A Meta-analysis of Transcranial Direct Current Stimulation Studies Examining the Reliability of Effects on Language Measures

Enhancement of phonemic verbal fluency in multilingual young adults by transcranial random noise stimulation

Several studies have analyzed the effects of transcranial direct current stimulation on verbal fluency tasks in non-clinical populations. Nevertheless, the reported effects on verbal fluency are inconsistent. In addition, the effect of other techniques such as transcranial random noise stimulation (tRNS) on verbal fluency enhancement has yet to be studied in healthy multilingual populations. This study aims to explore the effects of tRNS on verbal fluency in healthy multilingual individuals. Fifty healthy multilingual (Spanish, English and Basque) adults were randomly assigned to a tRNS or sham group. Electrodes were placed on the left dorsolateral prefrontal cortex and left inferior frontal gyrus. All participants performed phonemic and semantic verbal fluency tasks before, during (online assessment) and immediately after (offline assessment) stimulation in three different languages. The results showed significantly better performance by participants who received tRNS in the phonemic verbal fluency tasks in Spanish (in the online and offline assessment) and English (in the offline assessment). No differences between conditions were found in Basque nor semantic verbal fluency. These findings suggests that tRNS on the left prefrontal cortex could help improve phonemic, yet not semantic, fluency in healthy multilingual adults.

A modified neural circuit framework for semantic memory retrieval with implications for circuit modulation to treat verbal retrieval deficits

Word finding difficulty is a frequent complaint in older age and disease states, but treatment options are lacking for such verbal retrieval deficits. Better understanding of the neurophysiological and neuroanatomical basis of verbal retrieval function may inform effective interventions. In this article, we review the current evidence of a neural retrieval circuit central to verbal production, including words and semantic memory, that involves the pre-supplementary motor area (pre-SMA), striatum (particularly caudate nucleus), and thalamus. We aim to offer a modified neural circuit framework expanded upon a memory retrieval model proposed in 2013 by Hart et al., as evidence from electrophysiological, functional brain imaging, and noninvasive electrical brain stimulation studies have provided additional pieces of information that converge on a shared neural circuit for retrieval of memory and words. We propose that both the left inferior frontal gyrus and fronto-polar regions should be included in the expanded circuit. All these regions have their respective functional roles during verbal retrieval, such as selection and inhibition during search, initiation and termination of search, maintenance of co-activation across cortical regions, as well as final activation of the retrieved information. We will also highlight the structural connectivity from and to the pre-SMA (e.g., frontal aslant tract and fronto-striatal tract) that facilitates communication between the regions within this circuit. Finally, we will discuss how this circuit and its correlated activity may be affected by disease states and how this circuit may serve as a novel target engagement for neuromodulatory treatment of verbal retrieval deficits.

Combined anodal transcranial direct current stimulation and behavioural naming treatment improves language performance in patients with post-stroke aphasia

PRIMARY OBJECTIVE

During the last decade, studies using anodal transcranial Direct Current Stimulation (atDCS) have yielded promising results in patients with aphasia. The main aim of the present pilot study was to assess the effects of combined atDCS over the left posterior perisylvian region and behavioral naming training on the behavioral outcomes of language comprehension and production of patients with post-stroke aphasia.

RESEARCH DESIGN

A 2 × 2 quasi-experimental design was conducted, optimal to compare changes after treatment in experimental versus control group.

METHODS AND PROCEDURES

Ten patients with post-stroke aphasia were enrolled in this study: half received atDCS on the left posterior perisylvian region while they underwent a 2-week behavioral naming training. The other half received sham stimulation. The outcomes were measured using the abbreviated form of the Boston Diagnostic Aphasia Examination and analyzed using ANOVAs.

MAIN OUTCOMES AND RESULTS

Both groups improved their performance in Oral comprehension, Narrative writing and Language Competence Index, but only those that received anodal tDCS presented better results in the Naming category after the treatment.

CONCLUSIONS

AtDCS on the left posterior perisylvian area seems to be a promising tool for boosting the outcomes of behavioral naming therapy in patients with post-stroke aphasia.

Effects of tDCS applied over the left IFG and pSTG language areas on verb recognition task performance

Knowledge about the relevance of the left inferior frontal gyrus (lIFG) and the left posterior superior temporal gyrus (lpSTG) in visual recognition of word categories is limited at present. tDCS is a non-invasive brain stimulation method that alters cortical activity and excitability, and thus might be a useful tool for delineating the specific impact of both areas on word recognition. The objective of this study was to explore whether the visual recognition process of verb categories is improved by a single tDCS session. lIFG and lpSTG areas were separately modulated by anodal tDCS to evaluate its effects on verbal recognition. Compared to sham stimulation, motor reaction times (RTs) were reduced after anodal tDCS over the lpSTG, and this effect was independent of the performing hand (right/left). These findings suggest that this region is involved in visual word recognition independently from the performing hand.

More focal, less heterogeneous? Multi-level meta-analysis of cathodal high-definition transcranial direct current stimulation effects on language and cognition

High-definition transcranial direct current stimulation (HD-tDCS) is a relatively focal, novel non-invasive brain stimulation method with the potential to investigate the causal contributions of specific cortical brain regions to language and cognition. Studies with HD-tDCS typically employ a 4 × 1 electrode design with a single central target electrode surrounded by four return electrodes, among which return current intensity is evenly distributed. With cathodal HD-tDCS, neural excitability in the target region is assumed to be reduced, which offers interesting perspectives for neuropsychological research and interventions. This multi-level meta-analysis compiles published studies using cathodal HD-tDCS in 4 × 1 configuration to modulate cognition and behavior. Regarding HD-tDCS, 77 effect sizes were gathered from 11 eligible reports. We extended this database with 52 effect sizes from 11 comparable reports using conventional tDCS with cathodal polarity. We observed no significant overall effect and no moderation by within-study and between-study variables in HD. In the extended analysis, results suggested a non-linear moderation of cathodal tDCS effects by intensity, driven by negative effect sizes at 1.5 mA. However, studies varied tremendously in task parameters, outcomes, and even technical parameters. Interestingly, within-study heterogeneity exceeded between-study heterogeneity in the present sample, and moderators hardly reduced the residual heterogeneity. Across domains and configurations, both positive and negative effect sizes are possible. We discuss the findings in relation to conventional cathodal tDCS and the framework of polarity specificity. Fundamental aspects of cathodal HD-tDCS are still to be addressed in future research.

Cerebellar tDCS does not modulate language processing performance in healthy individuals

Clinical and neuroscientific studies in healthy volunteers have established that the cerebellum contributes to language comprehension and production. Yet most evidence is correlational and the exact role of the cerebellum remains unclear. The aim of this study was to investigate the role of the right cerebellum in unimpaired language comprehension and production using non-invasive brain stimulation. In this double-blind, sham-controlled experiment, thirty-six healthy participants received anodal or sham transcranial direct current (tDCS) stimulation to the right cerebellum while performing a lexical decision, sentence comprehension, verbal fluency and a non-language control task. Active tDCS did not modulate performance in any of the tasks. Additional exploratory analyses suggest difficulty-specific performance modulation in the sentence comprehension and lexical decision task, with tDCS improving performance in easy trials of the sentence comprehension task and difficult trials in the lexical decision task. Overall, our findings provide no evidence for the involvement of the right posterior cerebellum in language processing. Further research is needed to dissociate the influence of task difficulty of the underlying cognitive processes.

Effects of Transcranial Direct Current Stimulation on Cognitive and Affective Outcomes Using Virtual Stimuli: A Systematic Review

Transcranial direct current stimulation (tDCS) is a noninvasive form of brain stimulation used to influence neural activity. While early tDCS studies primarily used static stimuli, there is growing interest in dynamic stimulus presentations using virtual environments (VEs). This review attempts to convey the state of the field. This is not a quantitative meta-analysis as there are not yet enough studies following consistent protocols and/or reporting adequate data. In addition to reviewing the state of the literature, this review includes an exploratory analysis of the available data. Following preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines, studies were culled from several databases. Results from this review reveal differences between online and offline stimulation. While offline stimulation did not influence affective and cognitive outcomes, online stimulation led to small changes in affect and cognition. Future studies should include randomized controlled trials with larger samples. Furthermore, greater care needs to be applied to full data reporting (e.g., means, standard deviations, and data for their nonsignificant findings) to improve our understanding of the combined effects of virtual stimuli with tDCS.

Baseline delayed verbal recall predicts response to high definition transcranial direct current stimulation targeting the superior medial frontal cortex

Anodal high definition transcranial direct current stimulation (HD-tDCS) targeting the pre-supplementary motor area/dorsal anterior cingulate cortex (pre-SMA/dACC) has recently been shown to improve verbal retrieval deficits in veterans with chronic traumatic brain injury (TBI) (Motes et al., 2020), but predictors of treatment response are unclear. We hypothesized that baseline delayed verbal recall, a sensitive measure for post-TBI chronic cognitive decline, would predict therapeutic effects of HD-tDCS targeting the pre-SMA/dACC for verbal retrieval deficits. Standardized verbal retrieval measures were administered at baseline, immediately after and 8 weeks after treatment completion. We applied mixed generalized linear modeling as a post-hoc subgroup analysis to the verbal retrieval scores that showed significant improvement in Motes at el. (2020) to examine effects of active stimulation across the groups with baseline-intact delayed recall (N = 10) and baseline-impaired delayed recall (N = 8), compared to sham (N = 7). Individuals with impaired baseline delayed recall showed significant improvement (compared to baseline) in both category fluency and color-word inhibition/switch, while individuals with intact delayed recall showed significant improvement only in color-word inhibition/switch. Baseline delayed verbal recall may therefore be considered as a predictor for future electromodulation studies targeting frontal structures to treat TBI-related verbal deficits.

A Systematic Review of Transcranial Direct Current Stimulation in Primary Progressive Aphasia: Methodological Considerations

A variety of tDCS approaches has been used to investigate the potential of tDCS to improve language outcomes, or slow down the decay of language competences caused by Primary Progressive Aphasia (PPA). The employed stimulation protocols and study designs in PPA are generally speaking similar to those deployed in post-stroke aphasic populations. These two etiologies of aphasia however differ substantially in their pathophysiology, and for both conditions the optimal stimulation paradigm still needs to be established. A systematic review was done and after applying inclusion and exclusion criteria, 15 articles were analyzed focusing on differences and similarities across studies especially focusing on PPA patient characteristics (age, PPA variant, language background), tDCS stimulation protocols (intensity, frequency, combined therapy, electrode configuration) and study design as recent reviews and group outcomes for individual studies suggest tDCS is an effective tool to improve language outcomes, while methodological approach and patient characteristics are mentioned as moderators that may influence treatment effects. We found that studies of tDCS in PPA have clinical and methodological and heterogeneity regarding patient populations, stimulation protocols and study design. While positive group results are usually found irrespective of these differences, the magnitude, duration and generalization of these outcomes differ when comparing stimulation locations, and when results are stratified according to the clinical variant of PPA. We interpret the results of included studies in light of patient characteristics and methodological decisions. Further, we highlight the role neuroimaging can play in study protocols and interpreting results and make recommendations for future work.

Using transcranial direct current stimulation (tDCS) on the dorsolateral prefrontal cortex to promote long-term foreign language vocabulary learning

Transcranial direct current stimulation (tDCS) on the dorsolateral prefrontal cortex (DLPFC) was used to improve foreign-langue learning while using mental imagery. Participants underwent two sessions of 1 mA, 1.5 mA, or sham stimulation prior to learning Swahili-English word pairs two consecutive days. During learning, participants were encouraged to create a mental image of the associated English word. Twenty-four hours after learning and one week later, participants received a cued recall test. A linear dose-response effect of stimulation was found across both tests that occurred long after the immediate effects of stimulation. Follow-up comparisons revealed that only the 1.5 mA condition differed from the sham group. Exploratory moderating effects revealed interactions with sleep quality and handedness. Those with poorer sleep and who were left-handed showed greater recall after 1.5 mA of stimulation than those with better sleep and right-handers. A follow-up behavioral study probing strategy usage indicated that mental imagery strategy use did not strongly impact learning but point to other possible mechanisms including the importance of attending to multimodal perceptual details and memory consolidation. This preliminary evidence supports the role of the DLPFC or connected regions in foreign language vocabulary learning and verbal memory encoding.

Effects of transcranial electrical stimulation techniques on second and foreign language learning enhancement in healthy adults: A systematic review and meta-analysis

BACKGROUND

Transcranial electrical stimulation (tES) techniques have been used to enhance different cognitive domains such as language in healthy adults. While several reviews and meta-analysis have been conducted on the effects of tES on different language skills (picture naming, verbal fluency, word reading), there has been little research conducted to date on the effects of tES on the processes involved in foreign language learning.

OBJECTIVE

A meta-analysis was performed to quantify the effects of tES on foreign language learning processes (non-words, artificial grammar, and foreign languages), focusing on accuracy, response times and 1-week follow-up effects, if reported by the studies.

RESULTS

Eleven studies that had sham condition were reviewed. Nine of them were analyzed, including five using within-participant design, and four that employed between-participant design. The final analysis encompassed nine studies with 279 healthy participants. The analysis showed moderate enhancing effects of tES on overall language learning (g = 0.50, 95 % CI [0.29, 0.71], p = .0001). However, results were not significant on follow up data (g = 0.54, 95 % CI [-0.12, 1.20], p = .07), and on response times (g = 0.50, 95 % CI [-0.1, 1.18], p = .10). The effects were significantly moderated by years of education.

CONCLUSIONS

The results suggest that tES seems to enhance the mechanisms involved in foreign language learning; however, more research is needed to understand the impact scope of these techniques on language learning processes.

Transcranial direct current stimulation in Autism Spectrum Disorder: A systematic review and meta-analysis

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has gained relevance in recent years as an alternative treatment for neuropsychiatric conditions. The aim of this study is to conduct a systematic review of the use of tDCS in Autism Spectrum Disorder (ASD). Both electronic and manual searches were conducted to identify studies published in peer-reviewed scientific journals addressing the use of tDCS in ASD population. A total of 16 studies fulfilled the criteria to be included in the review. Studies were conducted both in child and adult population. Anodal stimulation on the left dorsolateral prefrontal cortex was the most commonly chosen methodology. Outcomes addressed ASD symptoms and neuropsychological functions. Meta-analytic synthesis identified improvements in social, health, and behavioral problem domains of the Autism Treatment Evaluation Checklist. Limitations included high heterogeneity in the methodology and low-efficacy study designs (pre-post and single-case studies). Recent controlled trials shed promising results for the use of tDCS in ASD. A standardized stimulation protocol and a consensus in the measures used in the evaluation of the efficacy are imperative.

Verbal Fluency in Mild Alzheimer's Disease: Transcranial Direct Current Stimulation over the Dorsolateral Prefrontal Cortex

BACKGROUND

Recent studies showed that in healthy controls and in aphasic patients, inhibitory trains of repetitive transcranial magnetic stimulation (rTMS) over the right prefrontal cortex can improve phonemic fluency performance, while anodal transcranial direct current stimulation (tDCS) over the left prefrontal cortex can improve performance in naming and semantic fluency tasks.

OBJECTIVE

This study aimed at investigating the effects of cathodal tDCS over the left or the right dorsolateral prefrontal cortex (DLPFC) on verbal fluency tasks (VFT) in patients with mild Alzheimer's disease (AD).

METHODS

Forty mild AD patients participated in the study (mean age 73.17±5.61 years). All participants underwent cognitive baseline tasks and a VFT twice. Twenty patients randomly received cathodal tDCS to the left or the right DLPFC, and twenty patients were assigned to a control group in which only the two measures of VFT were taken, without the administration of the tDCS.

RESULTS

A significant improvement of performance on the VFT in AD patients was present after tDCS over the right DLPFC (p = 0.001). Instead, no difference was detected between the two VFTs sessions after tDCS over the left DLPFC (p = 0.42). Furthermore, these results cannot be related to task learning effects, since no significant difference was found between the two VFT sessions in the control group (p = 0.73).

CONCLUSION

These data suggest that tDCS over DLPFC can improve VFT performance in AD patients. A hypothesis is that tDCS enhances adaptive patterns of brain activity between functionally connected areas.

Facilitative Effects of Transcranial Direct Current Stimulation on Semantic Memory Examined by Text-Mining Analysis in Patients With Schizophrenia

Background: Beneficial effects of transcranial direct current stimulation (tDCS) are relevant to cognition and functional capacity, in addition to psychiatric symptoms in patients with schizophrenia. However, whether tDCS would improve higher-order cognition, e.g., semantic memory organization, has remained unclear. Recently, text-mining analyses have been shown to reveal semantic memory. The purpose of the current study was to determine whether tDCS would improve semantic memory, as evaluated by text-mining analyses of category fluency data, in patients with schizophrenia. Methods: Twenty-eight patients entered the study. Cognitive assessment including the category fluency task was conducted at baseline (before tDCS treatment) and 1 month after t administration of tDCS (2 mA × 20 min, twice per day) for 5 days, according to our previous study. The category fluency data were also obtained from 335 healthy control subjects. The verbal outputs (i.e., animal names) from the category fluency task were submitted to singular valued decomposition (SVD) analysis. Semantic memory structures were estimated by calculating inter-item cosines (i.e., similarities) among animal names frequently produced in the category fluency task. Data were analyzed longitudinally and cross-sectionally to compare the semantic structure within the patient group (i.e., baseline vs. follow-up) and between groups (patients vs. healthy controls). In the former, semantic associations for frequent items were compared in the form of cosine profiles, while in the latter, the difference in the magnitude of the correlations for inter-item cosines between healthy controls and patients (baseline, follow-up) was examined. Results: Cosine profiles in the patient group became more cluster-based (i.e., pet, carnivores, and herbivores) at follow-up compared to those at baseline, yielding higher cosines within subcategories. The correlational coefficient of inter-item cosines between healthy controls and patients was significantly greater at follow-up compared to baseline; semantic associations in patients approached the normality status after multi-session tDCS. Conclusions: To our knowledge, this is the first study to demonstrate the facilitative effect of tDCS on semantic memory organization in patients with schizophrenia. Text-mining analysis was indicated to effectively evaluate semantic memory structures in patients with psychiatric disorders.

Effects of transcranial direct current stimulation over the posterior parietal cortex on novice X-ray screening performance

Existing theories of visual search are generally deduced from lab-based studies involving the identification of a target object among similar distractors. The role of the right parietal cortex in visual search is well-established. However, less is known about real-world visual search tasks, such as X-ray screening, which require targets to be disembedded from their background. Research has shown variations in the cognitive abilities required for these tasks and typical lab-based visual search tasks. Thus, the findings of traditional visual search studies do not always transfer into the applied domain. Although brain imaging studies have offered insights into visual search tasks involving disembedding, highlighting an association between the left parietal cortex and disembedding performance, no causal link has yet been established. To this end, we carried out a pilot study (n = 34, between-subjects) administering non-invasive brain stimulation over the posterior parietal cortex (PPC) prior to completing a security X-ray screening task. The findings suggested that anodal left PPC tDCS enhanced novice performance in X-ray screening over that of sham stimulation, in line with brain imaging findings. However, the efficacy of tDCS is under question, with a growing number of failed replications. With this in mind, this study aims to re-test our original hypothesis by examining the effects of left-side parietal stimulation on novice X-ray screener performance and comparing them to those of sham stimulation and of stimulation on a control site (right PPC). As such, this within-subjects study comprised three sessions (2 mA left PPC, 2 mA right PPC, low-intensity sham stimulation left PPC), to investigate effects of anodal tDCS on X-ray screening performance. The pre-registered analysis did not detect any significant differences between left PPC tDCS and sham tDCS or left PPC tDCS and right PPC tDCS on novice performance (d') in X-ray screening. Further exploratory analyses detected no effects of left PPC tDCS on any other indices of performance in the X-ray security screening task (c, RTs and accuracy), or a disembedding control task (RTs and accuracy). The use of alternative stimulation techniques, with replicable behavioural effects on the parietal lobe (or a multi-technique approach), and well-powered studies with a systematic variation of stimulation parameters, could help to choose between two possible interpretations: that neither left nor right PPC are causally related to either tasks or that tDCS was ineffective. Finally, low-intensity sham stimulation (.016 mA), previously shown to outperform other sham conditions in between-subjects designs, was found to be ineffective for blinding participants in a within-subjects design. Our findings raise concerns for the current lack of optimal control conditions and add to the growing literature highlighting the need for replication in the field.

Neuromodulation of cursing in American English: A combined tDCS and pupillometry study

Many neurological disorders are associated with excessive and/or uncontrolled cursing. The right prefrontal cortex has long been implicated in a diverse range of cognitive processes that underlie the propensity for cursing, including non-propositional language representation, emotion regulation, theory of mind, and affective arousal. Neurogenic cursing often poses significant negative social consequences, and there is no known behavioral intervention for this communicative disorder. We examined whether right vs. left lateralized prefrontal neurostimultion via tDCS could modulate taboo word production in neurotypical adults. We employed a pre/post design with a bilateral frontal electrode montage. Half the participants received left anodal and right cathodal stimulation; the remainder received the opposite polarity stimulation at the same anatomical loci. We employed physiological (pupillometry) and behavioral (reaction time) dependent measures as participants read aloud taboo and non-taboo words. Pupillary responses demonstrated a crossover reaction, suggestive of modulation of phasic arousal during cursing. Participants in the right anodal condition showed elevated pupil responses for taboo words post stimulation. In contrast, participants in the right cathodal condition showed relative dampening of pupil responses for taboo words post stimulation. We observed no effects of stimulation on response times. We interpret these findings as supporting modulation of right hemisphere affective arousal that disproportionately impacts taboo word processing. We discuss alternate accounts of the data and future applications to neurological disorders.

Evidence-based Umbrella Review of Cognitive Effects of Prefrontal tDCS

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique, which has been increasingly used as an investigational tool in neuroscience. In social and affective neuroscience research, the prefrontal cortex has been primarily targeted, since this brain region is critically involved in complex psychobiological processes subserving both Șhotș and Școldș domains. Although several studies have suggested that prefrontal tDCS can enhance neuropsychological outcomes, meta-analyses have reported conflicting results. Therefore, we aimed to assess the available evidence by performing an umbrella review of meta-analyses. We evaluated the effects of prefrontal active vs sham tDCS on different domains of cognition among healthy and neuropsychiatric individuals. A MeaSurement Tool to Assess Systematic Reviews 2 was employed to evaluate the quality of meta-analyses, and the GRADE system was employed to grade the quality of evidence of every comparison from each meta-analysis. PubMed/MEDLINE, PsycINFO and the Cochrane Database of Systematic Reviews were searched, and 11 meta-analyses were included resulting in 55 comparisons. Only 16 comparisons reported significant effects favoring tDCS, but 13 of them had either very low or low quality of evidence. Of the remaining 39 comparisons which reported non-significant effects, 38 had either very low or low quality of evidence. Meta-analyses were rated as having critically low and low quality. Among several reasons to explain these findings, the lack of consensus and reproducibility in tDCS research is discussed.

Effects of Combined Transcranial Direct Current Stimulation with Cognitive Training in Girls with Rett Syndrome

Background: Transcranial Direct Current Stimulation (tDCS) combined with traditional rehabilitative techniques has not been widely applied to Rett Syndrome (RTT). The aim of this study was to examine the effects of combined cognitive traditional training with tDCS applied to attention and language measures in subjects with RTT. Methods: 31 subjects with RTT were randomly allocated into two groups: non-sham tDCS (n = 18) and sham tDCS (n = 13). The former received the integrated intervention non-sham tDCS plus cognitive empowerment during the treatment phase. The latter received sham stimulation plus cognitive empowerment. All participants underwent neurological and cognitive assessment to evaluate attention and language measures: before integrated treatment (pre-test phase), at the conclusion of the treatment (post-test phase), and at 1 month after the conclusion of the treatment (follow-up phase). Results: the results indicated longer attention time in the non-sham tDCS group compared to the sham tDCS group with a stable trend also in the follow-up phase; an increase of the number of vowel/phoneme sounds in the non-sham tDCS group; and an improvement in the neurophysiological parameters in the non-sham tDCS group. Conclusions: This study supports the use of tDCS as a promising and alternative approach in the RTT rehabilitation field.

Neuroethical issues in cognitive enhancement: Modafinil as the example of a workplace drug?

The use of cognitive-enhancing drugs by healthy individuals has been a feature for much of recorded history. Cocaine and amphetamine are modern cases of drugs initially enthusiastically acclaimed for enhancing cognition and mood. Today, an increasing number of healthy people are reported to use cognitive-enhancing drugs, as well as other interventions, such as non-invasive brain stimulation, to maintain or improve work performance. Cognitive-enhancing drugs, such as methylphenidate and modafinil, which were developed as treatments, are increasingly being used by healthy people. Modafinil not only affects ‘cold’ cognition, but also improves ‘hot’ cognition, such as emotion recognition and task-related motivation. The lifestyle use of ‘smart drugs’ raises both safety concerns as well as ethical issues, including coercion and increasing disparity in society. As a society, we need to consider which forms of cognitive enhancement (e.g. pharmacological, exercise, lifelong learning) are acceptable and for which groups under what conditions and by what methods we would wish to improve and flourish.

Left Prefrontal Cortex Supports the Recognition of Meaningful Patterns in Ambiguous Stimuli

Processing of ambiguous visual stimuli has been associated with an increased activation of the left lateral prefrontal cortex (PFC) in neuroimaging studies. Nevertheless, the functional role of prefrontal activity in this process is not fully understood. In this experiment we asked participants to evaluate ambiguous inkblots from the Rorschach test, while stimulating the left lateral PFC using excitatory anodal transcranial direct current stimulation (tDCS). In addition, visual insight ability was assessed as a control measure requiring visual and conceptual restructuring and convergent thinking rather than divergent idea generation employed to interpret the equivocal Rorschach inkblots. Using a randomized double-blind design, we demonstrated that anodal tDCS increased the number of meaningful patterns recognized in the inkblots but had no significant effect on visual insight. These findings support the role of left lateral PFC in the processing of ambiguous visual information and object recognition. More generally, we discuss that the PFC may be involved in the mechanisms supporting the activation of stored visual and semantic representations in order to compensate for less informative bottom-up inputs and thus facilitate flexible cognition and idea generation.

Transcranial direct current stimulation in attention-deficit hyperactivity disorder: A meta-analysis of neuropsychological deficits

Transcranial direct current stimulation (tDCS) is a promising method for altering cortical excitability with clinical implications in neuropsychiatric diseases. Its application in neurodevelopmental disorders especially attention-deficit hyperactivity disorder (ADHD), is in early stage and promising but its effectiveness has not been systematically examined yet. We conducted a meta-analysis on the effectiveness of tDCS on the most studied neuropsychological symptoms of ADHD, which is the first reported meta-analysis of tDCS studies on ADHD. Data from 10 randomized controlled studies (including 11 separate experiments) targeting inhibitory control, and/or working memory (WM) in ADHD were included. Results show that overall tDCS significantly improved inhibitory control. Sub-analyses further show that dorsolateral prefrontal cortex (dlPFC) (but not right inferior frontal gyrus) tDCS and anodal (but not cathodal) tDCS significantly improved inhibitory control with a small effect size. Anodal dlPFC-tDCS had the largest significant effect on inhibitory control with a small-to-medium effect size. Additionally, a significant improving effect of tDCS on inhibitory control accuracy (but not response time) and WM speed (but not accuracy) were found. Overall, this meta-analysis supports a beneficial effect of tDCS on inhibitory control and WM in ADHD with a small-to-medium effect size. TDCS seems to be a promising method for improving neuropsychological and cognitive deficits in ADHD. However, there might be a dissociation between neuropsychological deficits and clinical symptoms of ADHD and therefore, the significance of this meta-analysis for clinical purposes is limited. Future studies should systematically evaluate the role of inter-individual factors (i.e., ADHD subtype, types of the deficit) and stimulation parameters (i.e., site, polarity, intensity, duration, repetition rate) on tDCS efficacy in ADHD population and examine whether benefits are long-term.

The immediate and delayed effects of single tDCS session over posterior parietal cortex on face-word associative memory

Associative memory (AM), an ability to form and retrieve associations between information units is crucial for everyday functioning and is affected by aging as well as by different neurological conditions. It was shown that rTMS over posterior parietal cortex (PPC) can improve AM of face-word pairs. Therefore, we examined if tDCS will produce comparable effects and explore whether the effect would persist one and five days following the stimulation. Thirty-seven healthy participants took part in cross-over sham-controlled study in which they received 20 min of anodal (1.5 mA) or sham tDCS over left PPC. Following tDCS participants completed face-cued word recall and verbal fluency tasks. A randomly selected subsample (N = 18) has completed follow up memory assessments one and five days after the stimulation. Anodal tDCS facilitated AM performance in comparison to sham with the same trend persisting during the 5-day follow-up period. Additionally, participants with lower AM scores had higher relative gain following anodal tDCS. Anodal tDCS had no effect on the control task (verbal fluency). Results support the existence of a specific enhancing effect on AM produced by facilitatory neuromodulation of the PPC. The effect was more prominent in low-performers and it persisted at least 5 days post-stimulation. These findings support the robustness of tDCS effect on AM and provide a foundation for future research that could lead to its future clinical application.

A systematic review and meta-analysis of the effects of transcranial direct current stimulation (tDCS) on episodic memory

BACKGROUND

In the past decade, several studies have examined the effects of transcranial direct current stimulation (tDCS) on long-term episodic memory formation and retrieval. These studies yielded conflicting results, likely due to differences in stimulation parameters, experimental design and outcome measures.

OBJECTIVES

In this work we aimed to assess the robustness of tDCS effects on long-term episodic memory using a meta-analytical approach.

METHODS

We conducted four meta-analyses to analyse the effects of anodal and cathodal tDCS on memory accuracy and response times. We also used a moderator analysis to examine whether the size of tDCS effects varied as a function of specific stimulation parameters and experimental conditions.

RESULTS

Although all selected studies reported a significant effect of tDCS in at least one condition in the published paper, the results of the four meta-analyses showed only statistically non-significant close-to-zero effects. A moderator analysis suggested that for anodal tDCS, the duration of the stimulation and the task used to probe memory moderated the effectiveness of tDCS. For cathodal tDCS, site of stimulation was a significant moderator, although this result was based on only a few observations.

CONCLUSIONS

To warrant theoretical advancement and practical implications, more rigorous research is needed to fully understand whether tDCS reliably modulates episodic memory, and the specific circumstances under which this modulation does, and does not, occur.

Verbal long-term memory is enhanced by retrieval practice but impaired by prefrontal direct current stimulation

Retrieval practice involves repeatedly testing a student during the learning experience, reliably conferring learning advantages relative to repeated study. Transcranial direct current stimulation (tDCS) of the left dorsolateral prefrontal cortex (dlPFC) has also been shown to confer learning advantages for verbal memory, though research is equivocal. The present study examined the effects of retrieval versus study practice with or without left dlPFC tDCS on verbal episodic memory. Participants (N = 150) experienced either retrieval practice or study practice, and active anodal, active cathodal, or sham tDCS while encoding word lists, and then returned two days later for a final recall test. Three primary patterns emerged: first, during encoding, tDCS did not influence recall rates in the retrieval practice group. Second, during final recall, participants in the retrieval practice groups recalled more than those in the study practice groups. Finally, during final recall, anodal tDCS decreased recall relative to sham and cathodal stimulation, suggesting that it interfered with developing highly detailed memories that could be relied upon for subsequent recollection. Data support existing research demonstrating the effectiveness of retrieval practice as a learning strategy, but also suggest that anodal dlPFC stimulation can induce long-term negative impacts on verbal episodic memory retrieval.

Modulating the interhemispheric balance in healthy participants with transcranial direct current stimulation: No significant effects on word or sentence processing

Patient studies and brain stimulation evidence suggest that language processing can be enhanced by altering the interhemispheric balance: namely, preferentially enhancing left-hemisphere activity while suppressing right-hemisphere activity. To our knowledge, no study has yet compared the effects of such bilateral brain stimulation to both logically necessary control conditions (separate left- and right-hemisphere stimulation). This study did so in a between-group sham-controlled design, applying transcranial direct current stimulation over Broca's area and/or its homologue in 72 healthy participants. The effects were measured not only in a single-word-level task but also in a sentence-level task, rarely tested previously. We did not find either any significant overall effects of stimulation or greater stimulation effects in the bilateral compared to control groups. This null result, obtained in a large sample, contributes to the debate on whether tDCS can modulate language processing in healthy individuals.

Investigating the roles of medial prefrontal and superior temporal cortex in source monitoring

Source monitoring, or the ability to recall the origin of information, is a crucial aspect of remembering past experience. One facet of this, reality monitoring, refers to the ability to distinguish between internally generated and externally generated information, biases in which have previously been associated with auditory verbal hallucinations in schizophrenia. Neuroimaging evidence suggests that medial prefrontal and superior temporal (STG) regions may play a role in reality monitoring for auditory verbal information, with evidence from a previous neurostimulation experiment also suggesting that modulation of excitability in STG may affect reality monitoring task performance. Here, two experiments are reported that used transcranial direct current stimulation (tDCS) to modulate excitability in medial prefrontal and superior temporal cortex, to further investigate the role of these brain regions in reality monitoring. In the first experiment (N = 36), tDCS was applied during the encoding stage of the task, while in the second experiment, in a separate sample (N = 36), it was applied during the test stage. There was no effect of tDCS compared to a sham condition in either experiment, with Bayesian analysis providing evidence for the null hypothesis in both cases. This suggests that tDCS applied to superior temporal or medial prefrontal regions may not affect reality monitoring performance, and has implications for theoretical models that link reality monitoring to the therapeutic effect of tDCS on auditory verbal hallucinations.

Effects of Prefrontal Transcranial Direct Current Stimulation on Lexical Access in Chronic Poststroke Aphasia

Background. A successful interplay between prefrontal and domain-specific language areas is critical for language processing. Previous studies involving people with aphasia have shown that executive control processes might act on lexical-semantic representations during retrieval. Modulating the prefrontal control network by means of noninvasive brain stimulation might, therefore, improve lexical access in people with aphasia. Objective. The present study investigates the effects of prefrontal transcranial direct current stimulation (tDCS) on lexical access in chronic poststroke aphasia. Methods. We report data of 14 participants with chronic poststroke aphasia. We used a sham-tDCS (S-tDCS) controlled and double-blind within-subjects design. Performances in picture naming, verbal fluency, and word repetition were assessed immediately after stimulation. Results. As compared with S-tDCS, anodal tDCS (A-tDCS) improved verbal fluency as well as the speed of naming high frequency words, but not word repetition. Conclusion. The results of our study suggest that the brain network dedicated to lexical retrieval processing can be facilitated by A-tDCS over the left dorsolateral prefrontal cortex. This finding supports the notion that strengthening executive control functions after stroke could complement speech and language-focused therapy.

Neuromodulation of Right Auditory Cortex Selectively Increases Activation in Speech-Related Brain Areas in Brainstem Auditory Agnosia

Auditory agnosia is an inability to make sense of sound that cannot be explained by deficits in low-level hearing. In view of recent promising results in the area of neurorehabilitation of language disorders after stroke, we examined the effect of transcranial direct current stimulation (tDCS) in a young woman with general auditory agnosia caused by traumatic injury to the left inferior colliculus. Specifically, we studied activations to sound embedded in a block design using functional magnetic resonance imaging before and after application of anodal tDCS to the right auditory cortex. Before tDCS, auditory discrimination deficits were associated with abnormally reduced activations of the auditory cortex and bilateral unresponsiveness of the anterior superior temporal sulci and gyri. This session replicated a previous functional scan with the same paradigm a year before the current experiment. We then applied anodal tDCS over right auditory cortex for 20 min-utes and immediately re-scanned the patient. We found increased activation of bilateral auditory cortices and, for speech sounds, selectively increased activation in Broca's and Wernicke's areas. Future research might consider the long-term behavioral effects after neurostimulation in auditory agnosia and its potential use in the neurorehabilitation of more general auditory disorders.

Putting focus on transcranial direct current stimulation in language production studies

Previous language production studies targeting the inferior frontal and superior temporal gyrus using anodal tDCS have provided mixed results. Part of this heterogeneity may be explained by limited target region focality of conventionally used electrode montages. We examined the focality of conventionally and alternative electrode montages. Electrical field distributions of anodal tDCS targeting IFG and pSTG were simulated in conventional setups (anodal electrode over left IFG/pSTG, reference electrode over right supraorbital region) and an alternative electrode montage in four different brains. Conventional montages showed maximum field strengths outside of the target regions. Results from alternative electrode montages showed that focality of tDCS could be improved by adjustments in electrode placement. Heterogeneity of findings of language production studies deploying conventional montages may in part be explained by diffuse electrical field distributions. Alternative montages may improve focality and provide more unequivocal results.

tDCS Facilitation of Picture Naming: Item-Specific, Task General, or Neither?

The aim of the present study was to clarify the conditions under which anodal tDCS applied to left hemisphere language sites may facilitate picture naming latencies in healthy young adults. We built upon previous studies by directly testing for item-specific and generalized effects of tDCS through manipulation of item-familiarization and through testing for both online and offline effects of stimulation, in the same paradigm. In addition, we tested for the robustness of these effects by comparing two left hemisphere sites critical for lexical retrieval. Twenty-eight healthy young adults completed two testing sessions receiving either anodal (1.5 mA, 20 min) or sham stimulation (1.5 mA, 30 s) in each session. Half of the participants received tDCS over the left inferior frontal region and the other half over the left posterior superior temporal region. All participants were asked to a name a set of pictures and their response latencies were compared at three time points (before, during, and after the end of stimulation). The stimulus set was constructed so that some items were presented at all time points, some before and after stimulation, and some during stimulation only. A parsimonious linear mixed effects model (LMM) revealed robust repetition priming effects as latencies were reliably faster for previously named items in all conditions. However, active tDCS did not produce any additional facilitation in relation to sham, and even led to slower performance in the IFG group when the stimulated items differed from those tested at baseline and post-test. Our findings add to the present debate about the efficacy of single-session tDCS for modulation of lexical retrieval in healthy young adults. We conclude that future research should take a more systematic, step-wise approach to the application of tDCS to the study of language and that more sensitive experimental paradigms, which include a training element, are more adapted to the study of cognitive processes in populations with optimal levels of cortical excitability.

Adaptability and reproducibility of a memory disruption rTMS protocol in the PharmaCog IMI European project

Transcranial magnetic stimulation (TMS) can interfere with cognitive processes, such as transiently impairing memory. As part of a multi-center European project, we investigated the adaptability and reproducibility of a previously published TMS memory interfering protocol in two centers using EEG or fMRI scenarios. Participants were invited to attend three experimental sessions on different days, with sham repetitive TMS (rTMS) applied on day 1 and real rTMS on days 2 and 3. Sixty-eight healthy young men were included. On each experimental day, volunteers were instructed to remember visual pictures while receiving neuronavigated rTMS trains (20 Hz, 900 ms) during picture encoding at the left dorsolateral prefrontal cortex (L-DLPFC) and the vertex. Mixed ANOVA model analyses were performed. rTMS to the L-DLPFC significantly disrupted recognition memory on experimental day 2. No differences were found between centers or between fMRI and EEG recordings. Subjects with lower baseline memory performances were more susceptible to TMS disruption. No stability of TMS-induced memory interference could be demonstrated on day 3. Our data suggests that adapted cognitive rTMS protocols can be implemented in multi-center studies incorporating standardized experimental procedures. However, our center and modality effects analyses lacked sufficient statistical power, hence highlighting the need to conduct further studies with larger samples. In addition, inter and intra-subject variability in response to TMS might limit its application in crossover or longitudinal studies.

Frontotemporal stimulation modulates semantically-guided visual search during confrontation naming: A combined tDCS and eye tracking investigation

Transcranial direct current stimulation (tDCS) was paired with eye tracking to elucidate contributions of frontal, temporoparietal and anterior temporal cortex to early visual search patterns during picture naming (e.g., rapid visual scanning to diagnostic semantic features). Neurotypical adults named line drawings of objects prior to and following tDCS in three separate sessions, each employing a unique electrode montage. The gaze data revealed montage by stimulation (pre/post) interaction effects characterized by longer initial visual fixations (mean difference = 89 ms; Cohen's d = .8) and cumulative fixation durations (mean difference = 98 ms; Cohen's d = .9) on key semantic features (e.g., the head of an animal) after cathodal frontotemporal stimulation relative to the pre-stimulation baseline. We interpret these findings as reflecting a tDCS-induced modulation of semantic contributions of the anterior temporal lobe(s) to top-down influences on object recognition. Further, we discuss implications for the optimization of tDCS for the treatment of anomia in aphasia.

Blood perfusion in left inferior and middle frontal gyrus predicts communication skills in schizophrenia

Schizophrenia is one of the most debilitating mental disorders, severely affecting patients' community functioning. Functional capacity (FC) describes the skills necessary for successful functional outcome (FO), such as financial knowledge or communication skills. We investigated whether resting-state cerebral blood flow (rCBF) measured at baseline with arterial spin labeling (ASL) would predict FC or FO at a 6-months follow-up in 27 schizophrenia patients. A negative linear association was noted between FC and rCBF in the left inferior and middle frontal gyrus, areas involved in language processing, suggesting a potential relationship between cerebral perfusion and prospective FC in schizophrenia.

Anodal tDCS over Primary Motor Cortex Provides No Advantage to Learning Motor Sequences via Observation

When learning a new motor skill, we benefit from watching others. It has been suggested that observation of others' actions can build a motor representation in the observer, and as such, physical and observational learning might share a similar neural basis. If physical and observational learning share a similar neural basis, then motor cortex stimulation during observational practice should similarly enhance learning by observation as it does through physical practice. Here, we used transcranial direct-current stimulation (tDCS) to address whether anodal stimulation to M1 during observational training facilitates skill acquisition. Participants learned keypress sequences across four consecutive days of observational practice while receiving active or sham stimulation over M1. The results demonstrated that active stimulation provided no advantage to skill learning over sham stimulation. Further, Bayesian analyses revealed evidence in favour of the null hypothesis across our dependent measures. Our findings therefore provide no support for the hypothesis that excitatory M1 stimulation can enhance observational learning in a similar manner to physical learning. More generally, the results add to a growing literature that suggests that the effects of tDCS tend to be small, inconsistent, and hard to replicate. Future tDCS research should consider these factors when designing experimental procedures.

Null Effects on Working Memory and Verbal Fluency Tasks When Applying Anodal tDCS to the Inferior Frontal Gyrus of Healthy Participants

Transcranial direct current stimulation (tDCS) is a technique used to modify cognition by modulating underlying cortical excitability via weak electric current applied through the scalp. Although many studies have reported positive effects with tDCS, a number of recent studies highlight that tDCS effects can be small and difficult to reproduce. This is especially the case when attempting to modulate performance using single applications of tDCS in healthy participants. Possible reasons may be that optimal stimulation parameters have yet to be identified, and that individual variation in cortical activity and/or level of ability confound outcomes. To address these points, we carried out a series of experiments in which we attempted to modulate performance in fluency and working memory probe tasks using stimulation parameters which have been associated with positive outcomes: we targeted the left inferior frontal gyrus (LIFG) and compared performance when applying a 1.5 mA anodal current for 25 min and with sham stimulation. There is evidence that LIFG plays a role in these tasks and previous studies have found positive effects of stimulation. We also compared our experimental group (N = 19–20) with a control group receiving no stimulation (n = 24). More importantly, we also considered effects on subgroups subdivided according to memory span as well as to more direct measures of executive function abilities and motivational levels. We found no systematic effect of stimulation. Our findings are in line with a growing body of evidence that tDCS produces unreliable effects. We acknowledge that our findings speak to the conditions we investigated, and that alternative protocols (e.g., multiple sessions, clinical samples, and different stimulation polarities) may be more effective. We encourage further research to explore optimal conditions for tDCS efficacy, given the potential benefits that this technique poses for understanding and enhancing cognition.

The Role of Left Dorsolateral Prefrontal Cortex in Language Processing

In addition to the role of left frontotemporal areas in language processing, there is increasing evidence that language comprehension and production require cognitive control and working memory resources involving the left dorsolateral prefrontal cortex (DLPFC). The aim of this study was to investigate the role of the left DLPFC in both language comprehension and production. In a double-blind, sham-controlled crossover experiment, thirty-two participants received cathodal or sham transcranial direct current stimulation (tDCS) to the left DLPFC while performing a language comprehension and a language production task. Results showed that cathodal tDCS increases reaction times in the language comprehension task, but decreases naming latencies in the language production task. However, additional analyses revealed that the polarity of tDCS effects was highly correlated across tasks, implying differential individual susceptibility to the effect of tDCS within participants. Overall, our findings demonstrate that left DLPFC is part of the complex cortical network associated with language processing.

The Effect of Transcranial Direct Current Stimulation on Jaw Motor Function Is Task Dependent: Speech, Syllable Repetition and Chewing

Motor cortex transcranial direct current stimulation (tDCS) has been shown to enhance motor learning in healthy adults as well as various neurological conditions. However, there has been limited data on whether tDCS enhances jaw motor performance during different oral behaviors such as speech, maximum syllable repetition, and chewing. Because the effects of anodal and cathodal stimulation are known to be dependent on task demands, we hypothesized that tDCS would have a distinct effect on the jaw motor performance during these disparate oral behaviors. Ten healthy adults completed speech, maximum syllable repetition, and chewing tasks as their jaw movements were recorded using 3D optical motion capture during sham, anodal, and cathodal tDCS. Our findings showed that compared to the sham condition, jaw displacements during speech and syllable repetition were smaller during anodal stimulation, but larger during cathodal stimulation for syllable repetition and chewing indicating improved performance during anodal tDCS. On the other hand, there were no effects of anodal tDCS during chewing. These results confirm our hypotheses that: (1) tDCS induces a significant effect on jaw motor function; (2) its effects are polarity dependent; and (3) its effects are dependent on the task demands on jaw motor function. These findings support future studies exploring the effects of tDCS on persons with oral sensorimotor impairments and the development of therapeutic protocols.

Cathodal tDCS of the bilateral anterior temporal lobes facilitates semantically-driven verbal fluency

In a verbal fluency task, a person is required to produce as many exemplars of a given category (e.g., 'animals', or words starting with 'f') as possible within a fixed duration. Successful verbal fluency performance relies both on the depth of search within semantic/phonological neighborhoods ('clustering') and the ability to flexibly disengage between exhausted clusters ('switching'). Convergent evidence from functional imaging and neuropsychology suggests that cluster-switch behaviors engage dissociable brain regions. Switching has been linked to a frontoparietal network dedicated to executive functioning and controlled lexical retrieval, whereas clustering is more commonly associated with temporal lobe regions dedicated to semantic and phonological processing. Here we attempted to modulate cluster-switch dynamics among neurotypical adults (N = 24) using transcranial direct current stimulation (tDCS) delivered at three sites: a) anterior temporal cortex; b) frontal cortex; and c) temporoparietal cortex. Participants completed letter-guided and semantic category verbal fluency tasks pre/post stimulation. Cathodal stimulation of anterior temporal cortex facilitated the total number of words generated and the number of words generated within clusters during semantic category verbal fluency. These neuromodulatory effects were specific to stimulation of the one anatomical site. Our findings highlight the role of the anterior temporal lobes in representing semantic category structure and support the claim that clustering and switching behaviors have distinct substrates. We discuss implications both for theory and application to neurorehabilitation.

Transcranial electric stimulation for the investigation of speech perception and comprehension

Transcranial electric stimulation (tES), comprising transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), involves applying weak electrical current to the scalp, which can be used to modulate membrane potentials and thereby modify neural activity. Critically, behavioural or perceptual consequences of this modulation provide evidence for a causal role of neural activity in the stimulated brain region for the observed outcome. We present tES as a tool for the investigation of which neural responses are necessary for successful speech perception and comprehension. We summarise existing studies, along with challenges that need to be overcome, potential solutions, and future directions. We conclude that, although standardised stimulation parameters still need to be established, tES is a promising tool for revealing the neural basis of speech processing. Future research can use this method to explore the causal role of brain regions and neural processes for the perception and comprehension of speech.

No evidential value in samples of transcranial direct current stimulation (tDCS) studies of cognition and working memory in healthy populations

A substantial number of studies have been published over the last decade, claiming that transcranial direct current stimulation (tDCS) can influence performance on cognitive tasks. However, there is some skepticism regarding the efficacy of tDCS, and evidence from meta-analyses are mixed. One major weakness of these meta-analyses is that they only examine outcomes in published studies. Given biases towards publishing positive results in the scientific literature, there may be a substantial "file-drawer" of unpublished negative results in the tDCS literature. Furthermore, multiple researcher degrees of freedom can also inflate published p-values. Recently, Simonsohn, Nelson and Simmons (2014) created a novel meta-analytic tool that examines the distribution of significant p-values in a literature, and compares it to expected distributions with different effect sizes. Using this tool, one can assess whether the selected studies have evidential value. Therefore, we examined a random selection of studies that used tDCS to alter performance on cognitive tasks, and tDCS studies on working memory in a recently published meta-analysis (Mancuso et al., 2016). Using a p-curve analysis, we found no evidence that the tDCS studies had evidential value (33% power or greater), with the estimate of statistical power of these studies being approximately 14% for the cognitive studies, and 5% (what would be expected from randomly generated data) for the working memory studies. It is likely that previous tDCS studies are substantially underpowered, and we provide suggestions for future research to increase the evidential value of future tDCS studies.