Transcranial direct current stimulation (tDCS) of frontal cortex decreases performance on the WAIS-IV intelligence test

Transcranial electrical stimulation for procedural learning and rehabilitation

Procedural learning is the acquisition of motor and non-motor skills through a gradual process that increases with practice. Impairments in procedural learning have been consistently demonstrated in neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Considering that noninvasive brain stimulation modulates brain activity and boosts neuroplastic mechanisms, we reviewed the effects of coupling transcranial direct current stimulation (tDCS) with training methods for motor and non-motor procedural learning to explore tDCS potential use as a tool for enhancing implicit learning in healthy and clinical populations. The review covers tDCS effects over i. motor procedural learning, from basic to complex activities; ii. non-motor procedural learning; iii. procedural rehabilitation in several clinical populations. We conclude that targeting the primary motor cortex and prefrontal areas seems the most promising for motor and non-motor procedural learning, respectively. For procedural rehabilitation, the use of tDCS is yet at an early stage but some effectiveness has been reported for implicit motor and memory learning. Still, systematic comparisons of stimulation parameters and target areas are recommended for maximising the effectiveness of tDCS and its robustness for procedural rehabilitation.

Non-invasive brain stimulation and neuroenhancement

Attempts to enhance human memory and learning ability have a long tradition in science. This topic has recently gained substantial attention because of the increasing percentage of older individuals worldwide and the predicted rise of age-associated cognitive decline in brain functions. Transcranial brain stimulation methods, such as transcranial magnetic (TMS) and transcranial electric (tES) stimulation, have been extensively used in an effort to improve cognitive functions in humans. Here we summarize the available data on low-intensity tES for this purpose, in comparison to repetitive TMS and some pharmacological agents, such as caffeine and nicotine. There is no single area in the brain stimulation field in which only positive outcomes have been reported. For self-directed tES devices, how to restrict variability with regard to efficacy is an essential aspect of device design and function. As with any technique, reproducible outcomes depend on the equipment and how well this is matched to the experience and skill of the operator. For self-administered non-invasive brain stimulation, this requires device designs that rigorously incorporate human operator factors. The wide parameter space of non-invasive brain stimulation, including dose (e.g., duration, intensity (current density), number of repetitions), inclusion/exclusion (e.g., subject's age), and homeostatic effects, administration of tasks before and during stimulation, and, most importantly, placebo or nocebo effects, have to be taken into account. The outcomes of stimulation are expected to depend on these parameters and should be strictly controlled. The consensus among experts is that low-intensity tES is safe as long as tested and accepted protocols (including, for example, dose, inclusion/exclusion) are followed and devices are used which follow established engineering risk-management procedures. Devices and protocols that allow stimulation outside these parameters cannot claim to be "safe" where they are applying stimulation beyond that examined in published studies that also investigated potential side effects. Brain stimulation devices marketed for consumer use are distinct from medical devices because they do not make medical claims and are therefore not necessarily subject to the same level of regulation as medical devices (i.e., by government agencies tasked with regulating medical devices). Manufacturers must follow ethical and best practices in marketing tES stimulators, including not misleading users by referencing effects from human trials using devices and protocols not similar to theirs.

Cognitive outcomes after tDCS in schizophrenia patients with prominent negative symptoms: Results from the placebo-controlled STARTS trial

Cognitive deficits and negative symptoms in schizophrenia are associated with poor functional outcomes and limited in terms of treatment. The Schizophrenia Treatment With Electric Transcranial Stimulation (STARTS) trial has shown efficacy of transcranial direct current stimulation (tDCS) for improving negative symptoms. In this secondary analysis, we investigate its effects on cognitive performance. In STARTS, a double-blinded, sham-controlled, randomized clinical trial, patients were treated with twice-daily, 20-min, 2-mA fronto-temporal tDCS over 5 days or sham-tDCS. In 90 patients, we evaluated the cognitive performance up to 12 weeks post-treatment. We found that active-tDCS showed no beneficial effects over sham-tDCS in any of the tests. Based on a 5-factor cognitive model, improvements of executive functions and delayed memory were observed in favor of sham-tDCS. Overall, the applied active-tDCS protocol, primarily designed to improve negative symptoms, did not promote cognitive improvement. We discuss possible protocol modification potentially required to increase tDCS effects on cognition. ClinicalTrials.gov identifier: NCT02535676.

The effects of offline and online prefrontal vs parietal transcranial direct current stimulation (tDCS) on verbal and spatial working memory

Working memory (WM) is a limited-capacity system or set of processes that enables temporary storage and manipulation of information essential for complex cognitive processes. The WM performance is supported by a widespread neural network in which fronto-parietal functional connections have a pivotal role. Transcranial direct current stimulation (tDCS) is rapidly emerging as a promising tool for understanding the role of various cortical areas and their functional networks on cognitive performance. Here we comprehensively evaluated the effects of tDCS on WM by conducting three cross-over counterbalanced sham-controlled experiments in which we contrasted the effects and interactions of the anodal (i.e. facilitatory) tDCS across anterior-posterior (i.e. DLPFC vs PPC) and left-right (i.e. the lateralization) axes, and across online and offline protocols using both verbal and spatial WM (3-back) tasks as outcomes. In the offline protocols, left DLPFC stimulation affected neither verbal nor spatial WM, while left PPC stimulation increased spatial WM. When applied offline over right DLPFC, tDCS improved verbal WM task and marginally enhanced spatial WM; while when tDCS was applied over the right PPC, facilitatory effects were observed on verbal WM. In the online protocol, tDCS did not modulate WM regardless of the task modality or stimulation loci. In summary, the study did not replicate the left DLPFC tDCS effect on WM, found in some of the previous studies, but demonstrated positive effects of stimulation of the right DLPFC as well as PPC bilaterally. The observed effects varied across modality of the 3-back task, and tDCS protocol applied. The results of this study argue for moving towards targeting the lesser-explored stimulation sites within the fronto-parietal network, such as PPC, to gain a better understanding of the usefulness of tDCS for WM neuromodulation.

Precision non-implantable neuromodulation therapies: a perspective for the depressed brain

Current first-line treatments for major depressive disorder (MDD) include pharmacotherapy and cognitive-behavioral therapy. However, one-third of depressed patients do not achieve remission after multiple medication trials, and psychotherapy can be costly and time-consuming. Although non-implantable neuromodulation (NIN) techniques such as transcranial magnetic stimulation, transcranial direct current stimulation, electroconvulsive therapy, and magnetic seizure therapy are gaining momentum for treating MDD, the efficacy of non-convulsive techniques is still modest, whereas use of convulsive modalities is limited by their cognitive side effects. In this context, we propose that NIN techniques could benefit from a precision-oriented approach. In this review, we discuss the challenges and opportunities in implementing such a framework, focusing on enhancing NIN effects via a combination of individualized cognitive interventions, using closed-loop approaches, identifying multimodal biomarkers, using computer electric field modeling to guide targeting and quantify dosage, and using machine learning algorithms to integrate data collected at multiple biological levels and identify clinical responders. Though promising, this framework is currently limited, as previous studies have employed small samples and did not sufficiently explore pathophysiological mechanisms associated with NIN response and side effects. Moreover, cost-effectiveness analyses have not been performed. Nevertheless, further advancements in clinical trials of NIN could shift the field toward a more “precision-oriented” practice.

Accessing the development and heritability of the capacity of cognitive control

Cognitive control serves as a core construct, with limited capacity, to support executive functions and other higher-level mental processes such as intellectual activity. Although previous studies have investigated the development of executive functions during specific age periods, the development of the capacity of cognitive control (CCC) from early childhood to late adolescence and the heritability of the CCC have yet to be delineated. In this study, we estimated the CCC based on the performance of a perceptual decision-making task in monozygotic (n = 95) and dizygotic (n = 81) twin pairs with an age range from 6 to 18 years and in a reference young adult group (n = 41, mean age = 26.15 years). In addition, the intelligence quotient (IQ) of these participants was assessed using the Wechsler Intelligence Scales. We found an increase in the CCC from 1.55 bits per second (bps) at age 6 years to its 95% capacity of 3.87 bps at age 21 years, with a reduced growth rate as a function of age. The estimated heritability of the CCC was 0.66, and shared and non-shared environmental influences on the CCC were 0.18 and 0.16, respectively. The CCC was significantly correlated to IQ (r = 0.34). These findings indicate that the CCC is developed throughout the school years, is highly heritable, and is associated with higher-level cognition.

Neurocognitive effects of transcranial direct current stimulation (tDCS) in unipolar and bipolar depression: Findings from an international randomized controlled trial

OBJECTIVE

Transcranial direct current stimulation (tDCS) has been found to have antidepressant effects and may have beneficial neurocognitive effects. However, prior research has produced an unclear understanding of the neurocognitive effects of repeated exposure to tDCS. The study's aim was to determine the neurocognitive effects following tDCS treatment in participants with unipolar or bipolar depression.

METHOD

The study was a triple-masked, randomized, controlled clinical trial across six international academic medical centers. Participants were randomized to high dose (2.5 mA for 30 min) or low dose (0.034 mA, for 30 min) tDCS for 20 sessions over 4 weeks, followed by an optional 4 weeks of open-label high dose treatment. The tDCS anode was centered over the left dorsolateral prefrontal cortex at F3 (10/20 EEG system) and the cathode over F8. Participants completed clinical and neurocognitive assessments before and after tDCS. Genotype (BDNF Val66Met and catechol-o-methyltransferase [COMT] Val158Met polymorphisms) were explored as potential moderators of neurocognitive effects.

RESULTS

The study randomized 130 participants. Across the participants, tDCS treatment (high and low dose) resulted in improvements in verbal learning and recall, selective attention, information processing speed, and working memory, which were independent of mood effects. Similar improvements were observed in the subsample of participants with bipolar disorder. There was no observed significant effect of tDCS dose. However, BDNF Val66Met and COMT Val158Met polymorphisms interacted with tDCS dose and affected verbal memory and verbal fluency outcomes, respectively.

CONCLUSIONS

These findings suggest that tDCS could have positive neurocognitive effects in unipolar and bipolar depression. Thus, tDCS stimulation parameters may interact with interindividual differences in BDNF and COMT polymorphisms to affect neurocognitive outcomes, which warrants further investigation.

Cognitive changes after tDCS and escitalopram treatment in major depressive disorder: Results from the placebo-controlled ELECT-TDCS trial

BACKGROUND

Cognitive deficits in major depressive disorder (MDD) are associated with low quality of life and higher suicide risk. Antidepressant drugs have modest to null effects in improving such deficits. Therefore, we investigated the cognitive effects of transcranial direct current stimulation (tDCS), which is a promising antidepressant non-pharmacological intervention, in MDD.

METHODS

An exploratory analysis on cognitive performance was conducted in 243 depressed patients from the Escitalopram vs. Electric Current Therapy for Treating Depression Clinical Study (ELECT-TDCS), a sham-controlled study comparing the efficacy of tDCS vs. escitalopram. A neuropsychological battery was applied at baseline and endpoint (10 weeks of treatment) to create composite cognitive scores (processing speed, working memory, and verbal fluency). Linear mixed regression models were used to evaluate changes according to intervention groups, adjusted for confounding variables (age, years of schooling, gender, and benzodiazepine use) and depression improvement.

RESULTS

No cognitive deterioration was observed in any group. Patients receiving tDCS presented reduced practice gains compared to placebo in processing speed. In patients receiving escitalopram vs. placebo and in the subgroup of clinical responders (>50% depression improvement from baseline), those receiving tDCS vs. placebo presented increased performance in verbal fluency. No significant differences between tDCS and escitalopram groups were detected.

LIMITATIONS

Absence of healthy controls.

CONCLUSION

Prefrontal tDCS did not lead to cognitive deficits in depressed patients, although it reduced practice effects in processing speed. tDCS responders presented increased performance in verbal fluency. Further investigation of tDCS cognitive effects in depression is warranted.

General fluid/inductive reasoning battery for a high-ability population

The validity of studies investigating interventions to enhance fluid intelligence (Gf) depends on the adequacy of the Gf measures administered. Such studies have yielded mixed results, with a suggestion that Gf measurement issues may be partly responsible. The purpose of this study was to develop a Gf test battery comprising tests meeting the following criteria: (a) strong construct validity evidence, based on prior research; (b) reliable and sensitive to change; (c) varying in item types and content; (d) producing parallel tests, so that pretest-posttest comparisons could be made; (e) appropriate time limits; (f) unidimensional, to facilitate interpretation; and (g) appropriate in difficulty for a high-ability population, to detect change. A battery comprising letter, number, and figure series and figural matrix item types was developed and evaluated in three large-N studies (N = 3,067, 2,511, and 801, respectively). Items were generated algorithmically on the basis of proven item models from the literature, to achieve high reliability at the targeted difficulty levels. An item response theory approach was used to calibrate the items in the first two studies and to establish conditional reliability targets for the tests and the battery. On the basis of those calibrations, fixed parallel forms were assembled for the third study, using linear programming methods. Analyses showed that the tests and test battery achieved the proposed criteria. We suggest that the battery as constructed is a promising tool for measuring the effectiveness of cognitive enhancement interventions, and that its algorithmic item construction enables tailoring the battery to different difficulty targets, for even wider applications.

Behavioural and electrophysiological modulations induced by transcranial direct current stimulation in healthy elderly and Alzheimer's disease patients: A pilot study

OBJECTIVE

To investigate whether anodal and cathodal transcranial direct current stimulation (tDCS) can modify cognitive performance and neural activity in healthy elderly and Alzheimer's disease (AD) patients.

METHODS

Fourteen healthy elderly and twelve AD patients performed a working memory task during an electroencephalogram recording before and after receiving anodal, cathodal, and sham tDCS over the left dorsolateral prefrontal cortex. Behavioural performance, event-related potentials (P200, P300) and evoked cortical oscillations were studied as correlates of working memory.

RESULTS

Anodal tDCS increased P200 and P300 amplitudes in healthy elderly. Cathodal tDCS increased P200 amplitude and frontal theta activity between 150 and 300 ms in AD patients. Improved working memory after anodal tDCS correlated with increased P300 in healthy elderly. In AD patients, slight tendencies between enhanced working memory and increased P200 after cathodal tDCS were observed.

CONCLUSIONS

Functional neural modulations were promoted by anodal tDCS in healthy elderly and by cathodal tDCS in AD patients.

SIGNIFICANCE

Interaction between tDCS polarity and the neural state (e.g., hyper-excitability exhibited by AD patients) suggests that appropriate tDCS parameters (in terms of tDCS polarity) to induce behavioural improvements should be chosen based on the participant's characteristics. Future studies using higher sample sizes should confirm and extend the present findings.

FAST: A Novel, Executive Function-Based Approach to Cognitive Enhancement

The present study introduces a novel cognitive intervention aimed at improving fluid intelligence (Gf), based on a framework we refer to as FAST: Flexible, Adaptive, Synergistic Training. FAST leverages a combination of novel game-based executive function (EF) training—designed specifically to enhance the likelihood of transfer—and transcranial electrical stimulation (tES), with aims to synergistically activate and strengthen mechanisms of cognitive control critical to Gf. To test our intervention, we collected three Gf measures from 113 participants [the advanced short Bochumer Matrizen-Test (BOMAT), Raven’s Advanced Progressive Matrices (APM), and matrices similar to Raven’s generated by Sandia labs], prior to and following one of three interventions: (1) the FAST + tRNS intervention, a combination of 30 min of daily training with our novel training game, Robot Factory, and 20 min of concurrent transcranial random noise stimulation applied to bilateral dorsolateral prefrontal cortex (DLPFC); (2) an adaptively difficult Active Control intervention comprised of visuospatial tasks that specifically do not target Gf; or (3) a no-contact control condition. Analyses of changes in a Gf factor from pre- to post-test found numerical increases for the FAST + tRNS group compared to the two control conditions, with a 0.3 SD increase relative to Active Control (p = 0.07), and a 0.19 SD increase relative to a No-contact control condition (p = 0.26). This increase was found to be largely driven by significant differences in pre- and post-test Gf as measured on the BOMAT test. Progression through the FAST training game (Robot Factory) was significantly correlated with changes in Gf. This is in contrast with progress in the Active Control condition, as well as with changes in individual EFs during FAST training, which did not significantly correlate with changes in Gf. Taken together, this research represents a useful step forward in providing new insights into, and new methods for studying, the nature of Gf and its malleability. Though our results await replication and extension, they provide preliminary evidence that the crucial characteristic of Gf may, in fact, be the ability to combine EFs rapidly and adaptively according to changing demand, and that Gf may be susceptible to targeted training.

Transcranial electrical stimulation nomenclature

Transcranial electrical stimulation (tES) aims to alter brain function non-invasively by applying current to electrodes on the scalp. Decades of research and technological advancement are associated with a growing diversity of tES methods and the associated nomenclature for describing these methods. Whether intended to produce a specific response so the brain can be studied or lead to a more enduring change in behavior (e.g. for treatment), the motivations for using tES have themselves influenced the evolution of nomenclature, leading to some scientific, clinical, and public confusion. This ambiguity arises from (i) the infinite parameter space available in designing tES methods of application and (ii) varied naming conventions based upon the intended effects and/or methods of application. Here, we compile a cohesive nomenclature for contemporary tES technologies that respects existing and historical norms, while incorporating insight and classifications based on state-of-the-art findings. We consolidate and clarify existing terminology conventions, but do not aim to create new nomenclature. The presented nomenclature aims to balance adopting broad definitions that encourage flexibility and innovation in research approaches, against classification specificity that minimizes ambiguity about protocols but can hinder progress. Constructive research around tES classification, such as transcranial direct current stimulation (tDCS), should allow some variations in protocol but also distinguish from approaches that bear so little resemblance that their safety and efficacy should not be compared directly. The proposed framework includes terms in contemporary use across peer-reviewed publications, including relatively new nomenclature introduced in the past decade, such as transcranial alternating current stimulation (tACS) and transcranial pulsed current stimulation (tPCS), as well as terms with long historical use such as electroconvulsive therapy (ECT). We also define commonly used terms-of-the-trade including electrode, lead, anode, and cathode, whose prior use, in varied contexts, can also be a source of confusion. This comprehensive clarification of nomenclature and associated preliminary proposals for standardized terminology can support the development of consensus on efficacy, safety, and regulatory standards.

Anodal Transcranial Direct Current Stimulation Induces High Gamma-Band Activity in the Left Dorsolateral Prefrontal Cortex During a Working Memory Task: A Double-Blind, Randomized, Crossover Study

Transcranial direct current stimulation (tDCS) has been shown to have mixed effects on working memory (WM) capacity in healthy individuals. Different stimulation paradigms may account for these discrepancies, with certain features being favored. To determine the effect in the context of anodal tDCS, we investigated whether anodal tDCS induced cortical oscillatory changes during a WM task. Specifically, we tested whether anodal offline tDCS over the left prefrontal cortex (PFC) enhances WM capacity by modulating the oscillatory activity in the left dorsolateral PFC (DLPFC) using magnetoencephalography (MEG). This study employed a double-blind, randomized, crossover design, in which 24 healthy right-handed participants conducted MEG recordings during a 3-back task after administration of 2 mA tDCS or sham stimulation as a placebo. Our results showed that the effect of tDCS did not appear in the behavioral indices—WM accuracy (d′) or reaction time (RT). From the results of the time-frequency analysis, significant event-related synchronization (ERS) in the high-gamma band (82–84 Hz) of the left DLPFC was found under the tDCS condition; however, ERS was not correlated with WM capacity. Furthermore, we calculated the modulation index (MI), which indicates the strength of phase-amplitude coupling (PAC). tDCS significantly decreased MI of the left DLPFC, representing the theta-gamma PAC during the n-back task using color names as verbal stimuli. Our results suggest that although tDCS increased the gamma-band oscillation indicating greater neural activity in the left DLPFC, it did not lead to an improvement of WM capacity; this may be due to the inability of gamma-band oscillation to couple with the task-induced theta wave. WM capacity might not increase unless theta-gamma PAC is not enhanced by tDCS.

Testing a Cognitive Control Model of Human Intelligence

The definition of human intelligence and its underlying psychological constructs have long been debated. Although previous studies have investigated the fundamental cognitive functions determining intellectual abilities, such as the broadly defined executive functions including working memory, the core process has yet to be identified. A potential candidate for such a role might be cognitive control, a psychological construct for the coordination of thoughts and actions under conditions of uncertainty. In this study, we tested a cognitive control model of intellectual ability by examining the association between cognitive control, measured by a perceptual decision-making task and by the attention network test, and general intelligence including components of fluid intelligence (Gf, concerning the ability to solve problems by abstraction) and crystalized intelligence (Gc, related to learning from prior knowledge and experience) measured by the Wechsler Adult Intelligence Scale. We also examined the potential role of cognitive control as a core process involved in another determinant of intellectual abilities, the working memory, measured by the N-back tasks and the working memory complex span tasks. The relationship among intelligence, cognitive control, and working memory was examined using structural equation modeling. Results showed that cognitive control shared a large amount of variance with working memory and both measures were strongly associated with Gf and Gc, with a stronger association with Gf than Gc. These findings suggest that cognitive control, serving as a core construct of executive functions, contributes substantially to general intellectual ability, especially fluid intelligence.

Cognitive Effects of Transcranial Direct Current Stimulation in Healthy and Clinical Populations: An Overview

Transcranial direct current stimulation (tDCS) is a neuromodulatory approach that is affordable, safe, and well tolerated. This review article summarizes the research and clinically relevant findings from meta-analyses and studies investigating the cognitive effects of tDCS in healthy and clinical populations. We recapitulate findings from recent studies where cognitive performance paired with tDCS was compared with performance under placebo (sham stimulation) in single sessions and longitudinal designs where cognitive effects were evaluated following repeated sessions. In summary, the tDCS literature currently indicates that the effects of tDCS on cognitive measures are less robust and less predictable compared with the more consistent effects on motor outcomes. There is also a notable difference in the consistency of single-session and longitudinal designs. In single-session tDCS designs, there are small effects amid high variability confounded by individual differences and potential sham stimulation effects. In contrast, longitudinal studies provide more consistent benefits in healthy and clinical populations, particularly when tDCS is paired with a concurrent task. Yet, these studies are few in number, thereby impeding design optimization. While there is good evidence that tDCS can modulate cognitive functioning and potentially produce longer-term benefits, a major challenge to widespread translation of tDCS is the absence of a complete mechanistic account for observed effects. Significant future work is needed to identify a priori responders from nonresponders for every cognitive task and tDCS protocol.

Effects of prefrontal tDCS on executive function: Methodological considerations revealed by meta-analysis

A meta-analysis of studies using single-session transcranial direct current stimulation (tDCS) to target the dorsolateral prefrontal cortex (DLPFC) was undertaken to examine the effect of stimulation on executive function (EF) in healthy samples. 27 studies were included in analyses, yielding 71 effect sizes. The most relevant measure for each task was determined a priori and used to calculate Hedge's g. Methodological characteristics of each study were examined individually as potential moderators of effect size. Stimulation effects on three domains of EF (inhibition of prepotent responses, mental set shifting, and information updating and monitoring) were analyzed separately. In line with previous work, the current study found no significant effect of anodal unilateral tDCS, cathodal unilateral tDCS, or bilateral tDCS on EF. Further moderator and subgroup analyses were only carried out for anodal unilateral montages due to the small number of studies using other montages. Subgroup analyses revealed a significant effect of anodal unilateral tDCS on updating tasks, but not on inhibition or set-shifting tasks. Cathode location significantly moderated the effect of anodal unilateral tDCS. Extracranial cathodes yielded a significant effect on EF while cranial cathodes yielded no effect. Anode size also significantly moderated effect of anodal unilateral tDCS, with smaller anodes being more effective than larger anodes. In summary, anodal DLPFC stimulation is more effective at improving updating ability than inhibition and set-shifting ability, but anodal stimulation can significantly improve general executive function when extracranial cathodes or small anodes are used. Future meta-analyses may examine how stimulation's effects on specific behavioral tasks, rather than broader domains, might be affected by methodological moderators.

Anodal Transcranial Direct Current Stimulation Promotes Frontal Compensatory Mechanisms in Healthy Elderly Subjects

Recent studies have demonstrated that transcranial direct current stimulation (tDCS) is potentially useful to improve working memory. In the present study, young and elderly subjects performed a working memory task (n-back task) during an electroencephalogram recording before and after receiving anodal, cathodal, and sham tDCS over the left dorsolateral prefrontal cortex (DLPFC). We investigated modulations of behavioral performance and electrophysiological correlates of working memory processes (frontal and parietal P300 event-related potentials). A strong tendency to modulated working memory performance was observed after the application of tDCS. In detail, young, but not elderly, subjects benefited from additional practice in the absence of real tDCS, as indicated by their more accurate responses after sham tDCS. The cathodal tDCS had no effect in any group of participants. Importantly, anodal tDCS improved accuracy in elderly. Moreover, increased accuracy after anodal tDCS was correlated with a larger frontal P300 amplitude. These findings suggest that, in elderly subjects, improved working memory after anodal tDCS applied over the left DLPFC may be related to the promotion of frontal compensatory mechanisms, which are related to attentional processes.

Primum non nocere or primum facere meliorem? Hacking the brain in the 21st century

Abstract Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modulates cortical excitability. It is devoid of serious adverse events and exerts variable effects on cognition, with several research findings suggesting that it can improve memory, verbal and mathematical skills. Because tDCS devices are low-cost, portable and relatively easy to assemble, they have become available outside of the medical setting and used for non-medical (“cosmetic”) purposes by laypersons. In this sense, tDCS has become a popular technique aiming to improve cognition and the achievement of a better performance not only at work, but also in other fields such as sports, leisure activities (video games) and even the military. In spite of these unforeseen developments, there has been a general paralysis of the medical and regulatory agencies to develop guidelines for the use of tDCS for cosmetic purposes. Several challenges are present, most importantly, how to restrict tDCS use outside of the medical setting in face of variable and sometimes conflicting results from scientific research. This article aims to describe the popular use of tDCS, in light of the pillars of neuroethics, a branch of bioethics relative to brain research. Between two possible but extreme solutions – total release or total restriction of tDCS – it is paramount to develop a spectrum of alternatives, which may vary over time and in different cultural backgrounds.

Effects of alpha and gamma transcranial alternating current stimulation (tACS) on verbal creativity and intelligence test performance

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation method that allows to directly modulate brain oscillations of a given frequency. Using this method, it was recently shown that increasing alpha (10Hz) oscillations improved creative ideation with figural material and that increasing gamma (40Hz) oscillations speeded up performance in a figural matrices intelligence task. The aim of the present study was to examine whether these findings generalize to verbal creativity and intelligence tasks. In addition, we explored whether the stimulation effects are moderated by individual differences in creative potential and intelligence. Twenty-two adults received 10Hz, 40Hz and sham tACS while they worked on a verbal creativity (alternate uses) task and a verbal intelligence (anagram) task. Analyses revealed that 10Hz stimulation had a marginally significant effect on ideational fluency in the alternate uses task, whereas originality was unaffected. The beneficial effect of stimulation on fluency tended to emerge mainly in the individuals with higher creative potential. In the verbal intelligence task, in contrast, 40Hz stimulation did neither impact on performance nor interacted with individual differences in intelligence. These findings provide first tentative evidence that enhancing alpha oscillations through tACS may improve creative thinking not only in the figural but also in the verbal domain. The previously reported beneficial effect of gamma tACS on figural intelligence, however, could not be observed in a verbal task. In sum, the present study further corroborates the causal link between alpha oscillations and creative thinking and suggests that tACS may be a promising tool to enhance cognitive processes.

Effect of transcranial direct current stimulation combined with cognitive training on cognitive functioning in older adults with HIV: A pilot study

The objective of this study was to examine combination speed of processing (SOP) cognitive remediation therapy (CRT) and transcranial direct stimulation (tDCS) as neurorehabilitation in older HIV+ adults. Thirty-three HIV+ adults aged 50+ completed neurocognitive testing and were randomized to either active (n = 17) or sham (n = 16) tDCS. Both conditions received 10 1-hour sessions of SOP CRT, with either active or sham tDCS for the first 20 minutes. Participants then completed a posttest assessment. Repeated measures analysis of variance examining Time X Condition showed small-to-medium effects in the expected direction for an executive (d = 0.36), and SOP measure (d = 0.49), while medium-to-large effects were observed for an executive/attention (d = 0.60) and oral reading measure (d = 0.75). The only statistically significant interaction was the oral reading measure. Small-to-medium and medium-to-large effects (ds = 0.32, 0.58) were found for two SOP measures in the opposite direction (sham group showing greater improvements). Further trials of CRT and tDCS in this population are needed, including larger samples and a nonactive control and tDCS only condition, as is determination of which parameters of each technique (e.g., tDCS montage, timing of tDCS, domain targeted in CRT, number of sessions) are most effective in improving cognitive outcomes, durability of training gains, and translation to everyday functioning.

Transcranial direct current stimulation versus caffeine as a fatigue countermeasure

BACKGROUND

To assess the efficacy of using transcranial direct current stimulation (tDCS) to remediate the deleterious effects of fatigue induced by sleep deprivation and compare these results to caffeine, a commonly used fatigue countermeasure.

OBJECTIVE/HYPOTHESIS

Based on previous research, tDCS of the dorsolateral prefrontal cortex (DLPFC) can modulate attention and arousal. The authors hypothesize that tDCS can be an effective fatigue countermeasure.

METHODS

Five groups of ten participants each received either active tDCS and placebo gum at 1800, caffeine gum with sham tDCS at 1800, active tDCS and placebo gum at 0400, caffeine gum with sham tDCS at 0400, or sham tDCS with placebo gum at 1800 and 0400 during 36-h of sustained wakefulness. Participants completed a vigilance task, working memory task, psychomotor vigilance task (PVT), and a procedural game beginning at 1800 h and continued every two hours throughout the night until 1900 the next day.

RESULTS

tDCS dosed at 1800 provided 6 h of improved attentional accuracy and reaction times compared to the control group. Caffeine did not produce an effect. Both tDCS groups also had an improved effect on mood. Participants receiving tDCS reported feeling more vigor, less fatigue, and less bored throughout the night compared to the control and caffeine groups.

CONCLUSIONS

We believe tDCS could be a powerful fatigue countermeasure. The effects appear to be comparable or possibly more beneficial than caffeine because they are longer lasting and mood remains more positive.

Low-Intensity Transcranial Current Stimulation in Psychiatry

Neurostimulation is rapidly emerging as an important treatment modality for psychiatric disorders. One of the fastest-growing and least-regulated approaches to noninvasive therapeutic stimulation involves the application of weak electrical currents. Widespread enthusiasm for low-intensity transcranial electrical current stimulation (tCS) is reflected by the recent surge in direct-to-consumer device marketing, do-it-yourself enthusiasm, and an escalating number of clinical trials. In the wake of this rapid growth, clinicians may lack sufficient information about tCS to inform their clinical practices. Interpretation of tCS clinical trial data is aided by familiarity with basic neurophysiological principles, potential mechanisms of action of tCS, and the complicated regulatory history governing tCS devices. A growing literature includes randomized controlled trials of tCS for major depression, schizophrenia, cognitive disorders, and substance use disorders. The relative ease of use and abundant access to tCS may represent a broad-reaching and important advance for future mental health care. Evidence supports application of one type of tCS, transcranial direct current stimulation (tDCS), for major depression. However, tDCS devices do not have regulatory approval for treating medical disorders, evidence is largely inconclusive for other therapeutic areas, and their use is associated with some physical and psychiatric risks. One unexpected finding to arise from this review is that the use of cranial electrotherapy stimulation devices-the only category of tCS devices cleared for use in psychiatric disorders-is supported by low-quality evidence.

Novel Neuromodulation Techniques to Assess Interhemispheric Communication in Neural Injury and Neurodegenerative Diseases

Interhemispheric interaction has a major role in various neurobehavioral functions. Its disruption is a major contributor to the pathological changes in the setting of brain injury such as traumatic brain injury, peripheral nerve injury, and stroke, as well as neurodegenerative diseases. Because interhemispheric interaction has a crucial role in functional consequence in these neuropathological states, a review of noninvasive and state-of-the-art molecular based neuromodulation methods that focus on or have the potential to elucidate interhemispheric interaction have been performed. This yielded approximately 170 relevant articles on human subjects or animal models. There has been a recent surge of reports on noninvasive methods such as transcranial magnetic stimulation and transcranial direct current stimulation. Since these are noninvasive techniques with little to no side effects, their widespread use in clinical studies can be easily justified. The overview of novel neuromodulation methods and how they can be applied to study the role of interhemispheric communication in neural injury and neurodegenerative disease is provided. Additionally, the potential of each method in therapeutic use as well as investigating the pathophysiology of interhemispheric interaction in neurodegenerative diseases and brain injury is discussed. New technologies such as transcranial magnetic stimulation or transcranial direct current stimulation could have a great impact in understanding interhemispheric pathophysiology associated with acquired injury and neurodegenerative diseases, as well as designing improved rehabilitation therapies. Also, advances in molecular based neuromodulation techniques such as optogenetics and other chemical, thermal, and magnetic based methods provide new capabilities to stimulate or inhibit a specific brain location and a specific neuronal population.

Effects of Noninvasive Brain Stimulation on Food Cravings and Consumption: A Meta-Analytic Review

OBJECTIVE

The primary aim of this review was to evaluate the effectiveness of noninvasive brain stimulation to the dorsolateral prefrontal cortex (dlPFC) for modulating appetitive food cravings and consumption in laboratory (via meta-analysis) and therapeutic (via systematic review) contexts.

METHODS

Keyword searches of electronic databases (PubMed, Scopus, Web of Science, PsychoInfo, and EMBASE) and searches of previous quantitative reviews were used to identify studies (experimental [single-session] or randomized trials [multi-session]) that examined the effects of neuromodulation to the dlPFC on food cravings (n = 9) and/or consumption (n = 7). Random-effects models were employed to estimate the overall and method-specific (repetitive transcranial magnetic stimulation [rTMS] and transcranial direct current stimulation [tDCS]) effect sizes. Age and body mass index were examined as potential moderators. Two studies involving multisession therapeutic stimulation were considered in a separate systematic review.

RESULTS

Findings revealed a moderate-sized effect of modulation on cravings across studies (g, -0.516; p = .037); this effect was subject to significant heterogeneity (Q, 33.086; p < .001). Although no statistically significant moderators were identified, the stimulation effect on cravings was statistically significant for rTMS (g, -0.834; p = .008) but not tDCS (g, -0.252; p = .37). There was not sufficient evidence to support a causal effect of neuromodulation and consumption in experimental studies; therapeutic studies reported mixed findings.

CONCLUSIONS

Stimulation of the dlPFC modulates cravings for appetitive foods in single-session laboratory paradigms; when estimated separately, the effect size is only significant for rTMS protocols. Effects on consumption in laboratory contexts were not reliable across studies, but this may reflect methodological variability in delivery of stimulation and assessment of eating behavior. Additional single- and multi-session studies assessing eating behavior outcomes are needed.

Making Brains run Faster: are they Becoming Smarter?

A brief overview of structural and functional brain characteristics related to g is presented in the light of major neurobiological theories of intelligence: Neural Efficiency, P-FIT and Multiple-Demand system. These theories provide a framework to discuss the main objective of the paper: what is the relationship between individual alpha frequency (IAF) and g? Three studies were conducted in order to investigate this relationship: two correlational studies and a third study in which we experimentally induced changes in IAF by means of transcranial alternating current stimulation (tACS). (1) In a large scale study (n = 417), no significant correlations between IAF and IQ were observed. However, in males IAF positively correlated with mental rotation and shape manipulation and with an attentional focus on detail. (2) The second study showed sex-specific correlations between IAF (obtained during task performance) and scope of attention in males and between IAF and reaction time in females. (3) In the third study, individuals’ IAF was increased with tACS. The induced changes in IAF had a disrupting effect on male performance on Raven’s matrices, whereas a mild positive effect was observed for females. Neuro-electric activity after verum tACS showed increased desynchronization in the upper alpha band and dissociation between fronto-parietal and right temporal brain areas during performance on Raven’s matrices. The results are discussed in the light of gender differences in brain structure and activity.

Evidence-based guidelines on the therapeutic use of transcranial direct current stimulation (tDCS)

A group of European experts was commissioned by the European Chapter of the International Federation of Clinical Neurophysiology to gather knowledge about the state of the art of the therapeutic use of transcranial direct current stimulation (tDCS) from studies published up until September 2016, regarding pain, Parkinson's disease, other movement disorders, motor stroke, poststroke aphasia, multiple sclerosis, epilepsy, consciousness disorders, Alzheimer's disease, tinnitus, depression, schizophrenia, and craving/addiction. The evidence-based analysis included only studies based on repeated tDCS sessions with sham tDCS control procedure; 25 patients or more having received active treatment was required for Class I, while a lower number of 10-24 patients was accepted for Class II studies. Current evidence does not allow making any recommendation of Level A (definite efficacy) for any indication. Level B recommendation (probable efficacy) is proposed for: (i) anodal tDCS of the left primary motor cortex (M1) (with right orbitofrontal cathode) in fibromyalgia; (ii) anodal tDCS of the left dorsolateral prefrontal cortex (DLPFC) (with right orbitofrontal cathode) in major depressive episode without drug resistance; (iii) anodal tDCS of the right DLPFC (with left DLPFC cathode) in addiction/craving. Level C recommendation (possible efficacy) is proposed for anodal tDCS of the left M1 (or contralateral to pain side, with right orbitofrontal cathode) in chronic lower limb neuropathic pain secondary to spinal cord lesion. Conversely, Level B recommendation (probable inefficacy) is conferred on the absence of clinical effects of: (i) anodal tDCS of the left temporal cortex (with right orbitofrontal cathode) in tinnitus; (ii) anodal tDCS of the left DLPFC (with right orbitofrontal cathode) in drug-resistant major depressive episode. It remains to be clarified whether the probable or possible therapeutic effects of tDCS are clinically meaningful and how to optimally perform tDCS in a therapeutic setting. In addition, the easy management and low cost of tDCS devices allow at home use by the patient, but this might raise ethical and legal concerns with regard to potential misuse or overuse. We must be careful to avoid inappropriate applications of this technique by ensuring rigorous training of the professionals and education of the patients.

Cognitive effects of transcranial direct current stimulation in depression: Results from the SELECT-TDCS trial and insights for further clinical trials

BACKGROUND

Cognitive dysfunction treatment remains an unmet clinical need in major depressive disorder (MDD). Transcranial direct current stimulation (tDCS) may improve cognitive symptoms in MDD. Our aim was to investigate the cognitive effects of tDCS in the Sertraline vs. Electric Current Therapy for Treating Depression Clinical Study (SELECT-TDCS). We also explored whether tDCS could have mood-independent cognitive effects.

METHODS

One hundred twenty MDD patients aged from 18 to 65 years received 12 sessions of active/sham tDCS (2mA for 30min) and real/placebo 50mg/d sertraline over 6 weeks in a factorial trial. We analyzed whether changes in performance of neuropsychological tests (Trail Making, Digit Span, Stroop Task, Mini-Mental Status Exam and Montreal Cognitive Assessment) occurred over time, according to treatment group and depression improvement. Exploratory analyses were carried out to verify the influence of clinical and demographic variables on the outcomes.

RESULTS

Cognitive improvement was showed in most tests used, although they occurred regardless of intervention type and depression improvement. Further exploratory analyses revealed that clinical response and education level could have mediated pro-cognitive tDCS effects on some of the tests used.

LIMITATIONS

The neuropsychological battery used might not have been sensitive to detect tDCS-induced effects on cognition. Lack of simultaneous cognitive training during application may have also limited its cognitive effects.

CONCLUSIONS

We found no evidence of beneficial or deleterious cognitive effects of tDCS as a treatment for depression. We discussed clinical trial design considerations for further tDCS studies assessing cognitive effects, including sample and outcomes considerations.

Phase Dependency of the Human Primary Motor Cortex and Cholinergic Inhibition Cancelation During Beta tACS

The human motor cortex has a tendency to resonant activity at about 20 Hz so stimulation should more readily entrain neuronal populations at this frequency. We investigated whether and how different interneuronal circuits contribute to such resonance by using transcranial magnetic stimulation (TMS) during transcranial alternating current stimulation (tACS) at motor (20 Hz) and a nonmotor resonance frequency (7 Hz). We tested different TMS interneuronal protocols and triggered TMS pulses at different tACS phases. The effect of cholinergic short-latency afferent inhibition (SAI) was abolished by 20 Hz tACS, linking cortical beta activity to sensorimotor integration. However, this effect occurred regardless of the tACS phase. In contrast, 20 Hz tACS selectively modulated MEP size according to the phase of tACS during single pulse, GABAAergic short-interval intracortical inhibition (SICI) and glutamatergic intracortical facilitation (ICF). For SICI this phase effect was more marked during 20 Hz stimulation. Phase modulation of SICI also depended on whether or not spontaneous beta activity occurred at ~20 Hz, supporting an interaction effect between tACS and underlying circuit resonances. The present study provides in vivo evidence linking cortical beta activity to sensorimotor integration, and for beta oscillations in motor cortex being promoted by resonance in GABAAergic interneuronal circuits.

Noninvasive Brain Stimulation Techniques Can Modulate Cognitive Processing

Recent methods that allow a noninvasive modulation of brain activity are able to modulate human cognitive behavior. Among these methods are transcranial electric stimulation and transcranial magnetic stimulation that both come in multiple variants. A property of both types of brain stimulation is that they modulate brain activity and in turn modulate cognitive behavior. Here, we describe the methods with their assumed neural mechanisms for readers from the economic and social sciences and little prior knowledge of these techniques. Our emphasis is on available protocols and experimental parameters to choose from when designing a study. We also review a selection of recent studies that have successfully applied them in the respective field. We provide short pointers to limitations that need to be considered and refer to the relevant papers where appropriate.

Transcranial stimulation over the left inferior frontal gyrus increases false alarms in an associative memory task in older adults

Background

Transcranial direct current stimulation (tDCS) is a potential tool for alleviating various forms of cognitive decline, including memory loss, in older adults. However, past effects of tDCS on cognitive ability have been mixed. One important potential moderator of tDCS effects is the baseline level of cognitive performance.

Methods

We tested the effects of tDCS on face-name associative memory in older adults, who suffer from performance deficits in this task relative to younger adults. Stimulation was applied to the left inferior prefrontal cortex during encoding of face-name pairs, and memory was assessed with both a recognition and recall task.

Results

Face–name memory performance was decreased with the use of tDCS. This result was driven by increased false alarms when recognizing rearranged face–name pairs.

Conclusions

This result suggests that tDCS can lead to increased false alarm rates in recognition memory, and that effects of tDCS on a specific cognitive task may depend upon cognitive capability for that task.

Tolerability of Repeated Application of Transcranial Electrical Stimulation with Limited Outputs to Healthy Subjects

BACKGROUND

The safety and tolerability of limited output transcranial electrical stimulation (tES) in clinical populations support a non-significant risk designation. The tolerability of long-term use in a healthy population had remained untested.

OBJECTIVE

We tested the tolerability and compliance of two tES waveforms, transcranial direct current stimulation (tDCS) and modulated high frequency transcranial pulsed current stimulation (MHF-tPCS) compared to sham-tDCS, applied to healthy subjects for three to five days (17-20 minutes per day) per week for up to six weeks in a communal setting. MHF-tPCS consisted of asymmetric high-frequency pulses (7-11 kHz) having a peak amplitude of 10-20 mA peak, adjusted by subject, resulting in an average current of 5-7 mA.

METHOD

A total of 100 treatment blind healthy subjects were randomly assigned to one of three treatment groups: tDCS (n = 33), MHF-tPCS (n = 30), or sham-tDCS (n = 37). In order to test the role of waveform, electrode type and montage were fixed across tES and sham-tDCS arms: high-capacity self-adhering electrodes on the right lateral forehead and back of the neck. We conducted 1905 sessions (636 sham-tDCS, 623 tDCS, and 646 MHF-tPCS sessions) on study volunteers over a period of six weeks.

RESULTS

Common adverse events were primarily restricted to influences upon the skin and included skin tingling, itching, and mild burning sensations. The incidence of these events in the active tES treatment arms (MHF-tPCS, tDCS) was equivalent or significantly lower than their incidence in the sham-tDCS treatment arm. Other adverse events had a rarity (<5% incidence) that could not be significantly distinguished across the treatment groups. Some subjects were withdrawn from the study due to atypical headache (sham-tDCS n = 2, tDCS n = 2, and MHF-tPCS n = 3), atypical discomfort (sham-tDCS n = 0, tDCS n = 1, and MHF-tPCS n = 1), or atypical skin irritation (sham-tDCS n = 2, tDCS n = 8, and MHF-tPCS n = 1). The rate of compliance, elected sessions completed, for the MHF-tPCS group was significantly greater than the sham-tDCS group's compliance (p = 0.007). There were no serious adverse events in any treatment condition.

CONCLUSION

We conclude that repeated application of limited output tES across extended periods, limited to the hardware, electrodes, and protocols tested here, is well tolerated in healthy subjects, as previously observed in clinical populations.

Improving Interference Control in ADHD Patients with Transcranial Direct Current Stimulation (tDCS)

The use of transcranial direct current stimulation (tDCS) in patients with attention deficit hyperactivity disorder (ADHD) has been suggested as a promising alternative to psychopharmacological treatment approaches due to its local and network effects on brain activation. In the current study, we investigated the impact of tDCS over the right inferior frontal gyrus (rIFG) on interference control in 21 male adolescents with ADHD and 21 age matched healthy controls aged 13–17 years, who underwent three separate sessions of tDCS (anodal, cathodal, and sham) while completing a Flanker task. Even though anodal stimulation appeared to diminish commission errors in the ADHD group, the overall analysis revealed no significant effect of tDCS. Since participants showed a considerable learning effect from the first to the second session, performance in the first session was separately analyzed. ADHD patients receiving sham stimulation in the first session showed impaired interference control compared to healthy control participants whereas ADHD patients who were exposed to anodal stimulation, showed comparable performance levels (commission errors, reaction time variability) to the control group. These results suggest that anodal tDCS of the right inferior frontal gyrus could improve interference control in patients with ADHD.

Transcranial magnetic stimulation and sedation as treatment for functional neurologic disorders

Functional neurologic disorder (FND), also known as conversion disorder, is common and often associated with a poor prognosis. It has been relatively neglected by research and as such there is a conspicuous lack of evidence-based treatments. Physical and psychologic therapies are the main treatment modalities, over and above reassurance and sensitive explanation of the diagnosis. However there are two other historic treatments that have seen a recent resurgence of interest and use. The first is electric stimulation, which was initially pioneered with direct stimulation of nerves but now used indirectly (and therefore noninvasively) in the form of transcranial magnetic stimulation (TMS). The second is (therapeutic) sedation, previously known as "abreaction," where it was mostly used in the context of psychologic investigation and treatment, but now increasingly advocated during rehabilitation as a way to therapeutically demonstrate reversibility of symptoms. This chapter introduces the background of these treatment modalities, their evolution into their current applications before critically evaluating their current evidence base and exploring possible mechanisms of action. It also tentatively suggests when they should be considered in current practice and briefly considers their future potential. In summary there is encouraging preliminary evidence to suggest that both TMS and sedation may be effective treatments for FNDs.