were randomly assigned to either a real-tACS or sham-tACS group. Before and after treatment, they completed several cognitive tasks, and their behavioral and EEG data were collected.

RESULTS

Only the real-tACS group improved in the oddball task and exhibited increased event-related EEG amplitude in the theta range.

CONCLUSIONS

These findings suggest that theta-tACS can improve cognitive performance in individuals with cognitive decline at both behavioral and psychophysiological levels, supporting its potential for alleviating cognitive decline in elderly populations.

Anodal transcranial direct current stimulation combined with physical exercise increases postural sway in Parkinson's disease: a double-blind and cross-over study

Transcranial direct current stimulation (tDCS) has shown promising effects on postural control in people with Parkinson's disease (PwPD). However, the characteristics of the stimulation, such as the specific cortical area targeted and combination with exercise, seem to influence the tDCS effects. Therefore, analyzing these factors is essential for identifying key characteristics and optimizing rehabilitation protocols for postural control in PD.We aimed to analyze the efficacy of tDCS over the primary motor (M1) and pre-frontal cortices (PFC) combined with aerobic exercise on postural control in PwPD. Twenty-one PwPD participated in this crossover, randomized, and double-blind study. The intervention consisted of exercising on a treadmill at moderate intensity for 30 min while receiving the stimulation. tDCS was applied during the central 20 min of exercise over M1, PFC, or sham on 3 different days. Three one-minute trials were conducted with participants standing still on a force platform to assess the center of pressure parameters in anteroposterior (AP) and mediolateral (ML) directions in pre- and post-intervention. Time*stimulation interaction was observed for sway area (p = 0.038) and sway mean amplitude in both the AP (p = 0.009) and ML directions (p = 0.059, marginal effect). Post-hoc analysis indicated a larger sway area and mean amplitude in both directions post-intervention compared to pre-intervention after tDCS application to the M1 and PFC. No significant differences were observed for the sham condition. Our findings suggest that the combination of exercise and tDCS, regardless of the area stimulated, modifies postural control in PwPD, leading to a larger sway.

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.

Transcranial alternating current stimulation in subjects with insomnia symptoms: A randomized, double-blind and controlled study

BACKGROUND

This study evaluated whether transcranial alternating current stimulation (tACS), a non-invasive brain stimulation technique, could alleviate insomnia symptoms.

METHODS

Participants exhibiting insomnia symptoms without meeting the criteria for insomnia disorder were recruited and randomized into 0.5 Hz, 100 Hz, or a sham group. To maximize the delivery of intracranial stimulation, a carrier frequency of 10 kHz was utilized. Participants were required to use the device for 30 min, twice daily for six weeks.

RESULTS

Eighty-seven participants (74 females, mean age = 54.15 ± 0.73 years) were randomized and completed the trial. The Insomnia Severity Index scores showed significant improvement across all three groups without a significant difference between groups (sham: 13.83 to 8.45, p < 0.05; 0.5 Hz: 12.03 to 8.79, p < 0.05; 100 Hz: 12.38 to 7.83, p < 0.05). In the average sleep diary over four days, sleep latency (SL) and wake after sleep onset (WASO) decreased in all three groups (sham, 0.5 Hz, 100 Hz) without significant group by visit interaction (SL: -5.74 min, -8.94 min, -16.53 min, respectively, p = 0.345; WASO: -10.74 min, -23.62 min, -16.73 min, respectively, p = 0.431). No significant improvements were observed in actigraphy-based sleep measures.

CONCLUSIONS

tACS did not demonstrate greater efficacy than sham treatment in ameliorating symptoms of insomnia. Future studies should account for the potent placebo effect on sleep and the potential for high carrier frequencies to obscure the target frequencies.

Evaluating the efficacy of transcranial direct current stimulation (tDCS) in managing neuropathic pain-induced emotional consequences: Insights from animal models

Neuropathic pain is a global health concern due to its severity and its detrimental impact on patients' quality of life. It is primarily characterized by sensory alterations, most commonly hyperalgesia and allodynia. As the disease progresses, patients with neuropathic pain develop co-occurring emotional disorders, such as anxiety and depression, which further complicate therapeutic management. While pharmacotherapy remains the first-line treatment, limitations in its efficacy and the prevalence of side effects often leave patients with insufficient pain relief. Transcranial direct current stimulation (tDCS), a non-invasive brain stimulation technique, has recently emerged as a promising alternative for chronic pain management. This review provides an overview of preclinical studies examining the effects of tDCS in rodent models of neuropathic pain. It specifically highlights the potential of tDCS to modulate the emotional-affective component of pain, with a focus on identifying optimal cortical targets for stimulation to enhance the translational application of tDCS in managing pain-related emotional disorders.

The role of 15 mA and 77.5 Hz transcranial alternating current stimulation in blood pressure regulation: A post hoc analysis of a randomized controlled trial

BACKGROUND

Transcranial alternating current stimulation (tACS) at 77.5 Hz and 15 mA, targeting the forehead and mastoid areas, has proven efficacious in patients with major depressive disorder (MDD) by simultaneously stimulating multiple brain nuclei and regions, many of which are critical for blood pressure regulation. This post hoc analysis aimed to assess the potential blood pressure-lowering effects of 77.5 Hz, 15 mA tACS in first-episode drug-naive MDD patients with normotension.

METHODS

Data from a previous randomized controlled trial (RCT) involving first-episode drug-naive MDD patients were analyzed. Participants underwent 20 sessions of either active tACS or sham stimulation. Vital signs, including systolic blood pressure (SBP) and diastolic blood pressure (DBP), were measured at baseline, after treatment (Week 4), and at follow-up (Week 8). Multivariate linear regression and Generalized Estimating Equations (GEE) models were used to evaluate the effects of the treatment on blood pressure.

RESULTS

Totally 68 participants were analysis (33 in the sham group and 35 in the active group). By Week 4, the active tACS group exhibited a significant reduction in both SBP and DBP compared to the sham group (coefficient - 2.04, 95 % CI -3.01 to -1.07, p < 0.001 on SBP, and coefficient - 1.92, 95 % CI -2.69 to -1.18, p < 0.001 on DBP).

CONCLUSIONS

tACS at 77.5 Hz and 15 mA can effectively reduce SBP and DBP in first-episode drug-naive depressive individuals with normotension, with greater reductions observed in those with higher baseline levels.

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.

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.

Boosting working memory in the elderly: driving prefrontal theta-gamma coupling via repeated neuromodulation

The escalating global burden of age-related neurodegenerative diseases and associated healthcare costs necessitates innovative interventions to stabilize or enhance cognitive functions. Deficits in working memory (WM) are linked to alterations in prefrontal theta-gamma cross-frequency coupling. Low-intensity transcranial alternating current stimulation (tACS) has emerged as a non-invasive, low-cost approach capable of modulating ongoing oscillations in targeted brain areas through entrainment. This study investigates the impact of multi-session peak-coupled theta-gamma cross-frequency tACS administered to the dorsolateral prefrontal cortex (DLPFC) on WM performance in older adults. In a randomized, sham-controlled, triple-blinded design, 77 participants underwent 16 stimulation sessions over six weeks while performing n-back tasks. Signal detection measures revealed increased 2-back sensitivity and robust modulations of response bias, indicating improved WM and decision-making adaptations, respectively. No effects were observed in the 1-back condition, emphasizing dependencies on cognitive load. Repeated tACS reinforces behavioral changes, indicated by increasing effect sizes. This study supports prior research correlating prefrontal theta-gamma coupling with WM processes and provides unique insights into the neurocognitive benefits of repeated tACS intervention. The well-tolerated and highly effective multi-session tACS intervention among the elderly underscores its therapeutic potential in vulnerable populations.

Effects of cerebellar transcranial alternating current stimulation on balance and gait in healthy subjects

BACKGROUND

Transcranial Alternating Current Stimulation (tACS) is a non-invasive brain stimulation technique that modulates cortical oscillations and influences behavior.

OBJECTIVES

This study aimed to explore the effects of cerebellar theta (5 Hz) and gamma (50 Hz) tACS on human balance and gait through kinematic analysis.

MATERIALS AND METHODS

Nineteen right-handed healthy subjects participated in three randomized motor tasks: postural standing (PS), gait initiation (GI), and gait cycle (GC). Participants underwent theta-, gamma-, or sham-tACS over the cerebellum while kinematic data were collected using a force platform and an 8-camera optoelectronic system.

RESULTS

Theta-tACS significantly influenced motor behavior during PS and GC, but not GI. Specifically, it reduced the Maximum Radius, Total Trace Length, Longitudinal Range, and Area during PS, and decreased Stride Width during GC. In contrast, cerebellar gamma-tACS had no significant effect on any kinematic parameters across the tasks.

CONCLUSIONS

Cerebellar theta-tACS may enhance postural stability and gait control in healthy individuals. We hypothesize that theta-tACS may entrain theta-resonant neurons in the cerebellar cortex, affecting motor control networks involved in balance and gait.

SIGNIFICANCE

This study highlights tACS's potential as a non-invasive treatment for balance and gait disorders associated with cerebellar dysfunction.

Alpha rhythm transcranial electrical stimulation to inferior parietal cortex increases alpha power and phase synchrony while attending to mind-body self-states

Self-referential processing (SRP) refers to the human brain's response to semantic and somatic self-related information. Recent developments in modulating semantic and somatic SRP using non-invasive brain stimulation supported the efficacy of transcranial direct current stimulation in modulating alpha electroencephalography (alpha-EEG) during SRP. Meanwhile, although alpha transcranial alternating current stimulation (alpha-tACS) shows greater efficacy in modulating alpha-EEG, the efficacy of alpha-tACS for modulating alpha-EEG during SRP has not been evaluated. The current study investigates the effects of alpha-tACS compared to sham stimulation over the medial prefrontal cortex and the bilateral inferior parietal lobule on alpha-EEG during both semantic and somatic SRP in two separate experiments. Semantic SRP was provoked by introspection on life roles (e.g., "friend"), while somatic SRP was provoked by interoception upon sensations occurring in the exterior body (e.g., "shoulders") during the experimental task, and alpha-EEG responses during SRP were compared to those occurring during resting state and an external attention control condition. Results indicated that while alpha-tACS to the medial prefrontal cortex did not produce significant source-level alpha-EEG changes, alpha-tACS to inferior parietal cortex increased alpha-EEG source power and phase synchrony when participants received real stimulation during the first experimental session. An exploratory analysis also indicated that real stimulation reduced alpha-EEG power during semantic but not somatic SRP during the first session but not the second session. Our results demonstrate that while alpha-tACS can modulate alpha-EEG during SRP, the effects may be dependent on the ordering of real vs. sham stimulation sessions and stimulation sites.

Human head models and populational framework for simulating brain stimulations

Noninvasive brain stimulation (NIBS) is pivotal in studying human brain-behavior relations and treating brain disorders. NIBS effectiveness relies on informed targeting of specific brain regions, a challenge due to anatomical differences between humans. Computational volumetric head modeling can capture individual effects and enable comparison across a population. However, most studies implementing modeling use a single-head model, ignoring morphological variability, potentially skewing interpretation, and realistic precision. We present a comprehensive dataset of 100 realistic head models with variable tissue conductivity values, lead-field matrices, standard-space co-registrations, and quality-assured tissue segmentations to provide a large sample of healthy adult head models with anatomical and tissue variance. Leveraging the Human Connectome Project s1200 release, this dataset powers population head modeling for stimulation target optimization, MEEG source modeling simulations, and advanced meta-analysis of brain stimulation studies. We performed a quality assessment for each head mesh, which included a semi-manual segmentation accuracy correction and finite-element analysis quality measures. This dataset will facilitate brain stimulation developments in academic and clinical research.

Learning from missteps: Potential of transcranial electrical stimulation in neuropsychological rehabilitation

Transcranial electrical stimulation (tES) holds promise for neuropsychological rehabilitation by leveraging the brain's inherent plasticity to enhance cognitive and motor functions. However, early results have been variable due to oversimplified approaches. This manuscript explores the potential and complexities of tES, particularly focusing on a protocol defined transcranial direct current stimulation as a reference model for all tES protocols, emphasising the need for precision in tailoring stimulation parameters to individual characteristics. By integrating intrinsic (i.e. the neuro-physiological system state) and extrinsic factors (i.e. experimental set up), highlighting the critical role of state-dependent effects and the synergy with cognitive tasks, we aim to refine tES protocols. This approach not only addresses the complexity of the brain system (as defined by its current state) but also highlights the importance of collaborative research and data sharing to understand the underlying mechanisms of tES-induced changes and optimising therapeutic efficacy. Emphasising the integration of tES with targeted tasks and clearer hypotheses, this work underscores the potential for more effective neurorehabilitation strategies.

HD-tDCS effects on social impairment in autism spectrum disorder with sensory processing abnormalities: a randomized controlled trial

This study examined the effects of high-definition transcranial direct current stimulation (HD-tDCS) on social impairment in children with autism spectrum disorder (ASD), focusing on those with and without sensory processing abnormalities. A randomized double-blind sham-controlled trial involved 72 children with ASD, divided into three groups based on sensory integration status. A post-hoc analysis of 51 children aged 4-8 years who received true HD-tDCS was conducted, categorizing them into hypo-tactile, hyper-tactile, and typical tactile sensitivity groups. Therapeutic efficacy was compared across these groups. (1) The randomized cntrolled Trial: The typical sensory integration group showed significant improvements in social awareness (t = 5.032, p < 0.000) and autistic mannerisms (t = 3.085, p = 0.004) compared to the sensory integration dysfunction group. (2)The result of the post-hoc analysis: The hypo-tactile and typical tactile sensitivity groups exhibited notable improvements in social awareness, cognition, communication, autistic mannerisms, and total SRS scores. In contrast, the hyper-tactile group only had a significant reduction in social communication (t = 2.385, p = 0.022) post-intervention. HD-tDCS effectively improved social impairment symptoms in children with ASD, particularly those with typical sensory integration and either typical or hypo-tactile responsiveness.

Effect of Transcranial Direct Current Stimulation on Memory and Emotional Recovery in Patients with Stroke and Traumatic Brain Injury: A Prospective, Multicenter, Interventional Pilot Study

Background/Objectives: Emotional and cognitive impairments are prevalent in patients with acute ischemic stroke (AIS) and traumatic brain injury (TBI), significantly affecting their quality of life and recovery potential. Transcranial direct current stimulation (tDCS) has emerged as a promising non-invasive method to enhance neurorehabilitation outcomes by modulating neural activity. Methods: This prospective, open-label, multicenter interventional study included 100 participants (50 AIS, 50 TBI) who underwent 10 sessions of tDCS. Emotional states, depression levels, and memory and learning outcomes were assessed pre- and post-intervention using the UWIST Mood Adjective Checklist (UMACL), Depression Measurement Questionnaire (DMQ), Benton Visual Retention Test (BVRT), and Brain Damage Diagnostic Test (BDDT). Results: Significant improvements in emotional states were observed post-tDCS. Hedonic tone increased (AIS: 2.5 to 5 stens; TBI: 1.5 to 4 stens), while tension arousal decreased (AIS: 8 to 6 stens; TBI: 8 to 6 stens; all p < 0.001). Depression levels dropped significantly, with the overall depression index decreasing from 131 to 100 points in AIS and from 126 to 104 points in TBI (both p < 0.001). Memory and learning scores improved significantly, evidenced by increased correct responses and reduced errors in BVRT and BDDT tests (all p < 0.001). Conclusions: tDCS effectively improved emotional states, reduced depression levels, and enhanced cognitive functions in AIS and TBI patients. These findings support the integration of tDCS into neurorehabilitation protocols, with further research needed to explore long-term benefits and individualized treatment strategies.

Exploring the classical and numerical Delboeuf illusion: the impact of transcranial alternating current stimulation on magnitude processing

Understanding cognitive and neural mechanisms underlying quantity processing is crucial for unraveling human cognition. The existence of a single magnitude system, encompassing non-symbolic number estimation alongside other magnitudes like time and space, is still highly debated since clear evidence is limited. Recent research examined whether spatial biases also influence numerosity judgments, using visual illusions like the Delboeuf illusion. While findings support a generalized magnitude system, direct comparisons of spatial and numerical Delboeuf illusions are missing. This study explored whether perceptual biases similarly affect different magnitude processing and whether transcranial alternating current stimulation (tACS) modulates these processes. Participants underwent three tACS conditions (seven Hz, 18 Hz, placebo) while performing tasks involving the classic and numerical Delboeuf illusions. We hypothesized that theta-frequency tACS (seven Hz) would enhance visual integration and illusion strength, while beta tACS (18 Hz) would reduce it by promoting visual segregation. Results indicated higher discrimination accuracy in area-based tasks than numerical judgments. Nonetheless, a significant correlation between performances in spatial and numerical illusions supported the existence of a shared mechanism for magnitude processing. Contrary to expectations, seven Hz tACS reduced the perceptual illusion's strength. No significant interaction emerged between tACS frequency and discrimination abilities. These findings deepen our understanding of the cognitive processes involved in magnitude perception, potentially supporting the hypothesis of a generalized magnitude system. They also highlight the potential and limitations of non-invasive brain stimulation techniques, such as tACS, in modulating perceptual processes, offering insights into the neural underpinnings of quantity perception.

Transcutaneous and transcranial electrical stimulation for enhancing military performance: an update and systematic review

Introduction

Electrical stimulation (ES), including transcranial electrical stimulation (tES) and transcutaneous vagus nerve stimulation (tVNS), has shown potential for cognitive enhancement in military contexts. Various types of ES, such as transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), modulate neuronal membrane potentials and cortical excitability, potentially improving cognitive functions relevant to military training and operations.

Methods

This systematic review updates previous findings by examining studies published between 2019 and 2024 that investigated electrical stimulation effects on cognitive performance in military personnel and tasks. We focused on whether the studies addressed key questions about the generalizability of lab findings to military tasks, the frequency and intensity of adverse effects, the impact of repeated ES administration, and the ethical and regulatory considerations for its use in potentially vulnerable military populations.

Results

Eleven studies met the inclusion criteria; most demonstrated overall low to some concerns, however, two of these had overall high risk of bias. While tES and tVNS showed some promise for enhancing multitasking and visual search performance, the results were mixed, with no reliable effects on vigilance tasks.

Discussion

The reviewed studies highlight the need for a better understanding of ES mechanisms, optimal stimulation parameters, and individual differences in response to ES. They also highlight the importance of conducting high-powered research in military settings to evaluate the efficacy, safety, and ethical implications of ES. Future research should address the generalizability of lab-based results to real-world military tasks, monitor the frequency and intensity of adverse effects, and explore the long-term impacts of repeated administration. Furthermore, ethical and regulatory considerations are crucial for the responsible application of ES in military contexts, and a series of outstanding questions is posed to guide continuing research in this domain.

tDCS cranial nerve Co-stimulation: Unveiling brainstem pathways involved in trigeminal nerve direct current stimulation in rats

BACKGROUND

The effects of transcranial direct current stimulation (tDCS) are generally thought to result from the polarization of cortical neurons by the weak electric fields it creates. However, recent evidence suggests that some tDCS effects may be mediated through co-stimulation of peripheral or cranial nerves, particularly the trigeminal nerve (TN). The TN projects to key brainstem nuclei that regulate neurotransmitter release throughout the central nervous system, but the specific pathways involved are not yet well understood.

METHODS

In this study, we examined the effects of acute transcutaneous TN direct current stimulation (TN-DCS) on tonic (i.e. mean spike rate) and phasic (number of bursts, spike rate per burst, burst duration, and inter-burst interval) activities while simultaneously recording single-neuron activity across three brainstem nuclei in rats: the locus coeruleus (LC; phasic and tonic activities), dorsal raphe nucleus (DRN; tonic activity), and median raphe nucleus (MnRN; tonic activity).

RESULTS

TN-DCS significantly modulated tonic activity in the LC and DRN, with interactions between amplitude, polarity, and time affecting mean spike rates. It also influenced phasic activity in the LC, altering burst number, duration, and inter-burst intervals. In contrast, MnRN tonic activity was unchanged. Blocking TN with xylocaine eliminated the effects on tonic activity in both the LC and DRN.

CONCLUSIONS

These results suggest that tDCS may modulate the TN, altering DRN and LC activity. Differential changes in tonic and phasic LC activity highlight their roles in TN-DCS effects on the cortex. This research offers insights to improve tDCS efficacy and understanding.

Transcranial pulsed current stimulation alleviates neuronal pyroptosis and neurological dysfunction following traumatic brain injury via the orexin-A/NLRP3 pathway

Traumatic brain injury (TBI) is a life-threatening condition with high incidence and mortality rates. The current pharmacological interventions for TBI exhibit limited efficacy, underscoring the necessity to explore novel and effective therapeutic approaches to ameliorate its impact. Previous studies have indicated that transcranial pulsed current stimulation (tPCS) can improve neurofunctional deficits in patients by modulating brain neuroplasticity. However, the exact mechanism underlying this neuroprotective effect remains elusive. In this study, mice with TBI induced by controlled cortical impact were subjected to 30 min of daily tPCS for 5 consecutive days and intraperitoneally administered an orexin receptor type 1 (OX1R) antagonist (SB334867). The neuroprotective effects of tPCS and its potential mechanisms were assessed through behavioral tests, histopathological examination, immunohistochemistry and Western blotting. In vitro experiments involved stimulating HT22 cells with LPS + ATP to assess the anti-neuroinflammatory effects of Orexin-A (OX-A) using CCK-8, Western blotting, and Flow cytometry. The results demonstrated that tPCS reduced the mNSS in TBI mice, ameliorated tissue damage, improved motor and cognitive deficits, and upregulated OX-A expression. Notably, SB334867 reversed the protective effects of tPCS. In vitro studies revealed that OX-A inhibited the formation and activation of NLRP3 inflammasomes, resulting in reduced levels of ROS and restoration of MMP. However, this effect could be reversed by the NLRP3 agonist BMS-986299. Our findings suggest that tPCS promotes the release of OX-A and modulates the OX1R/NLRP3 pathway to mitigate the inflammatory response following TBI, thereby exerting neuroprotective effects.

The application of non-invasive neuromodulation in stuttering: Current status and future directions

Non-invasive neuromodulation methods such as transcranial Direct Current Stimulation (tDCS) and Transcranial Magnetic Stimulation (TMS), have been extensively utilized to enhance treatment efficacy for various neurogenic communicative disorders. Recently, these methods have gained attention for their potential to reveal more about the underlying nature of stuttering and serve as adjunct therapeutic approaches for stuttering intervention. In this review, we present existing research and discuss critical factors that might influence the efficacy of these interventions, such as location, polarity, intensity, and duration of stimulation, as well as the impact of combined behavioral training. Additionally, we explore implications for future studies, including the application of different neuromodulation methods to address various aspects of stuttering such as speech fluency and associated psychological and cognitive aspects in people who stutter.

tDCS Combined with CIMT for Post-stroke Upper Extremity Rehabilitation: A Systematic Review and Meta-Analysis

BackgroundTranscranial direct current stimulation (tDCS) has been widely used as an adjunctive treatment for motor function after stroke.ObjectiveTo quantify the effect of tDCS combined with constraint-induced movement therapy (CIMT) on the functional recovery of the upper limb after stroke.MethodsBy May 2024, two independent authors screened relevant randomized controlled trials (RCTs) published in English from PubMed, Embase, Web of Science and the Cochrane Library. Publication bias was assessed using the Egger's test.ResultsOf the 221 retrieved records, seven publications met the criteria for systematic review and quantitative analysis. According to estimates of Hedges'g, significant effects were revealed from Fugl-Meyer Assessment for Upper Limbs (UL-FMA) for upper limb impairment (g = 0.587, 95% CI = 0.256 to 0.919, p < 0.05) and Motor Activity Log-Amount of Movement (MAL-AoM) for perceived amount of motor (g = 0.386, 95% CI = 0.030 to 0.743, p < 0.05). Significant results favoring combined therapy were not found in Motor Activity Log-Quality of Movement (MAL-QoM) (g = 0.181, 95% CI = -0.169 to 0.531, p > 0.05), grip strength (g = 0.135, 95% CI = -0.214 to 0.485, p > 0.05) or Wolf Motor Function Test (WMFT) (g = 0.210, 95% CI = -0.117 to 0.537, p > 0.05).ConclusionsOur findings confirmed that tDCS enhanced the effect of CIMT in improving upper limb impairment and perceived amount of motor in daily life after stroke.

Non-invasive brain stimulation for upper extremity dysfunction in children with cerebral palsy: a systematic review and meta-analysis

Background

Repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are the most commonly used non-invasive brain stimulation (NIBS) techniques. However, NIBS for upper extremity dysfunction remains unclear in children with cerebral palsy (CP). Thus, we aim to determine safety and effectiveness of NIBS for upper extremity dysfunction in children with CP.

Methods

Two reviewers conducted literature search on five databases including PubMed, Web of Science, ProQuest, Scopus, and Embase independently. Systematic review and meta-analyses of included studies were conducted. Studies used standardized mean difference (SMD) and 95% confidence interval (CI) to calculate pooled effect size between two groups. The statistics I2 was used to assess the heterogeneity between randomized controlled trials (RCTs).

Results

Fifteen studies were included, with seven of which examined rTMS and eight studied tDCS. Total 366 children with CP were included. Changes in Box and Block Test (BBT) of the affected hand changed significantly in post (SMD =0.68; 95% CI: 0.02 to 1.34; P=0.044; I2=0%) and 90-minute effect (SMD =0.69; 95% CI: 0.02 to 1.36; P=0.04; I2=0%), and Modified Ashworth Scale (MAS) (SMD =-0.51; 95% CI: -0.99 to -0.03; P=0.04; I2=0%) after using tDCS were statistically significant. There was no difference of total number of dropouts between each group. No patients experienced serious adverse events.

Conclusions

NIBS is safe and well tolerated in children with CP. And current evidence suggests that when safety guidelines are followed, NIBS does not induce seizures in pediatric patients with no history of epilepsy or stable epilepsy. tDCS is effective in improving upper extremity dysfunction such as fine motor function especially hand dexterity, and reducing upper extremity spasticity in children with CP. Due to insufficient studies, the effectiveness of rTMS is uncertain.

Understanding and targeting repetitive behaviors and restricted interests in autism spectrum disorder via high-definition transcranial direct current stimulation: a study-protocol

BACKGROUND

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by persistent deficits in social interaction and repetitive behaviors (RBs). Therapies specifically targeting RBs have been underexplored despite advances in understanding their neurobiological basis. This study aims to evaluate whether high-definition transcranial direct current stimulation (HD-tDCS) can reduce dysfunctional RBs in autistic children and investigate whether improvements differ between lower-order and higher-order RBs based on the brain regions stimulated.

METHODS

The study entails a multi-session, sham-controlled, site-controlled, double-blind, and between-subjects design. The study will include participants with an ASD diagnosis (aged 8-13 years; IQ ≥ 70), who will undergo the HD-tDCS intervention for 10 sessions. Participants will be randomly assigned to three conditions: (1) Pre-Motor Active Group (active HD-tDCS over pre-SMA cortex); (2) Frontal Active Group (active HD-tDCS over dlPFC); (3) Placebo Control Group. In the active HD-tDCS conditions, the current will be delivered through a 4 × 1 montage; small circular electrodes will be used with the cathode placed centrally with a current intensity of 0.5 mA for a total of 20 min (30 s ramp up/down) per session. Participants during the sham condition will undergo the same procedures as those in the both active conditions actual placement of electrodes, and turning on the HD-tDCS equipment (30 s). The assessment will be completed at baseline (T0), immediately after the end of the intervention (T1) and 3 months after the end of the intervention (T2). The primary outcome measure will be the Total Score of the Repetitive Behavior Scale-Revised. The secondary outcomes measures will comprise ASD symptoms, sensory processing pattern, emotional/behavioral problems, sleep functioning, parental stress, neuropsychological features and High-Density EEG connectivity. We hypothesize that active HD-tDCS will lead to significant reduction in the total score of the primary outcome compared to Sham Group, with site-specific effects on lower-order and higher-order RBs.

DISCUSSION

HD-tDCS is an easy-to-deliver, time-efficient, neurobiologically-driven intervention that could be performed as add-on to reduce the time of conventional therapy for ASD. Given the inherent limitations of specific interventions for RBs, tDCS represents an important "third" treatment arm to address the burden of interventions for ASD.

TRIAL REGISTRATION DETAILS

The trial has been registered at ClinicalTrials.gov (ID: NCT06645587). Registered 17 October 2024.

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.

Predicting the Beneficial Effects of Cognitive Stimulation and Transcranial Direct Current Stimulation in Amnestic Mild Cognitive Impairment with Clinical, Inflammation, and Human Microglia Exposed to Serum as Potential Markers: A Double-Blind Placebo-Controlled Randomized Clinical Trial

In amnestic mild cognitive impairment (aMCI), neuroinflammation evolves during disease progression, affecting microglial function and potentially accelerating the pathological process. Currently, no effective treatment exists, leading to explorations of various symptomatic approaches, though few target the underlying physiological mechanisms. Modulating inflammatory processes may be critical in slowing disease progression. Cognitive stimulation (CS) and transcranial direct current stimulation (tDCS) applied to the left dorsolateral prefrontal cortex (l-DLPFC) show promise, but the results are heterogeneous. Thus, a randomized, double-blind, placebo-controlled clinical trial is currently underway. The first-stage results were examined after three weeks of intervention in two groups: active tDCS combined with CS and sham tDCS combined with CS. Twenty-two participants underwent two assessments: T0 (baseline) and T1 (after 15 sessions of tDCS, active or sham, and 9 sessions of CS). The results demonstrated that CS improved cognition, increased brain-derived neurotrophic factor (BDNF) levels, and reduced peripheral proinflammatory cytokine levels (interleukin IL-6 and chemokine CX3CL1) in serum. This decrease in IL-6 may promote microglial proliferation and survival as a modulatory effect response, while the increase in BDNF might suggest a regulatory mechanism in microglia-neuron interaction responses. However, tDCS did not enhance the cognitive or modulatory effects of CS, suggesting that longer interventions might be required to achieve substantial benefits.

Vision Restoration through transorbital electrical stimulation in Optic Neuropathy in patients with significant optic atrophy due to primary open-angle glaucoma-a randomised, controlled, double-blind, multicentre clinical trial: the VIRON study protocol

INTRODUCTION

Glaucoma is one of the most common causes of blindness and affects more than 70 million people worldwide. The disease is characterised by the loss of retinal ganglion cells associated with a progressive optic neuropathy, resulting in an impairment of visual functions, for example, visual field loss. Nowadays, the only modifiable risk factor is the increase in intraocular pressure, and its treatment is to lower this pressure by medication, laser treatment or surgery to avoid disease progression. New methods for preventing and reversing vision loss are thus urgently needed. Several small and two multicentre studies have presented evidence that repetitive transorbital alternating current stimulation (rtACS) can lead to long-lasting visual field improvement. This could open a new and inexpensive therapeutic option for optic atrophy. However, the level of evidence for this method is still fairly rather poor, and further trials are needed. Therefore, this clinical trial aims to prove the effectiveness of rtACS compared with sham stimulation in patients with primary open-angle glaucoma (POAG).

METHODS AND ANALYSIS

VIRON (Vision Restoration in Optic Neuropathy) is a national, multicentre, prospective, randomised, placebo-controlled, double-blind trial with three arms. The primary objective is to assess the effectiveness of rtACS in patients with POAG compared with sham stimulation. The primary outcome is the change in mean defect (MD) in the visual field immediately after 10 sessions of rtACS (days 9, 16 and 23) compared with the values of initial perimetry (days -21 to -14 and 0) after applying electrical stimulation with a classical montage, compared with sham and electrical stimulation using individualised montage. Secondary outcome measures comprise a long-term effect with changes in MD at 24 weeks after stimulation, and data from the National Eye Institute Visual Function-25 and quality of life (Short Form 36) questionnaires. The target population are patients with glaucomatous optic atrophy and significant glaucomatous visual field defects (MD of 5-22 dB) due to POAG.After randomisation, patients received either classical rtACS (group 1), individual rtACS (group 2) or sham stimulation (group 3) in daily 25 min stimulation sessions in two series of five consecutive days separated by a weekend interval. In group 1, active stimulation will be via the routinely applied montage using two electrodes affixed on the right and left side of the head, next to the eyes, with straightforward fixation. In group 2, the current flow will be individually modelled (MRI-based) to target areas of partial visual field defects by optimising electrode positions in conjunction with an optimised visual fixation direction. Group 3 with sham stimulation will serve as control.The calculated sample size required to achieve a statistical power of 80% for a relevant effect size and allow for dropouts was 300 (100 per group). The trial has already begun with the first patient in July 2023. The planned recruitment period is 24 months with an estimated end of the study in November 2025 (last patient out). An adjusted extension of the study period is planned.

ETHICS AND DISSEMINATION

VIRON was approved by the Central Ethics Committee of the University Medical Center Göttingen (19 October 2022) and those of the individual participating centres (Bonn: 446/23-EP, Hamburg: 2023-200889-BO-bet, Cologne: 23-1487 and Mainz: 2023-17399-§23b). The study protocol complies with the Declaration of Helsinki, the national medicine device regulation (MDR) laws and the international standards of good clinical practice (GCP).The study protocol (V.5, 24 November 2023) was designed following the Standard Protocol Items: Recommendations for Interventional Trials guidelines and is registered on https://drks.de/search/de/trial/DRKS00029129.As study initiatior the University Medical Center Göttingen (UMG) is responsible for data ownership and data management of the VIRON study. The study data will be published within 6 months of the study being completed. After the publication of the primary results, all data are anonymised and published in an open-access journal to ensure access to the data for third parties.

TRIAL REGISTRATION NUMBER

https://drks.de/search/de/trial/DRKS00029129.

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.

Examining tolerability, safety, and blinding in 1032 transcranial electrical stimulation sessions for children and adolescents with neuropsychiatric and neurodevelopmental disorders

The present study first extensively evaluated the tolerability, safety, and blinding of transcranial direct current stimulation (tDCS) and transcranial random noise stimulation (tRNS) in paediatric clinical populations, composed of 92 children and adolescents (54 females, age range: 8-17 years), involving 1032 sessions across neuropsychiatric (i.e., anorexia nervosa) and neurodevelopmental (i.e., attention deficit and hyperactivity disorder, developmental dyscalculia) conditions. It compared adverse events (AEs) occurrence between active and sham transcranial electrical stimulation (tES) conditions (i.e., 528 active vs. 504 sham sessions) as well as tDCS and tRNS (i.e., 772 tDCS sessions vs. 260 tRNS sessions), while considering demographic and emotional-behavioural factors. Results showed tES safety with no "moderate" or "severe" AEs reported; about 77% of sessions were AE-free, supporting tES use in these populations. Itching was the most common symptom, and active sessions were found to be more likely to induce AEs compared to sham sessions. Notably, tRNS had a higher AE likelihood than tDCS, possibly due to experimental differences. In the current study, demographic and emotional-behavioural variables did not significantly affect AEs. Blinding procedures were moderately effective, with about half of participants correctly identifying their condition. As indicated in prior studies, tRNS seems to better preserve blinding integrity. In conclusion, this study provides comprehensive insights into tES tolerability and safety in paediatric clinical populations, emphasizing the need for further AEs exploration in tES and blinding procedure refinement in future research.

Gamma-frequency transcranial alternating current stimulation over the left posterior parietal cortex enhances the long-term retention of associative memory

Transcranial alternating current stimulation (tACS) has been reported to improve associative memory (AM) by modulating the frequency of neural oscillations in the brain; however, whether gamma-frequency (> 30 Hz) tACS in the left posterior parietal lobe (PPC) can enhance memory retention in AM remains unclear. This study aimed to investigate whether memory retention in AM could be improved after gamma-frequency tACS of the left PPC. We used a randomly assigned, double-blind, repeated-measures, sham-control design, in which 28 healthy adult participants were assigned to receive a single 20-min session of gamma-frequency (60 Hz) tACS or sham stimulation. The memory learning task consisted of studying and testing 50 unrelated word pairs three times on day 1. The number of correct responses in the cued recall task was measured at three time points: days 1, 7, and 28. The results revealed a significant difference in the number of correct responses between the interventions on day 7 and day 28. These data suggest that gamma-frequency tACS stimulation of the left PPC enhances the long-term retention of AM in healthy adults.

Research progress of tDCS in the treatment of ADHD

TDCS is one of the most widely used non-invasive neuromodulation techniques, which changes the excitability of local cortical tissue by applying weak continuous direct current to the scalp, effectively improves the attention and concentration of ADHD children, and improves the impulse disorder of patients, but related research is still in its infancy. Based on a review of a large number of existing literatures and an analysis of the pathogenesis and principle of ADHD, this paper summarized the research on tDCS in the treatment of ADHD in recent years from the aspects of treatment mechanism, safety and stimulation parameters, and simply compared the application of tDCS with other non-traumatic neuromodulation techniques in the treatment of ADHD. The future development direction of this technology is further discussed.

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.

Past and Present Role of Neurosurgical Interventions in the Management of Psychiatric Disorders: A Literature Review on the Evolution of Psychosurgery

Despite advancements in psychiatric treatments, many patients with treatment-resistant disorders are turning to neurosurgical interventions. These include neuromodulation-based surgeries such as deep brain stimulation (DBS) and ablative surgeries such as cingulotomy, offering relief for severe conditions such as post-traumatic stress disorder (PTSD), depression, schizophrenia, obsessive-compulsive disorder (OCD), anxiety, and substance use disorder. While "psychosurgery" has sparked debate due to concerns about patient well-being, recent studies indicate promising symptom improvement rates across various psychiatric conditions while also demonstrating overall safety. Neuromodulation techniques, such as DBS, transcranial magnetic stimulation (TMS), and electroconvulsive therapy (ECT), have evolved in regard to their sensitivity and their ability to target specific brain regions to alleviate psychiatric symptoms. Despite their benefits, these therapies have been shown to elicit side effects such as memory loss and seizures in patients, which has sparked controversy in the use of this technology across clinicians and patients. Ablative therapies, on the other hand, are concerning for being overly invasive in their approach toward psychiatric care. Despite the stigma associated with these neurosurgical interventions for psychiatric care, these procedures often remain a last resort for many patients, highlighting the need for continued research to improve these treatments and expand options for those in need. In this narrative review, we examine the current literature to elicit an understanding of neurosurgical history in regard to psychiatric disorder treatment and its implications for clinical practice.

Multi-session tDCS over the posterior parietal cortex and associative memory

Associative memory (AM) plays a crucial role in our ability to link disparate elements of our experiences, yet it is especially vulnerable to age-related decline and pathological conditions. Non-invasive brain stimulation (NIBS), particularly transcranial direct current stimulation (tDCS), has been investigated as a potential intervention to enhance cognitive functions, including AM. Previous tDCS studies yielded inconsistent results, often due to variations in stimulation sites and protocols. Nonetheless, enough evidence suggests that tDCS over the posterior parietal cortex (PPC) can improve AM performance. This study aimed to investigate the cumulative effects of multiple anodal tDCS over the PPC on AM performance alongside item memory and verbal fluency. In a randomized sham-controlled trial, 59 healthy young adults were assigned to either anodal or sham stimulation group, receiving tDCS (1.5 mA, for 20 minutes, at P3) over three consecutive days. Memory performance was assessed at four timepoints: pretest, immediately after the first session, posttest (Day 5), and follow-up (Day 9). Although tDCS was well tolerated, the anticipated enhancement of memory performance was not observed. We interpret these findings in the light of methodological considerations and propose potential explanations for the observed results emphasizing the large between-participants variability in memory performance as a significant factor that may have hindered the detection of tDCS effects.

Wearable Neurotechnology for the Treatment of Insomnia: The Study Protocol of a Prospective, Placebo-Controlled, Double-Blind, Crossover Clinical Trial of a Transcranial Electrical Stimulation Device

Sleep disruption and deprivation are epidemic problems in the United States, even among those without a clinically diagnosed sleep disorder. Military service members demonstrate an increased risk of insomnia, which doubles after deployment. This study will investigate the ability of a translational device, Teledyne PeakSleep™ (Teledyne Scientific & Imaging, Durham, NC, USA), to reduce sleep onset latency and the time spent awake after sleep onset, with improvement in the subjective benefits of sleep for patients with insomnia by enhancing the brain rhythms within the frontal lobe implicated in slow wave generation. During this crossover trial, patients will use the wearable neurotechnology prototype headband, which delivers < 14 min of frontal short duration repetitive-transcranial electrical stimulation over a 30 min period immediately before trying to fall asleep. Using active stimulation versus a sham paradigm, we will compare actigraphy data, physiological data, and subjective sleep measures against a pre-treatment baseline in the same patient over the course of the 8-week study. If successful, PeakSleep™ could address the final common pathway in insomnia, namely the onset and maintenance of slow-wave sleep (SWS), and accordingly has the potential to enhance sleep onset in a wide range of individuals, most importantly warfighters in whom efficient sleep onset may be critical for operational success.

Non-invasive brain stimulation in cognitive sciences and Alzheimer's disease

Over the last four decades, non-invasive brain stimulation techniques (NIBS) have significantly gained interest in the fields of cognitive sciences and dementia care, including neurorehabilitation, for its emerging potential in increasing the insights over brain functions and in boosting residual cognitive functions. In the present paper, basic physiological and technical mechanisms and different applications of NIBS were reviewed and discussed to highlight the importance of NIBS in multidisciplinary and translational approaches in clinical and research settings of cognitive sciences and neurodegenerative diseases, especially in Alzheimer's disease. Indeed, NIBS strategies may represent a promising opportunity to increase the potential of neuromodulation as efficacious interventions for individualized patients care.

Transcranial direct current stimulation (tDCS) in psychiatric disorders in early childhood (aged under 10 years): a systematic review

Transcranial direct current stimulation (tDCS) remains experimental for many psychiatric disorders in adults. Particularly in childhood, there is limited research on the evidence for the efficacy and mechanisms of action of tDCS on the developing brain. The objective of this review is to identify published experimental studies to examine the efficacy and mechanisms of tDCS in children with psychiatric or developmental disorders in early (prepubertal) childhood (aged under 10 years). Included Studies should meet the following criteria: (1) experimental studies (no reviews, no case reports), (2) studies published in international peer-reviewed journals, (3) written in English, (4) conducted on children under 10 under years of age, (5) at enrolment with a psychiatric or developmental disorder.Eight studies were identified that fulfilled the specified criteria. All studies investigated effect on children with autism-spectrum-disorder (ASD). Anodal tDCS, mainly targeting the left dorsolateral prefrontal cortex (dlPFC), showed positive effects on the reduction of ASD symptoms. There has also been evidence that these stimulations are feasible, have good tolerability and are safe. tDCS was found to be safe and partially effective, but a long-term effect of tDCS and changes in connectivity during tDCS in autism has not been proven. Other developmental or psychiatric diseases were not investigated. This results in a lack of knowledge regarding the reactivity of the brain during the prepubertal period, which is a critical phase in the pathogenesis of neurodevelopmental disorders such as attention deficit hyperactivity disorder (ADHD), ASD, Tourette's syndrome or dyslexia.

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.

Dissociable components of visual perceptual learning characterized by non-invasive brain stimulation: Stage 1 Registered Report

Visual perceptual learning (VPL), the training-induced improvement in visual tasks, has long been considered the product of neural plasticity at early and local stages of signal processing. However, recent evidence suggests that multiple networks and mechanisms, including stimulus- and task-specific plasticity, concur in generating VPL. Accordingly, early models of VPL, which characterized learning as being local and mostly involving early sensory areas, such as V1, have been updated to embrace these newfound complexities, acknowledging the involvement on parietal (i.e. intra-parietal sulcus) and frontal (i.e. dorsolateral prefrontal cortex) areas, in aspects concerning decision-making, feedback integration and task structure. However, evidence of multiple brain regions differentially involved in different aspects of learning is thus far mostly correlational, emerging from electrophysiological and neuroimaging techniques. To directly address these multiple components of VPL, we propose to use a causal neuromodulation technique, namely transcranial random noise stimulation, to selectively modulate the activity of different brain regions suggested to be involved in various aspects of learning. Specifically, we will target a region in the occipital cortex, which has been associated with stimulus-specific plasticity, and one in the parietal cortex, which has been associated with task-specific plasticity, in a between-subject design. Measures of transfer of learning to untrained stimuli and tasks will be used to evaluate the role of different regions and test for double dissociations between learning effects and stimulated area, shedding lights on learning mechanisms in the visual system. Evidence of dissociable mechanisms of learning can help refine current models of VPL and may help develop more effective visual training and rehabilitation protocols.

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.

Recommendations for the Safe Application of Temporal Interference Stimulation in the Human Brain Part II: Biophysics, Dosimetry, and Safety Recommendations

Temporal interference stimulation (TIS) is a new form of transcranial electrical stimulation (tES) that has been proposed as a method for targeted, noninvasive stimulation of deep brain structures. While TIS holds promise for a variety of clinical and nonclinical applications, little data is yet available regarding its effects in humans and its mechanisms of action. 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. In Part II of this effort, described here, we draw on a previously compiled list (see Part I) of adverse effects (AEs) reported for transcranial direct/alternating current stimulation (tDCS/ACS), deep brain stimulation (DBS), and TIS to determine biophysics-informed exposure metrics for assessing safety. Using an in silico approach, we conduct multiphysics simulations of various tACS, DBS, and TIS exposure scenarios in an anatomically detailed head and brain model. By matching the stimulation in terms of the identified exposure metrics, we infer frequency-dependent TIS parameters that produce exposure conditions equivalent to those known to be safe for tACS and DBS. Based on the results of our simulations and existing knowledge regarding tES and DBS safety, we propose frequency-dependent thresholds below which TIS voltages and currents are unlikely to pose a risk to humans. Safety-related data from ongoing and future human studies are required to verify and refine the thresholds proposed here.

Independent effects of transcranial direct current stimulation and social influence on pain

ABSTRACT

Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulatory technique with the potential to provide pain relief. However, tDCS effects on pain are variable across existing studies, possibly related to differences in stimulation protocols and expectancy effects. We investigated the independent and joint effects of contralateral motor cortex tDCS (anodal vs cathodal) and socially induced expectations (analgesia vs hyperalgesia) about tDCS on thermal pain. We employed a double-blind, randomized 2 × 2 factorial cross-over design, with 5 sessions per participant on separate days. After calibration in Session 1, Sessions 2 to 5 crossed anodal or cathodal tDCS (20 minutes 2 mA) with socially induced analgesic or hyperalgesic expectations, with 6 to 7 days between the sessions. The social manipulation involved videos of previous "participants" (confederates) describing tDCS as inducing a low-pain state ("analgesic expectancy") or hypersensitivity to sensation ("hyperalgesic expectancy"). Anodal tDCS reduced pain compared with cathodal stimulation (F(1,19.9) = 19.53, P < 0.001, Cohen d = 0.86) and analgesic expectancy reduced pain compared with hyperalgesic expectancy (F(1,19.8) = 5.62, P = 0.027, Cohen d = 0.56). There was no significant interaction between tDCS and social expectations. Effects of social suggestions were related to expectations, whereas tDCS effects were unrelated to expectancies. The observed additive effects provide novel evidence that tDCS and socially induced expectations operate through independent processes. They extend clinical tDCS studies by showing tDCS effects on controlled nociceptive pain independent of expectancy effects. In addition, they show that social suggestions about neurostimulation effects can elicit potent placebo effects.

Transcranial Direct Current Stimulation for Cognitive Impairment Rehabilitation: A Bibliometric Analysis

BACKGROUND AND AIMS

As global demographics shift toward an older population, cognitive impairment is becoming increasingly critical. Transcranial Direct Current Stimulation (tDCS), an innovative brain stimulation technique, has the potential to significantly improve cognitive function. Our main aim is to comprehensively analyze the existing literature, identify key aspects of tDCS research in the rehabilitation of cognitive impairment, and predict future trends in this field.

METHODS

We used the Web of Science (WOS) database to search for English articles and reviews relevant to this topic. For visual analysis of the literature, we employed the WOS analysis tool, CiteSpace, along with VOSviewer software to ensure comprehensive analysis.

RESULTS

We included 2940 articles published between 1998 and 2023. Over 25 years, annual publications and citations in this field increased steadily, peaking at 379 articles in 2021. Michael A. Nitsche was a major contributor. Most articles came from developed countries, primarily North America and Europe, and journals generally had modest impact factors. Research in this field primarily aims to treat cognitive impairment resulting from pathological aging or neuropsychiatric disorders, with a particular focus on specific brain regions. Recently, researchers have integrated various treatment modalities with tDCS techniques to actively investigate effective strategies to mitigate cognitive impairments associated with pathological aging.

CONCLUSION

This study presents the first bibliometric analysis of the literature on tDCS in the rehabilitation of cognitive impairment, highlighting key areas of research and emerging trends. These findings provide critical insights for future tDCS interventions targeting cognitive impairment associated with pathological aging.

Bilateral tDCS over the DLPFC enhances baroreceptor reflex sensitivity and inhibits blood pressure-related hypoalgesia

OBJECTIVE

This study investigated the impact of transcranial direct stimulation (tDCS) on pain perception, baroreflex sensitivity (BRS), and blood pressure (BP)-related hypoalgesia.

METHOD

Fifty-eight healthy participants were randomized to receive 1) bi-hemispheric tDCS over the dorsolateral prefrontal cortex (DLPFC) at 2 mA for 20 min, or 2) non-stimulation (Sham). Pain measures (threshold, tolerance, intensity and unpleasantness), emotional state (anxiety and mood), continuous BP, and electrocardiogram (ECG) data were recorded before, during, and after stimulation.

RESULTS

tDCS stimulation was followed by increases in BRS, pain intensity and unpleasantness. Anxiety decreased in the Sham group, but not in the tDCS group. Positive correlations between BP and pain threshold and tolerance before stimulation were observed. These remained during stimulation in the Sham group, but not in the tDCS group. Moreover, negative associations between BRS and BP only persisted in the Sham group.

DISCUSSION

The results suggest that bilateral tDCS over the DLPFC enhances BRS and modulates pain perception and BP-related mechanisms. tDCS increases pain perception by inhibiting BP-related hypoalgesia and preventing habituation of anxiety.

SIGNIFICANCE

Low BRS is a powerful prognostic factor of cardiovascular disease, such that its increase via tDCS may be a new therapeutic strategy for cardiovascular health promotion.

Mapping the electric field of high-definition transcranial electrical stimulation across the lifespan

Transcranial electrical stimulation (tES) is a non-invasive technique widely used in modulating brain activity and behavior, but its effects differ across individuals and are influenced by head anatomy. In this study, we investigated how the electric field (EF) generated by high-definition tES varies across the lifespan among different demographic groups and its relationship with neural responses measured by functional magnetic resonance imaging (fMRI). We employed an MRI-guided finite element method to simulate the EF for the two most common tES montages (i.e., targeting the dorsolateral prefrontal cortex and motor cortex, respectively) in two large cohorts of white and Asian participants aged 12 to 100 years. We found that the EF intensity decreased with age, particularly in individuals under 25 years of age, and was influenced by gender and ethnicity. We identified skull thickness, scalp thickness, and epidural cerebrospinal fluid thickness, as the primary anatomical factors accounting for the inter-individual EF variability. Using a concurrent tES-fMRI approach, we observed a spatial consistency between the simulated EF and the brain activity changes induced by tES in the target region. Finally, we developed an open-source toolbox incorporating age-stratified head models to facilitate efficient EF calculations. These findings characterize and quantify the individual differences in tES-induced EF, offering a reference for implementing personalized neuromodulation strategies.

Boosting Psychotherapy With Noninvasive Brain Stimulation: The Whys and Wherefores of Modulating Neural Plasticity to Promote Therapeutic Change

The phenomenon of neural plasticity pertains to the intrinsic capacity of neurons to undergo structural and functional reconfiguration through learning and experiential interaction with the environment. These changes could manifest themselves not only as a consequence of various life experiences but also following therapeutic interventions, including the application of noninvasive brain stimulation (NIBS) and psychotherapy. As standalone therapies, both NIBS and psychotherapy have demonstrated their efficacy in the amelioration of psychiatric disorders' symptoms, with a certain variability in terms of effect sizes and duration. Consequently, scholars suggested the convenience of integrating the two interventions into a multimodal treatment to boost and prolong the therapeutic outcomes. Such an approach is still in its infancy, and the physiological underpinnings substantiating the effectiveness and utility of combined interventions are still to be clarified. Therefore, this opinion paper aims to provide a theoretical framework consisting of compelling arguments as to why adding NIBS to psychotherapy can promote therapeutic change. Namely, we will discuss the physiological effects of the two interventions, thus providing a rationale to explain the potential advantages of a combined approach.

High-density theta oscillatory-modulated tDCS over the parietal cortex for targeted memory enhancement

OBJECTIVES

Associative memory (AM) declines due to healthy aging as well as in various neurological conditions. Standard transcranial electrical stimulation (tES) protocols show inconclusive facilitatory effects on AM, often lacking function specificity and stimulation focality. We tested the effectiveness of high-density electrode montage delivering anodal theta oscillatory-modulated transcranial direct current stimulation (HD-Theta-otDCS) over the left posterior parietal cortex (PPC), aiming to target AM in a spatially focused and function-specific manner.

METHODS

In a sham-controlled cross-over experiment we explored the differential effects of HD-Theta-otDCS applied during either encoding or the retrieval phases of two AM tasks (Face-Word and Object-Location). The stimulation protocol consisted of an anode over the left PPC (P3) and four surrounding return electrodes (CP1, CP5, PO3, POz) with electrical current oscillating in theta rhythm (5 Hz, 1.5 ± 0.5 mA).

RESULTS

HD-Theta-otDCS stimulation applied during both encoding and retrieval increased AM performance compared to sham control in the Face-Word task. We found no differences between the two active stimulation conditions.

CONCLUSIONS

HD-Theta-otDCS showed to be a promising tool for enhancing AM, regardless of the stimulation timing. The results provide further support for our previous findings with bipolar otDCS and confirm that PPC stimulation can induce behaviorally relevant modulation in the memory-related cortico-subcortical networks.

SIGNIFICANCE

The presented approach is one step forward towards precision brain stimulation for memory neuromodulation. The novelty lies in the combination of increased focality and function-specific current waveform. Positive results set the ground for further research on HD-theta-otDCS effectiveness in clinical populations.