Transcranial direct current stimulation for memory enhancement: from clinical research to animal models

Anodal HD-tDCS on the dominant anterior temporal lobe and dorsolateral prefrontal cortex: clinical results in patients with mild cognitive impairment

OBJECTIVES

Mild cognitive impairment (MCI) is a neurocognitive disorder in which the cognitive and mental abilities of humans are declined. Transcranial direct-current stimulation (tDCS) is an emerging noninvasive brain stimulation technique aimed at neuromodulation. In this study, we investigate whether high-definition anodal tDCS stimulation (anodal HD-tDCS) in MCI patients in two different brain regions will be effective in improving cognitive function.

METHODS

This study was done as a randomized, double-blind clinical trial. Sixty MCI patients (clinically diagnosed by expert neurologists) were randomly divided into three groups. Two groups received 2-mA anodal HD-tDCS for 20 min for 2 weeks (5 consecutive days in each week, 10 days in total). In the first group (twenty patients), the left dorsolateral prefrontal cortex (left DLPFC) was targeted. In the second group (twenty patients), the target zone was the dominant anterior temporal lobe (DATL). The third group (twenty patients) formed the Sham group. The Montreal Cognitive Assessment (MoCA) and Quality of Life in Alzheimer's Disease (QoLAD) were considered as the outcome measures.

RESULTS

MCI patients obtained the highest MoCA mean scores in both left DLPFC and DATL groups versus the study baseline 2 weeks after the intervention. In addition, the MoCA mean scores of MCI patients were greater in both intervention groups compared to the Sham group up to 3 months post-stimulation (p-value ≤ 0.05). However, as we moved away from the first stimulation day, a decreasing trend in the MoCA mean scores was observed. Moreover, in the left DLPFC and DATL groups, higher QoLAD mean scores were observed 3-month post-stimulation, highlighting the effectiveness of anodal HD-tDCS in improving the quality of life in MCI patients.

CONCLUSION

In this research, it was shown that applying anodal HD-tDCS at left DLPFC and DATL brain regains for two successive weeks improves cognitive function in MCI patients (by obtaining higher values of MoCA scores) up to 3 months after the intervention compared to the Sham group. This illustrates the positive effects of HD-tDCS, as a non-pharmacological intervention, for improving cognitive function and quality of life in MCI patients.

SIGNIFICANCE

Two weeks after anodal HD-tDCS of the DLPFC and DATL brain regions, the MCI patients achieved the highest MoCA mean scores compared to the Sham group across all measurement intervals.

Repeated Anodal Transcranial Direct Current Stimulation (RA-tDCS) over the Left Frontal Lobe Increases Bilateral Hippocampal Cell Proliferation in Young Adult but Not Middle-Aged Female Mice

Repeated anodal transcranial direct current stimulation (RA-tDCS) is a neuromodulatory technique consisting of stimulating the cerebral cortex with a weak electric anodal current in a non-invasive manner. RA-tDCS over the dorsolateral prefrontal cortex has antidepressant-like properties and improves memory both in humans and laboratory animals. However, the mechanisms of action of RA-tDCS remain poorly understood. Since adult hippocampal neurogenesis is thought to be involved in the pathophysiology of depression and memory functioning, the purpose of this work was to evaluate the impact of RA-tDCS on hippocampal neurogenesis levels in mice. RA-tDCS was applied for 20 min per day for five consecutive days over the left frontal cortex of young adult (2-month-old, high basal level of neurogenesis) and middle-aged (10-month-old, low basal level of neurogenesis) female mice. Mice received three intraperitoneal injections of bromodeoxyuridine (BrdU) on the final day of RA-tDCS. The brains were collected either 1 day or 3 weeks after the BrdU injections to quantify cell proliferation and cell survival, respectively. RA-tDCS increased hippocampal cell proliferation in young adult female mice, preferentially (but not exclusively) in the dorsal part of the dentate gyrus. However, the number of cells that survived after 3 weeks was the same in both the Sham and the tDCS groups. This was due to a lower survival rate in the tDCS group, which suppressed the beneficial effects of tDCS on cell proliferation. No modulation of cell proliferation or survival was observed in middle-aged animals. Our RA-tDCS protocol may, therefore, influence the behavior of naïve female mice, as we previously described, but its effect on the hippocampus is only transient in young adult animals. Future studies using animal models for depression in male and female mice should provide further insights into RA-tDCS detailed age- and sex-dependent effects on hippocampal neurogenesis.

Rescue of astrocyte activity by the calcium sensor STIM1 restores long-term synaptic plasticity in female mice modelling Alzheimer's disease

Calcium dynamics in astrocytes represent a fundamental signal that through gliotransmitter release regulates synaptic plasticity and behaviour. Here we present a longitudinal study in the PS2APP mouse model of Alzheimer's disease (AD) linking astrocyte Ca²⁺ hypoactivity to memory loss. At the onset of plaque deposition, somatosensory cortical astrocytes of AD female mice exhibit a drastic reduction of Ca²⁺ signaling, closely associated with decreased endoplasmic reticulum Ca²⁺ concentration and reduced expression of the Ca²⁺ sensor STIM1. In parallel, astrocyte-dependent long-term synaptic plasticity declines in the somatosensory circuitry, anticipating specific tactile memory loss. Notably, we show that both astrocyte Ca²⁺ signaling and long-term synaptic plasticity are fully recovered by selective STIM1 overexpression in astrocytes. Our data unveil astrocyte Ca²⁺ hypoactivity in neocortical astrocytes as a functional hallmark of early AD stages and indicate astrocytic STIM1 as a target to rescue memory deficits.

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

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

Neural Mechanism Underlying Task-Specific Enhancement of Motor Learning by Concurrent Transcranial Direct Current Stimulation

The optimal protocol for neuromodulation by transcranial direct current stimulation (tDCS) remains unclear. Using the rotarod paradigm, we found that mouse motor learning was enhanced by anodal tDCS (3.2 mA/cm2) during but not before or after the performance of a task. Dual-task experiments showed that motor learning enhancement was specific to the task accompanied by anodal tDCS. Studies using a mouse model of stroke induced by middle cerebral artery occlusion showed that concurrent anodal tDCS restored motor learning capability in a task-specific manner. Transcranial in vivo Ca2+ imaging further showed that anodal tDCS elevated and cathodal tDCS suppressed neuronal activity in the primary motor cortex (M1). Anodal tDCS specifically promoted the activity of task-related M1 neurons during task performance, suggesting that elevated Hebbian synaptic potentiation in task-activated circuits accounts for the motor learning enhancement. Thus, application of tDCS concurrent with the targeted behavioral dysfunction could be an effective approach to treating brain disorders.

Effectiveness of Anodal otDCS Following with Anodal tDCS Rather than tDCS Alone for Increasing of Relative Power of Intrinsic Matched EEG Bands in Rat Brains

BACKGROUND

This study sought to determine whether (1) evidence is available of interactions between anodal tDCS and oscillated tDCS stimulation patterns to increase the power of endogenous brain oscillations and (2) the frequency matching the applied anodal otDCS's frequency and the brain's dominant intrinsic frequency influence power shifting during stimulation pattern sessions by both anodal DCS and anodal oscillated DCS.

METHOD

Rats received different anodal tDCS and otDCS stimulation patterns using 8.5 Hz and 13 Hz state-related dominant intrinsic frequencies of anodal otDCS. The rats were divided into groups with specific stimulation patterns: group A: tDCS-otDCS (8.5 Hz)-otDCS (13 Hz); group B: otDCS (8.5 Hz)-tDCS-otDCS (13 Hz); group C: otDCS (13 Hz)-tDCS-otDCS (8.5 Hz). Acute relative power changes (i.e., following 10 min stimulation sessions) in six frequency bands-delta (1.5-4 Hz), theta (4-7 Hz), alpha-1 (7-10 Hz), alpha-2 (10-12 Hz), beta-1 (12-15 Hz) and beta-2 (15-20 Hz)-were compared using three factors and repeated ANOVA measurement.

RESULTS

For each stimulation, tDCS increased theta power band and, above bands alpha and beta, a drop in delta power was observed. Anodal otDCS had a mild increasing power effect in both matched intrinsic and delta bands. In group pattern stimulations, increased power of endogenous frequencies matched exogenous otDCS frequencies-8.5 Hz or 13 Hz-with more potent effects in upper bands. The power was markedly more potent with the otDCS-tDCS stimulation pattern than the tDCS-otDCS pattern.

SIGNIFICANCE

The findings suggest that the otDCS-tDCS pattern stimulation increased the power in matched intrinsic oscillations and, significantly, in the above bands in an ascending order. We provide evidence for the successful corporation between otDCS (as frequency-matched guidance) and tDCS (as a power generator) rather than tDCS alone when stimulating a desired brain intrinsic band (herein, tES specificity).

The benefits of simultaneous tDCS and working memory training on transfer outcomes: A systematic review and meta-analysis

BACKGROUND

Transcranial direct current stimulation (tDCS) has shown potential as an effective aid to facilitate learning. A popular application of this technology has been in combination with working memory training (WMT) in order to enhance transfer effects to other cognitive measures after training.

OBJECTIVE

This meta-analytic review aims to synthesize the existing literature on tDCS-enhanced WMT to quantify the extent to which tDCS can improve performance on transfer tasks after training. Furthermore, we were interested to evaluate the moderating effects of assessment time point (immediate post-test vs. follow-up) and transfer distance, i.e., the degree of similarity between transfer and training tasks.

METHODS

Using robust variance estimation, we performed a systematic meta-analysis of all studies to date that compared WMT with tDCS to WMT with sham in healthy adults. All procedures conformed to PRISMA guidelines.

RESULTS

Across 265 transfer measures in 18 studies, we found a small positive net effect of tDCS on improving overall performance on transfer measures after WMT. These effects were sustained at follow-up, which ranged from 1 week to one year after training, with a median of 1 month. Additionally, although there were no significant differences as a function of transfer distance, effects were most pronounced for non-trained working memory tasks.

CONCLUSIONS

This review provides evidence that tDCS can be effective in promoting learning over and above WMT alone, and can durably improve performance on trained and untrained measures for weeks to months after the initial training and stimulation period. In particular, boosting performance on dissimilar working memory tasks may present the most promising target for tDCS-augmented WMT.

Inpatient suicide in psychiatric settings: Evaluation of current prevention measures

The risk of suicide in psychiatric hospitals is 50 times higher than in the general population, despite patient safety being a priority for any hospital. However, to date, due to the complexity of assessing suicide risk, there has been no consensus on the suicide prevention measures that should be in place in hospitals. The aim of this work is: To provide an overview of the progress that has been made in the field of inpatient suicide prevention in recent years; discuss the problems that remain; and suggest potential future developments. As new clinical dimensions (notably anhedonia, psychological pain and hopelessness) develop, they should become new therapeutic targets. Team training (like the Gatekeeper Training Program) and the latest advances in suicide risk assessment (such as the Collaborative Assessment and Management of Suicidality) should be implemented in psychiatric wards. Suicide prevention plans (e.g., ASSIP, SAFE-T, etc.) represent easy-to-administer, low-cost interventions. The Mental Health Environment of Care Checklist has been proven effective to reduce suicide risk at hospitals. Furthermore, the types of psychotherapy recommended to reduce suicide risk are cognitive behavioral therapy (CBT) and dialectical behavioral therapy (DBT). There are several pharmacological treatments for suicide risk, such as lithium and clozapine, which have been shown to be effective in the long term, as well as ketamine and esketamine, which are more effective in the short term. Following some encouraging recent results, buprenorphine may also be proposed to patients with a suicide risk. Triple chronotherapy rapidly improves depressive symptoms over 9 weeks. Regarding brain stimulation techniques, rTMS has proven to be effective in alleviating multiple dimensions of suicidality.

Direct current stimulation modulates gene expression in isolated astrocytes with implications for glia-mediated plasticity

While the applications of transcranial direct current stimulation (tDCS) across brain disease and cognition are diverse, they rely on changes in brain function outlasting stimulation. The cellular mechanisms of DCS leading to brain plasticity have been studied, but the role of astrocytes remains unaddressed. We previously predicted that during tDCS current is concentrated across the blood brain-barrier. This will amplify exposure of endothelial cells (ECs) that form blood vessels and of astrocytes that wrap around them. The objective of this study was to investigate the effect of tDCS on the gene expression by astrocytes or ECs. DCS (0.1 or 1 mA, 10 min) was applied to monolayers of mouse brain ECs or human astrocytes. Gene expression of a set of neuroactive genes were measured using RT-qPCR. Expression was assessed immediately or 1 h after DCS. Because we previously showed that DCS can produce electroosmotic flow and fluid shear stress known to influence EC and astrocyte function, we compared three interventions: pressure-driven flow across the monolayer alone, pressure-driven flow plus DCS, and DCS alone with flow blocked. We show that DCS can directly modulate gene expression in astrocytes (notably FOS and BDNF), independent of but synergistic with pressure-driven flow gene expression. In ECs, pressure-driven flow activates genes expression with no evidence of further contribution from DCS. In ECs, DCS alone produced mixed effects including an upregulation of FGF9 and downregulation of NTF3. We propose a new adjunct mechanism for tDCS based on glial meditated plasticity.

Transcranial direct current stimulation (tDCS) reduces motivation to drink ethanol and reacquisition of ethanol self-administration in female mice

Transcranial direct current stimulation (tDCS) is an emerging noninvasive brain neuromodulation technique aimed at relieving symptoms associated with psychiatric disorders, including addiction. The goal of the present study was to better identify which phase of alcohol-related behavior (hedonic effect, behavioral sensitization, self-administration, or motivation to obtain the drug) might be modulated by repeated anodal tDCS over the frontal cortex (0.2 mA, 20 min, twice a day for 5 consecutive days), using female mice as a model. Our data showed that tDCS did not modulate the hedonic effects of ethanol as assessed by a conditioned place preference test (CPP) or the expression of ethanol-induced behavioral sensitization. Interestingly, tDCS robustly reduced reacquisition of ethanol consumption (50% decrease) following extinction of self-administration in an operant paradigm. Furthermore, tDCS significantly decreased motivation to drink ethanol on a progressive ratio schedule (30% decrease). Taken together, our results show a dissociation between the effects of tDCS on “liking” (hedonic aspect; no effect in the CPP) and “wanting” (motivation; decreased consumption on a progressive ratio schedule). Our tDCS procedure in rodents will allow us to better understand its mechanisms of action in order to accelerate its use as a complementary and innovative tool to help alcohol-dependent patients maintain abstinence or reduce ethanol intake.

Effects of combining retrieval practice and tDCS over long-term memory: A randomized controlled trial

The ability to retain new information is important in daily life. In particular, two techniques have shown promise for improving long-term retention: retrieval practice (RP), which consists of actively retrieving information from long-term memory to make it more accessible in the future; and transcranial direct current stimulation (tDCS), which consists of non-invasive brain stimulation that modulates cognitive processes by increasing and decreasing neuronal excitability. Previous studies have implicated the left dorsolateral prefrontal cortex (l-dlPFC) in memory encoding and memory organization. We examined whether RP associated with a single 20-min tDCS session over the l-dlPFC could improve long-term memory retention. Participants (N = 119) repeatedly studied a list of related words either via RP or via restudy, while undergoing either anodal or sham stimulation. Participants returned 2 days later for a free-recall test. Results showed that the RP group outperformed the restudy group in all measures, regardless of stimulation type. Also, recall organization was higher in the RP group than in the restudy group. The data support previous findings and indicate that RP may enhance performance by improving the organization of the to-be-remembered list items.

The Effect of Adding Transcranial Direct Current Stimulation to Hyperbaric Oxygen Therapy in Patients With Delayed Encephalopathy After Carbon Monoxide Poisoning: A Randomised Controlled Trial

Objective: To investigate the effect of transcranial direct current stimulation (tDCS) combined with hyperbaric oxygen therapy (HBOT) in patients with delayed encephalopathy after carbon monoxide poisoning (DEACMP).

Design: A parallel-group, open-label randomised controlled study.

Setting: Hyperbaric Oxygen Therapy Room of the Second Hospital of Hebei Medical University.

Subjects: A total of 40 patients were recruited for the current study. Patients were randomly divided into a treatment group and a control group (20 cases/group).

Interventions: Control group: conventional, individualised rehabilitation therapy. Treatment group: conventional, individualised rehabilitation therapy and tDCS.

Main Measures: cognitive function of patients, the Barthel Index (BI).

Results: After treatment, significantly higher MMSE and BI scores, as well as a greater reduction in P300 latency and an increase in P300 amplitude, were observed in the treatment group compared to the control group (MMSE: 13 ± 7 vs. 9 ± 5; P300 latency: 342 ± 29 vs. 363 ± 17 ms; P300 amplitude: 7.0 ± 3.3 vs. 5.1 ± 2.7 μV; all P < 0.05). In both groups, however, MMSE and BI scores, in addition to P300 amplitude, were significantly improved; in contrast, there was a decrease in P300 latency in both groups after treatment compared to before treatment (all P < 0.05).

Conclusion: Combined with HBOT, tDCS can help improve cognitive function and ADL in patients with DEACMP. This combination therapy might be a helpful method to enhance the recovery of patients with DEACMP.

Cathodal transcranial direct current stimulation on the prefrontal cortex applied after reactivation attenuates fear memories and prevent reinstatement after extinction

BACKGROUND

In the last decade, pharmacological strategies targeting reconsolidation after memory retrieval have shown promising efforts to attenuate persistent memories and overcome fear recovery. However, most reconsolidation inhibiting agents have not been approved for human testing. While non-invasive neuromodulation can be considered an alternative approach to pharmacological treatments, there is a lack of evidence about the efficacy of these technologies when modifying memory traces via reactivation/reconsolidation mechanism.

OBJECTIVE

In this study, we evaluate the effect of cathodal (c-tDCS) and anodal (a-DCS) transcranial direct current stimulation applied after memory reactivation and extinction in rats.

METHODS

Male Wistar rats were randomly assigned into three groups: one sham group, one anodal tDCS group, and one cathodal tDCS group (500 μA, 20 min). Reconsolidation and extinction of fear memories were evaluated using a contextual fear conditioning.

RESULTS

Our results showed that c-tDCS and a-tDCS after memory reactivation can attenuate mild fear memories. However, only c-tDCS stimulation prevented both fear expression under strong fear learning and fear recovery after a reinstatement protocol without modification of learning rate or extinction retrieval. Nevertheless, the remote memories were resistant to modification through this type of neuromodulation. Our results are discussed considering the interaction between intrinsic excitability promoted by learning and memory retrieval and the electric field applied during tDCS.

CONCLUSION

These results point out some of the boundary conditions influencing the efficacy of tDCS in fear attenuation and open new ways for the development of noninvasive interventions aimed to control fear-related disorders via reconsolidation.

Neuromodulation in the Treatment of Alzheimer's Disease: Current and Emerging Approaches

Neuromodulation as a treatment strategy for psychiatric and neurological diseases has grown in popularity in recent years, with the approval of repetitive transcranial magnetic stimulation (rTMS) for the treatment of depression being one such example. These approaches offer new hope in the treatment of diseases that have proven largely intractable to traditional pharmacological approaches. For this reason, neuromodulation is increasingly being explored for the treatment of Alzheimer's disease. However, such approaches have variable, and, in many cases, very limited evidence for safety and efficacy, with most human evidence obtained in small clinical trials. Here we review work in animal models and humans with Alzheimer's disease exploring emerging neuromodulation modalities. Approaches reviewed include deep brain stimulation, transcranial magnetic stimulation, transcranial electrical stimulation, ultrasound stimulation, photobiomodulation, and visual or auditory stimulation. In doing so, we clarify the current evidence for these approaches in treating Alzheimer's disease and identify specific areas where additional work is needed to facilitate their clinical translation.

Frontoparietal anodal tDCS reduces ketamine-induced oscillopathies

During the prodromal phase of schizophrenia with its complex and insidious clinical picture, electroencephalographic recordings detect widespread oscillation disturbances (or oscillopathies) during the wake-sleep cycle. Neural oscillations are electrobiomarkers of the connectivity state within systems. A single-systemic administration of ketamine, a non-competitive NMDA glutamate receptor antagonist, transiently reproduces the oscillopathies with a clinical picture reminiscent of the psychosis prodrome. This acute pharmacological model may help the research and development of innovative treatments against psychotic transition. Transcranial electrical stimulation is recognized as an appropriate non-invasive therapeutic modality since it can increase cognitive performance and modulate neural oscillations with little or no side effects. Therefore, our objective was to set up, in the sedated adult rat, a stimulation method that is able to normalize ketamine-induced increase in gamma-frequency (30-80 Hz) oscillations and decrease in sigma-frequency (10-17 Hz) oscillations. Unilateral and bipolar frontoparietal (FP), transcranial anodal stimulation by direct current (<+1 mA) was applied in ketamine-treated rats. A concomitant bilateral electroencephalographic recording of the parietal cortex measured the stimulation effects on its spontaneously occurring oscillations. A 5 min FP anodal tDCS immediately and quickly reduced, significantly with an intensity-effect relationship, the ketamine-induced gamma hyperactivity, and sigma hypoactivity at least in the bilateral parietal cortex. A duration effect was also recorded. The tDCS also tended to diminish the ketamine-induced delta hypoactivity. These preliminary neurophysiological findings are promising for developing a therapeutic proof-of-concept against neuropsychiatric disorders.

The Neurostimulation of the Brain in Depression Trial: Protocol for a Randomized Controlled Trial of Transcranial Direct Current Stimulation in Treatment-Resistant Depression

Background

Major depressive disorder (MDD) is the second highest cause of disability worldwide. Standard treatments for MDD include medicine and talk therapy; however, approximately 1 in 5 Canadians fail to respond to these approaches and must consider alternatives. Transcranial direct current stimulation (tDCS) is a safe, noninvasive method that uses electrical stimulation to change the activation pattern of different brain regions. By targeting those regions known to be affected in MDD, tDCS may be useful in ameliorating treatment-resistant depression.

Objective

The objective of the Neurostimulation of the Brain in Depression trial is to compare the effectiveness of active versus sham tDCS in treating patients with ultraresistant MDD. The primary outcome will be the improvement in depressive symptoms, as measured by the change on the Mongtomery-Asberg Depression Rating Scale. Secondary outcomes will include changes in the Quick Inventory of Depressive Symptomatology Scale (subjective assessment), the World Health Organization Disability Assessment Schedule 2.0 (functional assessment), and the Screen for Cognitive Impairment in Psychiatry (cognitive assessment). Adverse events will be captured using the Young Mania Rating Scale; tDCS Adverse Events Questionnaire; Frequency, Intensity, and Burden of Side Effects Rating Scale; and Patient-Rated Inventory of Side Effects Scale. A parallel component of the study will involve assaying for baseline language function and the effect of treatment on language using an exploratory acoustic and semantic corpus analysis on recorded interviews. Participant accuracy and response latency on an auditory lexical decision task will also be evaluated.

Methods

We will recruit inpatients and outpatients in the city of Edmonton, Alberta, and will deliver the study interventions at the Grey Nuns and University of Alberta Hospitals. Written informed consent will be obtained from all participants before enrollment. Eligible participants will be randomly assigned, in a double-blinded fashion, to receive active or sham tDCS, and they will continue receiving their usual pharmacotherapy and psychotherapy throughout the trial. In both groups, participants will receive 30 weekday stimulation sessions, each session being 30 minutes in length, with the anode over the left dorsolateral prefrontal cortex and the cathode over the right. Participants in the active group will be stimulated at 2 mA throughout, whereas the sham group will receive only a brief period of stimulation to mimic skin sensations felt in the active group. Measurements will be conducted at regular points throughout the trial and 30 days after trial completion.

Results

The trial has been approved by the University of Alberta Research Ethics Board and is scheduled to commence in June 2021. The target sample size is 60 participants.

Conclusions

This is a protocol for a multicenter, double-blinded, randomized controlled superiority trial comparing active versus sham tDCS in patients with treatment-resistant MDD.

Trial Registration

ClinicalTrials.gov NCT04159012; http://clinicaltrials.gov/ct2/show/NCT04159012.

International Registered Report Identifier (IRRID)

PRR1-10.2196/22805

Efficacy and neurophysiological predictors of treatment response of adjunct bifrontal transcranial direct current stimulation (tDCS) in treating unipolar and bipolar depression

BACKGROUND

Although add-on transcranial direct current stimulation (tDCS) is a promising intervention for treating unipolar (UD) and bipolar depression (BD), its moderate antidepressant efficacy urges research into biomarkers for predicting therapeutic response and achieving highly targeted applications.

METHODS

This open-label trial enrolled UD (N=58) and BD (N=22) patients who had failed 1 or more trials of adequate pharmacologic interventions (ClinicalTrials.gov ID: NCT03287037). Bifrontal tDCS (anode/cathode: F3/F4) was applied using a 2 mA current for 20 min, twice daily, for 5 consecutive weekdays. Depression was measured with Hamilton Depression Rating Scale-17 (HAMD) at baseline, after 10-session stimulation, 1- and 4-week follow-ups. Heart rate (HR) and heart rate variability (HRV) was measured at baseline, during the initial 5 min of the 1^st session, after 10-session stimulation, 1- and 4-week follow-ups. Cognitive performance and other outcomes were also assessed.

RESULTS

Bifrontal tDCS rapidly and equally improved depression in both groups. The effects persisted until the end of the trial. Both groups had similar improvements in cognitive performance, anxiety, and psychosocial functioning. Compared with baseline, increased vagally-mediated HRV was observed one month after tDCS for both groups. A positive correlation was found between HR deceleration within the 1^st session and treatment response after 10-session tDCS only among UD patients, explaining 20% of the variance.

CONCLUSION

tDCS as an adjunct therapy is effective for both UD and BD. Data suggest that the greater the increase in parasympathetic signaling during the 1^st session, the better the clinical response after 10-session tDCS for UD patients.

The effects of lithium chloride and cathodal/anodal transcranial direct current stimulation on conditional fear memory changes and the level of p-mTOR/mTOR in PFC of male NMRI mice

Lithium chloride clinically used to treat mental diseases but it has some side effects like cognitive impairment, memory deficit. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that is able to change neural activity and gene transcription in the brain. The aim of the study is to provide a conceptual theoretical framework based on behavioral and molecular effects of tDCS on memory changes induced by lithium in male mice. we applied Anodal-tDCS and Cathodal-tDCS over the left PFC for 3 consecutive days tDCS for 20 min with 2 mA after injection of different doses of lithium/saline.Trained in fear condition and finally the day after that tested their memory persistency factors (freezing-latency) and other behavior such as grooming and rearing percentage time in the fear conditioning. P-mTOR/mTOR was analyzed using western blotting. The results obtained from the preliminary analysis of behavioral fear memory showed that lithium had destructive effect in higher doses and decreased freezing percentage time. However, both cathodal and anodal tDCS significantly improved memory and increased P-mTOR/mTOR level in the PFC. The results of this study indicate that cathodal and anodal tDCS upon the left prefrontal increased memory and reduced lithium side effects on memory consolidation and altered expression of plasticity-associated genes in the prefrontal cortex.

Effects of repeated anodal transcranial direct current stimulation on auditory fear extinction in C57BL/6J mice

BACKGROUND

Trauma-based psychotherapy is a first line treatment for post-traumatic stress disorder (PTSD) but not all patients achieve long-term remission. Transcranial direct current stimulation (tDCS) received considerable attention as a neuromodulation method that may improve trauma-based psychotherapy.

OBJECTIVE

We explored the effects of repeated anodal tDCS over the prefrontal cortex (PFC) on fear extinction in mice as a preclinical model for trauma-based psychotherapy.

METHODS

We performed auditory fear conditioning with moderate or high shock intensity on C57BL6/J mice. Next, mice received anodal tDCS (0.2 mA, 20 min) or sham stimulation over the PFC twice daily for five consecutive days. Extinction training was performed by repeatedly exposing mice to the auditory cue the day after the last stimulation session. Early and late retention of extinction were evaluated one day and three weeks after extinction training respectively.

RESULTS

We observed no significant effect of tDCS on the acquisition or retention of fear extinction in mice subjected to fear conditioning with moderate intensity. However, when the intensity of fear conditioning was high, tDCS significantly lowered freezing during the acquisition of extinction, regardless of the extinction protocol. Moreover, when tDCS was combined with a strong extinction protocol, we also observed a significant improvement of early extinction recall. Finally, we found that tDCS reduced generalized fear induced by contextual cues when the intensity of conditioning is high and extinction training limited.

CONCLUSIONS

Our data provide a rationale to further explore anodal tDCS over the PFC as potential support for trauma-based psychotherapy for PTSD.

Prior cortical activity differences during an action observation plus motor imagery task related to motor adaptation performance of a coordinated multi-limb complex task

Motor adaptation is the ability to develop new motor skills that makes performing a consolidated motor task under different psychophysical conditions possible. There exists a proven relationship between prior brain activity at rest and motor adaptation. However, the brain activity at rest is highly variable both between and within subjects. Here we hypothesize that the cortical activity during the original task to be later adapted is a more reliable and stronger determinant of motor adaptation. Consequently, we present a study to find cortical areas whose activity, both at rest and during first-person virtual reality simulation of bicycle riding, characterizes the subjects who did and did not adapt to ride a reverse steering bicycle, a complex motor adaptation task involving all limbs and balance. The results showed that cortical activity differences during the simulated task were higher, more significant, spatially larger, and spectrally wider than at rest for good performers. In this sense, the activity of the left anterior insula, left dorsolateral and ventrolateral inferior prefrontal areas, and left inferior premotor cortex (action understanding hub of the mirror neuron circuit) during simulated bicycle riding are the areas with the most descriptive power for the ability of adapting the motor task. Trials registration Trial was registered with the NIH Clinical Trials Registry (clinicaltrials.gov), with the registration number NCT02999516 (21/12/2016).

Open-tES: An open-source stimulator for transcranial electrical stimulation designed for rodent research

Non-invasive neuromodulatory techniques, including transcranial direct current stimulation (tDCS), have been shown to modulate neuronal function and are used both in cognitive neuroscience and to treat neuropsychiatric conditions. In this context, animal models provide a powerful tool to identify the neurobiological mechanisms of action of tDCS. However, finding a current generator that is easily usable and which allows a wide range of stimulation parameters can be difficult and/or expensive. Here, we introduce the Open-tES device, a project under a Creative Commons License (CC BY, SA 4.0) shared on the collaborative platform Git-Hub. This current generator allows tDCS (and other kinds of stimulations) to be realized, is suitable for rodents, is easy to use, and is low-cost. Characterization has been performed to measure the precision and accuracy of the current delivered. We also aimed to compare its effects with a commercial stimulator used in clinical trials (DC-Stimulator Plus, NeuroConn, Germany). To achieve this, a behavioral study was conducted to evaluate its efficacy for decreasing depression related-behavior in mice. The stimulator precision and accuracy were better than 250 nA and 25 nA, respectively. The behavioral evaluation performed in mice in the present study did not reveal any significant differences between the commercial stimulator used in clinical trials and the Open-tES device. Accuracy and precision of the stimulator ensure high repeatability of the stimulations. This current generator constitutes a reliable and inexpensive tool that is useful for preclinical studies in the field of non-invasive electrical brain stimulation.

Theory of Mind Deficits and Neurophysiological Operations in Autism Spectrum Disorders: A Review

Theory of Mind (ToM) is a multifaceted skill set which encompasses a variety of cognitive and neurobiological aspects. ToM deficits have long been regarded as one of the most disabling features in individuals with Autism Spectrum Disorder. One of the theories that attempts to account for these impairments is that of “broken mirror neurons”. The aim of this review is to present the most recent available studies with respect to the connection between the function of mirror neurons in individuals with ASD and ToM-reflecting sensorimotor, social and attentional stimuli. The majority of these studies approach the theory of broken mirror neurons critically. Only studies from the last 15 years have been taken into consideration. Findings from electroencephalography (EEG) studies so far indicate that further research is necessary to shed more light on the mechanisms underlying the connection(s) between ToM and neurophysiological operations.

Contralesional Application of Transcranial Direct Current Stimulation on Functional Improvement in Ischemic Stroke Mice

BACKGROUND AND PURPOSE

The therapeutic use of transcranial direct current stimulation (tDCS), an adjuvant tool for stroke, induces long-term changes in cortical excitability, for example, the secretion of activity-dependent growth factors. We assessed the proper therapeutic configuration of high-definition tDCS (HD-tDCS) in the subacute stage of ischemic stroke and its underlying expression profiling of growth factors to propose a new method for ensuring better therapeutic effects.

METHODS

Male C57BL/6J mice were subjected to middle cerebral artery occlusion, after which repetitive HD-tDCS (20 minutes, 55 µA/mm², charge density 66 000 C/m²) was applied from subacute phases of their ischemic insult. Behavioral tests assessing motor and cognitive functions were used to determine suitable conditions and HD-tDCS stimulation sites. Gene expression profiling of growth factors and their secretion and activation were analyzed to shed light on the underlying mechanisms.

RESULTS

Anodal HD-tDCS application over the contralesional cortex, especially the motor cortex, was more effective than ipsilesional stimulation in attenuating motor and cognitive deficits. In the HD-tDCS application over the contralesional motor cortex, positive changes in Bmp8b, Gdf5, Il4, Pdgfa, Pgf, and Vegfb were observed in the ipsilesional site. The expression of GDF5 (growth/differentiation factor 5) and PDGFA (platelet-derived growth factor subunit A) tended to similarly increase in both ipsi- and contralesional striata. However, higher expression levels of GDF5 and PDGFA and their receptors were observed in the peri-infarct regions of the striatum after HD-tDCS, especially in PDGFA expression. A higher number of proliferating or newly formed neuronal cells was detected in ipsilesional sites such as the subventricular zone.

CONCLUSIONS

Application of anodal HD-tDCS over the contralesional cortex may enhance beneficial recovery through the expression of growth factors, such as GDF5 and PDGFA, in the ipsilesional site. Therefore, this therapeutic configuration may be applied in the subacute stage of ischemic stroke to ameliorate neurological impairments.

Effect of transcranial direct current stimulation on sports performance for two profiles of athletes (power and endurance) (COMPETE): a protocol for a randomised, crossover, double blind, controlled exploratory trial

BACKGROUND

Transcranial direct current stimulation (tDCS) is promising for improving motor and cognitive performance. Nevertheless, its mechanisms of action are unclear and need to be better characterised according to the stimulated brain area and the type of exercise performed.

METHODS/DESIGN

This is a double-blind crossover study, organised into two parts: the first is to assess the effects of tDCS on explosive performance (jump task) and the second is to assess the effects on endurance performance (cycling time trial task). Participants, who are recreationally active or athletes (parkour practitioners, cyclists), will receive two active tDCS sessions (over the left dorsolateral prefrontal cortex and right motor cortex) and one sham tDCS session (part A), or two sequences (one active and one sham) of two daily tDCS sessions over 5 days (part B). Motor and cognitive performance will be compared before and after tDCS sessions (part A), and before and after the first session, after the last session and at day 12 and day 30 of each tDCS sequence (part B).

DISCUSSION

This study investigates the acute and repeated effects of tDCS on the motor and cognitive performance of healthy subjects. It will try to evaluate if tDCS could be considered as a neuroenhancement technology according to the physical task investigated (endurance versus explosive).

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03937115. Registered on 3 May 2019; retrospectively registered.

Electricity, Neurology, and Noninvasive Brain Stimulation: Looking Back, Looking Ahead

Electricity and neurology evolved synchronously over the past few centuries. This article looks at their origins and their journey into noninvasive brain stimulation technique of transcranial direct current stimulation (tDCS), which is now popular in neuroscience research.

Can transcranial direct current stimulation (tDCS) improve impulsivity in healthy and psychiatric adult populations? A systematic review

Impulsivity is a multidimensional phenomenon that remains hard to define. It compounds the core pathological construct of many neuropsychiatric illnesses, and despite its close relation to suicide risk, it currently has no specific treatment. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique whose application results in cognitive function improvement, both in healthy and psychiatric populations. Following PRISMA recommendations, a systematic review of the literature concerning tDCS's effects on impulsive behaviour was performed using the PubMed database. The research was based on the combination of the keyword 'tDCS' with 'impulsivity', 'response inhibition', 'risk-taking', 'planning', 'delay discounting' or 'craving'. The initial search yielded 309 articles, 92 of which were included. Seventy-four papers demonstrated improvement in task performance related to impulsivity in both healthy and clinical adult populations. However, results were often inconsistent. The conditions associated with improvement, such as tDCS parameters and other aspects that may influence tDCS's outcomes, are discussed. The overall effects of tDCS on impulsivity are promising. Yet further research is required to develop a more comprehensive understanding of impulsivity, allowing for a more accurate assessment of its behavioural outcomes as well as a definition of tDCS therapeutic protocols for impulsive disorders.

Neuromodulatory Effects of Transcranial Direct Current Stimulation on Motor Excitability in Rats

Transcranial direct current stimulation (tDCS) is a noninvasive technique for modulating neural plasticity and is considered to have therapeutic potential in neurological disorders. For the purpose of translational neuroscience research, a suitable animal model can be ideal for providing a stable condition for identifying mechanisms that can help to explore therapeutic strategies. Here, we developed a tDCS protocol for modulating motor excitability in anesthetized rats. To examine the responses of tDCS-elicited plasticity, the motor evoked potential (MEP) and MEP input-output (IO) curve elicited by epidural motor cortical electrical stimulus were evaluated at baseline and after 30 min of anodal tDCS or cathodal tDCS. Furthermore, a paired-pulse cortical electrical stimulus was applied to assess changes in the inhibitory network by measuring long-interval intracortical inhibition (LICI) before and after tDCS. In the results, analogous to those observed in humans, the present study demonstrates long-term potentiation- (LTP-) and long-term depression- (LTD-) like plasticity can be induced by tDCS protocol in anesthetized rats. We found that the MEPs were significantly enhanced immediately after anodal tDCS at 0.1 mA and 0.8 mA and remained enhanced for 30 min. Similarly, MEPs were suppressed immediately after cathodal tDCS at 0.8 mA and lasted for 30 min. No effect was noted on the MEP magnitude under sham tDCS stimulation. Furthermore, the IO curve slope was elevated following anodal tDCS and presented a trend toward diminished slope after cathodal tDCS. No significant differences in the LICI ratio of pre- to post-tDCS were observed. These results indicated that developed tDCS schemes can produce consistent, rapid, and controllable electrophysiological changes in corticomotor excitability in rats. This newly developed tDCS animal model could be useful to further explore mechanical insights and may serve as a translational platform bridging human and animal studies, establishing new therapeutic strategies for neurological disorders.

Effects of tDCS of Dorsolateral Prefrontal Cortex on Dual-Task Performance Involving Manual Dexterity and Cognitive Task in Healthy Older Adults

Healthy aging limits the activities of daily living and personal independence. Furthermore, cognitive-motor interference in dual-task (e.g., walking while talking) appears to be more pronounced in the elderly. Transcranial direct current stimulation (tDCS), a form of the non-invasive brain stimulation technique, is known to modify cortical excitability and has been investigated as a tool for enhancing motor and cognitive performance in health and disease. The present study examined whether tDCS targeting the dorsolateral prefrontal cortex (DLPFC) could improve dual-task performance in healthy older adults. The effects of tDCS, among other factors, depend on stimulation polarity (anodal vs. cathodal), electrode setup (unilateral vs. bilateral) and the time of application (off-line vs. on-line). We therefore explored the effects of unilateral and simultaneous bilateral tDCS (anodal and cathodal) of left DLPFC while performing (on-line) the Grooved Pegboard Test (GPT) and Serial Seven Subtraction Test (SSST) alone or together (dual-tasking). The number of pegs and the number of correct subtractions were recorded before, during and 30 min after tDCS. The dual-task performance was measured as the percent change from single- to the dual-task condition (dual-task cost DTC). Only bilateral, anode left tDCS, induced a significant increase in subtracted numbers while dual-tasking, i.e., it reduced the DTC of manual dexterity (GPT) to a cognitive task. Significant changes 30 min after the stimulation were only present after bilateral anode right (BAR) tDCS on GPT dual-task costs. These findings suggest that anodal tDCS applied on-line interacts with a dual-task performance involving demanding cognitive and manual dexterity tasks. The results support the potential use of non-invasive brain stimulation for improvement of cognitive functioning in daily activities in older individuals.

Transcranial direct current stimulation (tDCS) in elderly with mild cognitive impairment: A pilot study

Transcranial direct current stimulation (tDCS) is a non-invasive, painless and easy-to use-technology. It can be used in depression, schizophrenia and other neurological disorders. There are no studies about longer usage protocols regarding the ideal duration and weekly frequency of tDCS.

OBJECTIVE

to study the use of tDCS twice a week for longer periods to improve memory in elderly with MCI.

METHODS

a randomized double-blind controlled trial of anodal tDCS on cognition of 58 elderly aged over 60 years was conducted. A current of 2.0 mA was applied for 30 minutes for 10 sessions, twice a week. The anode was placed over the left dorsolateral prefrontal cortex (LDLFC). Subjects were evaluated before and after 10 sessions by the following tests: CAMCOG, Mini-Mental State Examination (MMSE), Trail Making, Semantic Verbal Fluency (Animals), Boston naming, Clock Drawing Test, Word list memory (WLMT), Direct and Indirect Digit Order (WAIS-III and WMS-III) and N-back.

RESULTS

After 10 sessions of tDCS, significant group-time interactions were found for the CAMCOG - executive functioning (χ2 = 3.961, p = 0.047), CAMCOG - verbal fluency (χ2 = 3.869, p = 0.049), CAMCOG - Memory recall (χ2 = 9.749, p = 0.004), and WMLT - recall (χ2 = 7.254, p = 0.007). A decline in performance on the CAMCOG - constructional praxis (χ2 = 4.371, p = 0.037) was found in the tDCS group after intervention. No significant differences were observed between the tDCS and Sham groups for any other tasks.

CONCLUSION

tDCS at 2 mA for 30 min twice a week over 5 consecutive weeks proved superior to placebo (Sham) for improving memory recall, verbal fluency and executive functioning in elderly with MCI.

Neurotechnologies for Human Cognitive Augmentation: Current State of the Art and Future Prospects

Recent advances in neuroscience have paved the way to innovative applications that cognitively augment and enhance humans in a variety of contexts. This paper aims at providing a snapshot of the current state of the art and a motivated forecast of the most likely developments in the next two decades. Firstly, we survey the main neuroscience technologies for both observing and influencing brain activity, which are necessary ingredients for human cognitive augmentation. We also compare and contrast such technologies, as their individual characteristics (e.g., spatio-temporal resolution, invasiveness, portability, energy requirements, and cost) influence their current and future role in human cognitive augmentation. Secondly, we chart the state of the art on neurotechnologies for human cognitive augmentation, keeping an eye both on the applications that already exist and those that are emerging or are likely to emerge in the next two decades. Particularly, we consider applications in the areas of communication, cognitive enhancement, memory, attention monitoring/enhancement, situation awareness and complex problem solving, and we look at what fraction of the population might benefit from such technologies and at the demands they impose in terms of user training. Thirdly, we briefly review the ethical issues associated with current neuroscience technologies. These are important because they may differentially influence both present and future research on (and adoption of) neurotechnologies for human cognitive augmentation: an inferior technology with no significant ethical issues may thrive while a superior technology causing widespread ethical concerns may end up being outlawed. Finally, based on the lessons learned in our analysis, using past trends and considering other related forecasts, we attempt to forecast the most likely future developments of neuroscience technology for human cognitive augmentation and provide informed recommendations for promising future research and exploitation avenues.

Long-Term Transcranial Direct Current Stimulation Does Not Improve Executive Function in Healthy Older Adults

Background: Executive function tends to decline as people age. Transcranial direct current stimulation (tDCS) is assumed to have beneficial effects on various cognitive functions. Some prior investigations have shown that repeated sessions of tDCS enhance the executive function performance of healthy elderly people by mediating cognitive training gains. However, studies of the effect of long-term stimulation on executive function without cognitive training are absent. Objective: The purpose of this study was to explore whether the executive function of healthy older adults could be enhanced with long-term tDCS alone applied on the prefrontal cortex. Methods: Sixty-five cognitively normal older adults were enrolled and randomly assigned to two groups: an anodal tDCS group and a sham tDCS group. The participants in the two groups received anodal stimulation or sham stimulation over the left dorsolateral prefrontal lobe, for 30 min per day for 10 consecutive days. Executive function was tested before stimulation, immediately after stimulation and 3 months after stimulation. Three core components of executive function were tested using a two-back task for updating, a flanker task for inhibition, and a switching task for shifting. Results: Across the three tasks, we failed to discover any differences between the anodal and sham stimulation. Moreover, we found no statistically significant stimulation effect in the follow-up session. Conclusion: Our study does not support the assumption that multiple sessions of tDCS that are independent of cognitive training have a beneficial effect on executive function in healthy older adults, presumably because the effect of the stimulation lies in its amplification of training gains. It indicates that combining traditional cognitive training methods with brain stimulation may be a better approach to improve older adults' executive function.

No Evidence That Baseline Prefrontal Cortical Excitability (3T-MRS) Predicts the Effects of Prefrontal tDCS on WM Performance

Transcranial direct current stimulation (tDCS) over the left dorsolateral prefrontal cortex (lDLPFC) is a promising tool to enhance working memory (WM) in clinical as well as healthy populations. Yet, tDCS does not affect everyone similarly: whereas tDCS improves WM in most individuals, some individuals do not, or actually show detriments in WM performance after stimulation. One hypothesis that has been put forward to account for individual differences in tDCS response is that baseline cortical excitability levels in the stimulated cortex may determine the strength and the direction of the effects of tDCS. Specifically, by locally affecting neuronal excitability, tDCS may interact with baseline cortical excitability levels, thereby pushing or pulling individuals toward or away from an optimal level of cortical functioning. In the current study, we put this hypothesis to the test with regard to prefrontal cortex stimulation and WM. In 20 healthy male participants, using magnetic resonance spectroscopy (MRS) at 3T, we measured concentrations of Glutamate and GABA in the lDLPFC and calculated individual Glutamate/GABA ratios as a measure for cortical excitability. Subsequently, in two stimulation sessions, we once applied anodal and once cathodal tDCS over the lDLPFC (20 min, 1 mA). Stimulation was always applied in the second block of three blocks of a WM updating task. Surprisingly, at the group-level, we found no effects of anodal or cathodal stimulation on WM performance. Yet, in line with previous studies, large individual variability was observed in the strength and direction of tDCS effects; whereas about half of the participants improved, the other half showed lower accuracy after stimulation. This was true for both anodal and cathodal tDCS. Nevertheless, contrary to our expectations, individual baseline prefrontal cortical excitability did not predict these individual differences in the effect of anodal or cathodal stimulation on WM accuracy. Future studies with larger sample sizes, which use higher magnetic field strengths (e.g., 7T) to measure cortical excitability and/or apply individualized stimulation protocols, are necessary to shed more light on the influence of baseline cortical excitability on effects of anodal and cathodal tDCS over lDLPFC on WM performance.

Role of BDNF Signaling in Memory Enhancement Induced by Transcranial Direct Current Stimulation

In the recent years numerous studies have provided encouraging results supporting the use of transcranial direct current stimulation (tDCS) as non-invasive brain stimulation technique to improve motor and cognitive functions in patients suffering from neurological and neuropsychiatric disorders as well as in healthy subjects. Among the multiple effects elicited by tDCS on cognitive functions, experimental evidence and clinical findings have highlighted the beneficial impact on long-term memory. Memory deficits occur during physiological aging as well as in neurological and neurodegenerative disorders, including Alzheimer’s disease (AD). In this scenario, non-invasive techniques for memory enhancement, such as tDCS, are receiving increasing attention. The knowledge of molecular mechanisms subtending tDCS effects is of pivotal importance for a more rationale use of this technique in clinical settings. Although we are still far from having a clear picture, recent literature on human and animal studies has pointed to the involvement of synaptic plasticity mechanisms in mediating tDCS effects on long-term memory. Here we review these studies focusing on the neurotrophin “brain-derived neurotrophic factor” (BDNF) as critical tDCS effector.

Anisotropic conductivity tensor imaging for transcranial direct current stimulation (tDCS) using magnetic resonance diffusion tensor imaging (MR-DTI)

Transcranial direct current stimulation (tDCS) is a widely used non-invasive brain stimulation technique by applying low-frequency weak direct current via electrodes attached on the head. The tDCS using a fixed current between 1 and 2 mA has relied on computational modelings to achieve optimal stimulation effects. Recently, by measuring the tDCS current induced magnetic field using an MRI scanner, the internal current pathway has been successfully recovered. However, up to now, there is no technique to visualize electrical properties including the electrical anisotropic conductivity, effective extracellular ion-concentration, and electric field using only the tDCS current in-vivo. By measuring the apparent diffusion coefficient (ADC) and the magnetic flux density induced by the tDCS, we propose a method to visualize the electrical properties. We reconstruct the scale parameter, which connects the anisotropic conductivity tensor to the diffusion tensor of water molecules, by introducing a repetitive scheme called the diffusion tensor J-substitution algorithm using the recovered current density and the measured ADCs. We investigate the proposed method to explain why the iterative scheme converges to the internal conductivity. We verified the proposed method with an anesthetized canine brain to visualize electrical properties including the electrical properties by tDCS current.

Transcranial Direct Current Stimulation (tDCS): A Promising Treatment for Major Depressive Disorder?

Background: Transcranial direct current stimulation (tDCS) opens new perspectives in the treatment of major depressive disorder (MDD), because of its ability to modulate cortical excitability and induce long-lasting effects. The aim of this review is to summarize the current status of knowledge regarding tDCS application in MDD. Methods: In this review, we searched for articles published in PubMed/MEDLINE from the earliest available date to February 2018 that explored clinical and cognitive effects of tDCS in MDD. Results: Despite differences in design and stimulation parameters, the examined studies indicated beneficial effects of tDCS for MDD. These preliminary results, the non-invasiveness of tDCS, and its good tolerability support the need for further research on this technique. Conclusions: tDCS constitutes a promising therapeutic alternative for patients with MDD, but its place in the therapeutic armamentarium remains to be determined.

Using animal models to improve the design and application of transcranial electrical stimulation in humans

Purpose of Review

Transcranial electrical stimulation (tES) is a non-invasive stimulation technique used for modulating brain function in humans. To help tES reach its full therapeutic potential, it is necessary to address a number of critical gaps in our knowledge. Here, we review studies that have taken advantage of animal models to provide invaluable insight about the basic science behind tES.

Recent Findings

Animal studies are playing a key role in elucidating the mechanisms implicated in tES, defining safety limits, validating computational models, inspiring new stimulation protocols, enhancing brain function and exploring new therapeutic applications.

Summary

Animal models provide a wealth of information that can facilitate the successful utilization of tES for clinical interventions in human subjects. To this end, tES experiments in animals should be carefully designed to maximize opportunities for applying discoveries to the treatment of human disease.

Repeated anodal transcranial direct current stimulation induces neural plasticity-associated gene expression in the rat cortex and hippocampus

BACKGROUND

Anodal transcranial direct current stimulation (A-tDCS) induces a long-lasting increase in cortical excitability that can increase gene transcription in the brain.

OBJECTIVE

The purpose of this study was to evaluate the expression of genes related to activity-dependent neuronal plasticity in the sensorimotor cortex and hippocampus of young Sprague-Dawley rats following A-tDCS.

METHODS

We applied A-tDCS over the right sensorimotor cortex epicranially with a circular electrode (3 mm diameter) at 250 μA for 20 min per day for 7 consecutive days. Levels of mRNA for brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), synapsin I, Ca2+/calmodulin-dependent protein kinase II (CaMKII), activity-regulated cytoskeleton-associated protein (Arc), and c-Fos were analyzed using SYBR Green quantitative real-time polymerase chain reaction (PCR).

RESULTS

We found that 7 days of unilateral A-tDCS resulted in significant increases in transcription of all plasticity-related genes tested in the ipsilateral cortex. Daily A-tDCS also resulted in a significant increase in c-Fos mRNA in the ipsilateral hippocampus.

CONCLUSION

These results indicate that altered expression of plasticity-associated genes in the cortex and hippocampus is a molecular substrate of A-tDCS-induced neural plasticity.

No Effects of Non-invasive Brain Stimulation on Multiple Sessions of Object-Location-Memory Training in Healthy Older Adults

Object-location memory (OLM) is known to decline with normal aging, a process accelerated in pathological conditions like mild cognitive impairment (MCI). In order to maintain cognitive health and to delay the transition from healthy to pathological conditions, novel strategies are being explored. Tentative evidence suggests that combining cognitive training and anodal transcranial direct current stimulation (atDCS), both reported to induce small and often inconsistent behavioral improvements, could generate larger or more consistent improvements or both, compared to each intervention alone. Here, we explored the combined efficacy of these techniques on OLM. In a subject-blind sham-controlled cross-over design 32 healthy older adults underwent a 3-day visuospatial training paired with either anodal (20 min) or sham (30 s) atDCS (1 mA, temporoparietal). Subjects were asked to learn the correct object-location pairings on a street map, shown over five learning blocks on each training day. Acquisition performance was assessed by accuracy on a given learning block in terms of percentage of correct responses. Training success (performance on last training day) and delayed memory after 1-month were analyzed by mixed model analysis and were controlled for gender, age, education, sequence of stimulation and baseline performance. Exploratory analysis of atDCS effects on within-session (online) and between-session (offline) memory performance were conducted. Moreover, transfer effects on similar trained (visuospatial) and less similar (visuo-constructive, verbal) untrained memory tasks were explored, both immediately after training, and on follow-up. We found that atDCS paired with OLM-training did not enhance success in training or performance in 1-month delayed memory or transfer tasks. In sum, this study did not support the notion that the combined atDCS-training approach improves immediate or delayed OLM in older adults. However, specifics of the experimental design, and a non-optimal timing of atDCS between sessions might have masked beneficial effects and should be more systematically addressed in future studies.

No effect of transcranial direct current stimulation of the dorsolateral prefrontal cortex on short-term memory

INTRODUCTION

Short-term memory refers to the capacity for holding information in mind for a short period of time with conscious memorization. It is an important ability for daily life and is impaired in several neurological and psychiatric disorders. Anodal transcranial direct current stimulation (tDCS) applied to the dorsolateral prefrontal cortex (DLPFC) was reported to enhance the capability of short-term memory in healthy subjects. However, results were not consistent and what is the possible impact factor is not known. One important factor that may significantly influence the effect of tDCS is the timing of tDCS administration.

AIMS

In order to explore whether tDCS impact short-term memory and the optimal timing of tDCS administration, we applied anodal tDCS to the left DLPFC to explore the modulatory effect of online and off-line tDCS on digit span as well as visual short-term memory performance in healthy subjects.

RESULTS

Results showed tDCS of the left DLPFC did not influence intentional digit span memory performance, whether before the task or during the task. In addition, tDCS of the DLPFC administered before the task showed no effect on visual short-term memory, while there was a trend of increase in false alarm when tDCS of the DLPFC administered during the task.

CONCLUSIONS

These results did not provide evidence for the enhancement of short-term memory by tDCS of the left DLPFC in healthy subjects, but it suggested an importance of administration time for visual short-term memory. Further studies are required to taking into account the baseline performance of subjects and time-dependence feature of tDCS.

Transcranial direct current stimulation produces long-lasting attenuation of cocaine-induced behavioral responses and gene regulation in corticostriatal circuits

Transcranial direct current stimulation (tDCS) is a non-invasive method to modulate cortical excitability. This technique is a promising emerging tool to treat several neuropathologies, including addiction. We have previously shown in mice that repeated tDCS normalizes pathological behaviors associated with chronic nicotine exposure. Here, we evaluated, in adult female mice, the impact of tDCS on cocaine-induced behavior and gene regulation in corticostriatal circuits implicated in psychostimulant addiction. Anodal tDCS was applied transcranially over the frontal cortex. Three weeks after repeated tDCS, we investigated the induction of a gene expression marker (Zif268) by cocaine (25 mg/kg) in 26 cortical and 23 striatal regions using in situ hybridization histochemistry. We also assessed place preference conditioning by cocaine (5, 10 and 25 mg/kg). tDCS pretreatment increased basal expression and attenuated cocaine (25 mg/kg)-induced expression of Zif268 in specific corticostriatal circuits. Cocaine-induced locomotor activation (25 mg/kg) and place preference conditioning (5 and 25 mg/kg) were also reduced. These results demonstrate that tDCS can attenuate molecular and behavioral responses to cocaine for several weeks. Together, our findings provide pre-clinical evidence that such electrical brain stimulation may be useful to modify the psychostimulant addiction risk.

The effect of left frontal transcranial direct-current stimulation on propranolol-induced fear memory acquisition and consolidation deficits

Accumulating evidence supports the efficacy of transcranial direct current stimulation (tDCS) in modulating numerous cognitive functions. Despite the fact that tDCS has been used for the enhancement of memory and cognition, very few animal studies have addressed its impact on the modulation of fear memory. This study was designed to determine whether pre/post-training frontal tDCS application would alter fear memory acquisition and/or consolidation deficits induced by propranolol in NMRI mice. Results indicated that administration of β1-adrenoceptor blocker propranolol (0.1mg/kg) impaired fear memory retrieval. Pre/post-training application of anodal tDCS when propranolol was administered prior to training reversed contextual memory retrieval whereas only the anodal application prior to training could induce the same result in the auditory test. Meanwhile, anodal stimulation had no effect on fear memories by itself. Moreover, regardless of when cathode was applied and propranolol administered, their combination restored contextual memory retrieval, while only cathodal stimulation prior to training facilitated the contextual memory retrieval. Also, auditory memory retrieval was restored when cathodal stimulation and propranolol occurred prior to training but it was abolished when stimulation occurred after training and propranolol was administered prior to training. Collectively, our findings show that tDCS applied on the left frontal cortex of mice affects fear memory performance. This alteration seems to be task-dependent and varies depending on the nature and timing of the stimulation. In certain conditions, tDCS reverses the effect of propranolol. These results provide initial evidence to support the timely use of tDCS for the modulation of fear-related memories.

Boosting Cognition: Effects of Multiple-Session Transcranial Direct Current Stimulation on Working Memory

Transcranial direct current stimulation (tDCS) is a promising tool for neurocognitive enhancement. Several studies have shown that just a single session of tDCS over the left dorsolateral pFC (lDLPFC) can improve the core cognitive function of working memory (WM) in healthy adults. Yet, recent studies combining multiple sessions of anodal tDCS over lDLPFC with verbal WM training did not observe additional benefits of tDCS in subsequent stimulation sessions nor transfer of benefits to novel WM tasks posttraining. Using an enhanced stimulation protocol as well as a design that included a baseline measure each day, the current study aimed to further investigate the effects of multiple sessions of tDCS on WM. Specifically, we investigated the effects of three subsequent days of stimulation with anodal (20 min, 1 mA) versus sham tDCS (1 min, 1 mA) over lDLPFC (with a right supraorbital reference) paired with a challenging verbal WM task. WM performance was measured with a verbal WM updating task (the letter n-back) in the stimulation sessions and several WM transfer tasks (different letter set n-back, spatial n-back, operation span) before and 2 days after stimulation. Anodal tDCS over lDLPFC enhanced WM performance in the first stimulation session, an effect that remained visible 24 hr later. However, no further gains of anodal tDCS were observed in the second and third stimulation sessions, nor did benefits transfer to other WM tasks at the group level. Yet, interestingly, post hoc individual difference analyses revealed that in the anodal stimulation group the extent of change in WM performance on the first day of stimulation predicted pre to post changes on both the verbal and the spatial transfer task. Notably, this relationship was not observed in the sham group. Performance of two individuals worsened during anodal stimulation and on the transfer tasks. Together, these findings suggest that repeated anodal tDCS over lDLPFC combined with a challenging WM task may be an effective method to enhance domain-independent WM functioning in some individuals, but not others, or can even impair WM. They thus call for a thorough investigation into individual differences in tDCS respondence as well as further research into the design of multisession tDCS protocols that may be optimal for boosting cognition across a wide range of individuals.

A Neurophysiological Perspective on a Preventive Treatment against Schizophrenia Using Transcranial Electric Stimulation of the Corticothalamic Pathway

Schizophrenia patients are waiting for a treatment free of detrimental effects. Psychotic disorders are devastating mental illnesses associated with dysfunctional brain networks. Ongoing brain network gamma frequency (30–80 Hz) oscillations, naturally implicated in integrative function, are excessively amplified during hallucinations, in at-risk mental states for psychosis and first-episode psychosis. So, gamma oscillations represent a bioelectrical marker for cerebral network disorders with prognostic and therapeutic potential. They accompany sensorimotor and cognitive deficits already present in prodromal schizophrenia. Abnormally amplified gamma oscillations are reproduced in the corticothalamic systems of healthy humans and rodents after a single systemic administration, at a psychotomimetic dose, of the glutamate N-methyl-d-aspartate receptor antagonist ketamine. These translational ketamine models of prodromal schizophrenia are thus promising to work out a preventive noninvasive treatment against first-episode psychosis and chronic schizophrenia. In the present essay, transcranial electric stimulation (TES) is considered an appropriate preventive therapeutic modality because it can influence cognitive performance and neural oscillations. Here, I highlight clinical and experimental findings showing that, together, the corticothalamic pathway, the thalamus, and the glutamatergic synaptic transmission form an etiopathophysiological backbone for schizophrenia and represent a potential therapeutic target for preventive TES of dysfunctional brain networks in at-risk mental state patients against psychotic disorders.

Transcranial Electrical Stimulation to Enhance Cognitive Performance of Healthy Minors: A Complex Governance Challenge

An increasing number of healthy adolescents are consuming products that can enhance their cognitive performance in educational settings. Currently, the use of pharmaceuticals is the most widely discussed enhancement method in the literature, but new evidence suggests that other methods based on Transcranial Electrical Stimulation (tES) also have potential as cognitive enhancer. Just like pharmaceutical enhancers, the availability and education-related use of tES-devices raise a broad range of ethical, legal, and societal issues that need to be addressed by policy-makers. Few studies, however, have specifically explored these issues in relation to child wellbeing. In this narrative review with systematic search, we describe the issues for child wellbeing that could arise from the availability and education-related use of tES-based enhancers by healthy minors. We demonstrate that the issues form a complex web of uncertainties and concerns, which are mainly incited by two factors. First is the high level of factual uncertainty due to gaps in empirical evidence about the exact working mechanisms and efficacy of tES. Moreover, a lack of insight into the technique’s (long-term) effects on healthy developing brains, and uncertainties about potential cognitive trade-offs have fueled concerns about the technique’s safety and impact. The second factor that contributes to the complexity of issues is the presence of moral diversity in our society. Different opinions exist on whether a certain enhancement effect would be desirable and whether potential risks would be acceptable. These opinions depend on one’s moral perspective, and the way one interprets and weights values such as the child’s autonomy and authenticity. The challenge for proper governance resides in the design of an appropriate framework that is capable of balancing the different moral perspectives in society, while recognizing the uncertainties that still exist. We therefore argue for a responsible innovation approach, which encourages an adaptive attitude toward emerging knowledge and dynamic societal values, to deal with the identified issues regarding tES-based enhancement appropriately.

Activating Developmental Reserve Capacity Via Cognitive Training or Non-invasive Brain Stimulation: Potentials for Promoting Fronto-Parietal and Hippocampal-Striatal Network Functions in Old Age

Existing neurocomputational and empirical data link deficient neuromodulation of the fronto-parietal and hippocampal-striatal circuitries with aging-related increase in processing noise and declines in various cognitive functions. Specifically, the theory of aging neuronal gain control postulates that aging-related suboptimal neuromodulation may attenuate neuronal gain control, which yields computational consequences on reducing the signal-to-noise-ratio of synaptic signal transmission and hampering information processing within and between cortical networks. Intervention methods such as cognitive training and non-invasive brain stimulation, e.g., transcranial direct current stimulation (tDCS), have been considered as means to buffer cognitive functions or delay cognitive decline in old age. However, to date the reported effect sizes of immediate training gains and maintenance effects of a variety of cognitive trainings are small to moderate at best; moreover, training-related transfer effects to non-trained but closely related (i.e., near-transfer) or other (i.e., far-transfer) cognitive functions are inconsistent or lacking. Similarly, although applying different tDCS protocols to reduce aging-related cognitive impairments by inducing temporary changes in cortical excitability seem somewhat promising, evidence of effects on short- and long-term plasticity is still equivocal. In this article, we will review and critically discuss existing findings of cognitive training- and stimulation-related behavioral and neural plasticity effects in the context of cognitive aging, focusing specifically on working memory and episodic memory functions, which are subserved by the fronto-parietal and hippocampal-striatal networks, respectively. Furthermore, in line with the theory of aging neuronal gain control we will highlight that developing age-specific brain stimulation protocols and the concurrent applications of tDCS during cognitive training may potentially facilitate short- and long-term cognitive and brain plasticity in old age.

Increased contextual cue utilization with tDCS over the prefrontal cortex during a recognition task

The precise role of the prefrontal and posterior parietal cortices in recognition performance remains controversial, with questions about whether these regions contribute to recognition via the availability of mnemonic evidence or via decision biases and retrieval orientation. Here we used an explicit memory cueing paradigm, whereby external cues probabilistically predict upcoming memoranda as old or new, in our case with 75% validity, and these cues affect recognition decision biases in the direction of the cue. The present study applied bilateral transcranial direct current stimulation (tDCS) over prefrontal or posterior parietal cortex, or sham tDCS, to test the causal role of these regions in recognition accuracy or decision biasing. Participants who received tDCS over prefrontal cortex showed increased cue utilization compared to tDCS over posterior parietal cortex and sham tDCS, suggesting that the prefrontal cortex is involved in processes that contribute to decision biases in memory.