Battery powered thought: enhancement of attention, learning, and memory in healthy adults using transcranial direct current stimulation

Combined transcranial direct current stimulation with virtual reality exposure for posttraumatic stress disorder: Feasibility and pilot results

BACKGROUND

Facilitating neural activity using non-invasive brain stimulation may improve extinction-based treatments for posttraumatic stress disorder (PTSD).

OBJECTIVE/HYPOTHESIS

Here, we examined the feasibility of simultaneous transcranial direct current stimulation (tDCS) application during virtual reality (VR) to reduce psychophysiological arousal and symptoms in Veterans with PTSD.

METHODS

Twelve Veterans with PTSD received six combat-related VR exposure sessions during sham-controlled tDCS targeting ventromedial prefrontal cortex. Primary outcome measures were changes in skin conductance-based arousal and self-reported PTSD symptom severity.

RESULTS

tDCS + VR components were combined without technical difficulty. We observed a significant interaction between reduction in arousal across sessions and tDCS group (p = .03), indicating that the decrease in physiological arousal was greater in the tDCS + VR versus sham group. We additionally observed a clinically meaningful reduction in PTSD symptom severity.

CONCLUSIONS

This study demonstrates feasibility of applying tDCS during VR. Preliminary data suggest a reduction in psychophysiological arousal and PTSD symptomatology, supporting future studies.

Safety of Transcranial Direct Current Stimulation of Frontal, Parietal, and Cerebellar Regions in Fasting Healthy Adults

(1) Background: Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation modality that has been investigated in a large number of studies in terms of it is effects on brain function, safety of use, and future implications. The principal aim of this study was to investigate the safety of 1.5-mA tDCS of three brain areas, that is, frontal, partial, and cerebellar cortices, in fasting healthy individuals during the month of Ramadan. (2) Methods: In a single-blinded, sham-controlled study, we assessed the safety of a 20-min tDCS current (1.5 mA, 35 cm²) over the right frontal, parietal, and cerebellar cortex areas after 8 h of fasting in healthy right-handed adult subjects using a standard safety questionnaire. (3) Results: A total of 49 subjects completed the tDCS sessions and safety questionnaire. None of the sessions were stopped due to pain or discomfort during stimulation. Moreover, no subject experienced serious adverse events such as seizures or loss of consciousness. (4) Conclusions: There was no significant difference in the frequency or type of side effects between active and sham stimulation sessions. The tDCS protocol applied in this study was found to be safe in fasting healthy adults.

No Evidence That Frontal Eye Field tDCS Affects Latency or Accuracy of Prosaccades

Transcranial direct current stimulation (tDCS) may be used to directly affect neural activity from outside of the skull. However, its exact physiological mechanisms remain elusive, particularly when applied to new brain areas. The frontal eye field (FEF) has rarely been targeted with tDCS, even though it plays a crucial role in control of overt and covert spatial attention. Here, we investigate whether tDCS over the FEF can affect the latency and accuracy of saccadic eye movements. Twenty-six participants performed a prosaccade task in which they made eye movements to a sudden-onset eccentric visual target (lateral saccades). After each lateral saccade, they made an eye movement back to the center (center saccades). The task was administered before, during, and after anodal or cathodal tDCS over the FEF, in a randomized, double-blind, within-subject design. One previous study (Kanai et al., 2012) found that anodal tDCS over the FEF decreased the latency of saccades contralateral to the stimulated hemisphere. We did not find the same effect: neither anodal nor cathodal tDCS influenced the latency of lateral saccades. tDCS also did not affect accuracy of lateral saccades (saccade endpoint deviation and saccade endpoint variability). For center saccades, we found some differences between the anodal and cathodal sessions, but these were not consistent across analyses (latency, endpoint variability), or were already present before tDCS onset (endpoint deviation). We tried to improve on the design of Kanai et al. (2012) in several ways, including the tDCS duration and electrode montage, which could explain the discrepant results. Our findings add to a growing number of null results, which have sparked concerns that tDCS outcomes are highly variable. Future studies should aim to establish the boundary conditions for FEF-tDCS to be effective, in addition to increasing sample size and adding additional controls such as a sham condition. At present, we conclude that it is unclear whether eye movements or other aspects of spatial attention can be affected through tDCS of the frontal eye fields.

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

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

Translating molecular advances in Down syndrome and Fragile X syndrome into therapies

Ongoing treatments for genetic developmental disorders of the central nervous system are mostly symptomatic and do not correct the genetic cause. Recent identification of common mechanisms between diseases has suggested that new therapeutic targets could be applied across intellectual disabilities with potential disease-modifying properties. The European Down syndrome and other genetic developmental disorders (DSG2D) network joined basic and clinical scientists to foster this research and carry out clinical trials. Here we discuss common mechanisms between several intellectual disabilities from genetic origin including Down's and Fragile X syndromes: i) how to model these complex diseases using neuronal cells and brain organoids derived from induced pluripotent stem cells; ii) how to integrate genomic, proteomic and interactome data to help defining common mechanisms and boundaries between diseases; iii) how to target common pathways for designing clinical trials and assessing their efficacy; iv) how to bring new neuro-therapies, such as noninvasive brain stimulations and cognitive training to clinical research. The basic and translational research efforts of the last years have utterly transformed our understanding of the molecular pathology of these diseases but much is left to be done to bring them to newborn babies and children to improve their quality of life.

Individual differences in learning correlate with modulation of brain activity induced by transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) has been shown to enhance cognitive performance on a variety of tasks. It is hypothesized that tDCS enhances performance by affecting task related cortical excitability changes in networks underlying or connected to the site of stimulation facilitating long term potentiation. However, many recent studies have called into question the reliability and efficacy of tDCS to induce modulatory changes in brain activity. In this study, our goal is to investigate the individual differences in tDCS induced modulatory effects on brain activity related to the degree of enhancement in performance, providing insight into this lack of reliability. In accomplishing this goal, we used functional magnetic resonance imaging (fMRI) concurrently with tDCS stimulation (1 mA, 30 minutes duration) using a visual search task simulating real world conditions. The experiment consisted of three fMRI sessions: pre-training (no performance feedback), training (performance feedback which included response accuracy and target location and either real tDCS or sham stimulation given), and post-training (no performance feedback). The right posterior parietal cortex was selected as the site of anodal tDCS based on its known role in visual search and spatial attention processing. Our results identified a region in the right precentral gyrus, known to be involved with visual spatial attention and orienting, that showed tDCS induced task related changes in cortical excitability that were associated with individual differences in improved performance. This same region showed greater activity during the training session for target feedback of incorrect (target-error feedback) over correct trials for the tDCS stim over sham group indicating greater attention to target features during training feedback when trials were incorrect. These results give important insight into the nature of neural excitability induced by tDCS as it relates to variability in individual differences in improved performance shedding some light the apparent lack of reliability found in tDCS research.

tDCS Modulates Visual Gamma Oscillations and Basal Alpha Activity in Occipital Cortices: Evidence from MEG

Transcranial direct-current stimulation (tDCS) is now a widely used method for modulating the human brain, but the resulting physiological effects are not understood. Recent studies have combined magnetoencephalography (MEG) with simultaneous tDCS to evaluate online changes in occipital alpha and gamma oscillations, but no study to date has quantified the offline (i.e., after tDCS) alterations in these responses. Thirty-five healthy adults received active or sham anodal tDCS to the occipital cortices, and then completed a visual stimulation paradigm during MEG that is known to elicit robust gamma and alpha oscillations. The resulting MEG data were imaged and peak voxel time series were extracted to evaluate tDCS effects. We found that tDCS to the occipital increased the amplitude of local gamma oscillations, and basal alpha levels during the baseline. tDCS was also associated with network-level effects, including increased gamma oscillations in the prefrontal cortex, parietal, and other visual attention regions. Finally, although tDCS did not modulate peak gamma frequency, this variable was inversely correlated with gamma amplitude, which is consistent with a GABA-gamma link. In conclusion, tDCS alters gamma oscillations and basal alpha levels. The net offline effects on gamma activity are consistent with the view that anodal tDCS decreases local GABA.

Transcranial Direct Current Stimulation (tDCS) Can Modulate EEG Complexity of Children With Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder which affects the developmental trajectory in several behavioral domains, including impairments of social communication, cognitive and language abilities. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique, and it was used for modulating the brain disorders. In this paper, we enrolled 13 ASD children (11 males and 2 females; mean ± SD age: 6.5 ± 1.7 years) to participate in our trial. Each patient received 10 treatments over the dorsolateral prefrontal cortex (DLPFC) once every 2 days. Also, we enrolled 13 ASD children (11 males and 2 females; mean ± SD age: 6.3 ± 1.7 years) waiting to receive therapy as controls. A maximum entropy ratio (MER) method was adapted to measure the change of complexity of EEG series. It was found that the MER value significantly increased after tDCS. This study suggests that tDCS may be a helpful tool for the rehabilitation of children with ASD.

Cognitive effects of transcranial direct current stimulation treatment in patients with major depressive disorder: An individual patient data meta-analysis of randomised, sham-controlled trials

Transcranial direct current stimulation (tDCS) has emerged as a promising new treatment for major depression. While recent randomised, sham-controlled studies found tDCS to have antidepressant effects, it remains to be determined whether a tDCS treatment course may also enhance cognitive function independent of mood effects in depressed patients. This systematic review and individual patient data (IPD) meta-analysis examined cognitive outcomes from randomised, sham-controlled trials of tDCS treatment for major depression. Seven randomised, sham-controlled trials (n = 478 participants, 260 in active and 218 in sham) of tDCS for major depression were included. Results showed no cognitive enhancement after active tDCS compared to sham for the 12 cognitive outcomes investigated. Active relative to sham tDCS treatment was associated with reduced performance gains on a measure of processing speed (β = -0.33, 95% CI -0.58; -0.08, p = 0.011). Active tDCS treatment for depression did not show cognitive benefits independent of mood effects. Rather, tDCS treatment relative to sham stimulation for major depression may instead be associated with a reduced practice effect for processing speed.

Null tDCS Effects in a Sustained Attention Task: The Modulating Role of Learning

The purpose of this study was to investigate sustained attention through modulation of the fronto-cerebral network with transcranial direct current stimulation (tDCS) in adults with attention-deficit/hyperactivity disorder (ADHD) and control participants. Thirty-seven participants (21 with ADHD) underwent three separate sessions (baseline, active tDCS, and sham) and performed the MOXO Continuous Performance Test (CPT). We applied double anodal stimulation of 1.8 mA tDCS for 20 min over the left and right dorsolateral prefrontal cortex (DLPFC), with the cathode over the cerebellum. Baseline session revealed significant differences between ADHD and control participants in the MOXO-CPT attention and hyperactivity scores, validating the MOXO as a diagnostic tool. However, there were no tDCS effects in most MOXO-CPT measures, except hyperactivity, due to a significant learning effect. We conclude that learning and repetition effects in cognitive tasks need to be considered when designing within-subjects tDCS experiments, as there are natural improvements between sessions that conceal potential stimulation effects.

Gamma-band auditory steady-state response after frontal tDCS: A double-blind, randomized, crossover study

The effects of transcranial direct current stimulation (tDCS) likely depend on cortical N-methyl-D-aspartic acid (NMDA) neurotransmission; however, no previous studies have reported tDCS-mediated modulation of cortical NMDA neurotransmission in humans. The gamma-band auditory steady-state response (ASSR) to a 40 Hz stimulation likely reflects the integrity of cortical NMDA neurotransmission. The present study tested whether the effect of tDCS is reflected in gamma-band ASSRs during a 40 Hz stimulation. Using a double-blind, randomized, crossover study, we performed magnetoencephalography (MEG) and measured the ASSR in 24 healthy participants during 40 Hz of auditory stimulation after prefrontal tDCS (2 mA) or sham (i.e., placebo) treatment. Our results failed to reveal significant differences in any brain between the two conditions after the application of a frequency of approximately 40 Hz. Based on these results, the ASSR is an insufficient method to detect the effect of tDCS on cortical NMDA neurotransmission. Unexpectedly, the results revealed an enhanced beta-band event-related spectral perturbation (ERSP) in the left motor cortex after tDCS compared with that observed after the sham stimuli. Given that beta-band oscillations reflect many functions in motor cortices, the tDCS for the frontal areas had some effect on the left motor cortex while the participants were focusing on not pressing the button with their right index finger. An additional study with an adequate psychological task is necessary to draw a conclusion regarding this unexpected result.

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.

High-definition transcranial direct current stimulation (HD-tDCS) of left dorsolateral prefrontal cortex affects performance in Balloon Analogue Risk Task (BART)

Background

Studies on risk preferences have long been of great concern and have examined the neural basis underlying risk-based decision making. However, studies using conventional transcranial direct current stimulation (tDCS) revealed that bilateral stimulation could change risk propensity with limited evidence of precisely focalized unilateral high-definition transcranial direct current stimulation (HD-tDCS). The aim of this experiment was to investigate the effect of HD-tDCS focalizing the left dorsal lateral prefrontal cortex (DLPFC) on risk-taking behavior during the Balloon Analogue Risk Task (BART).

Methods

This study was designed as a between-subject, single-blind, sham-controlled experiment. University students were randomly assigned to three groups: the anodal group (F3 anode, AF3, F1, F5, FC3 returned), the cathodal group (F3 cathodal, AF3, F1, F5, FC3 returned) and the sham group. Subsequently, 1.5-mA 20-min HD-tDCS was applied during the BART, and the Positive Affect and Negative Affect Scale (PANAS), the Sensation Seeking Scale-5 (SSS-5), and the Behavioral Inhibition System and Behavioral Approach System scale (BIS/BAS) were measured as control variables.

Results

The cathodal group earned less total money than the sham group, and no significant difference was observed between the anodal group and the sham group.

Conclusions

These results showed that, to some extent, focalized unilateral cathodal HD-tDCS on left DLPFC could change performance during risky tasks and diminish risky decision making. Further studies are needed to investigate the dose effect and electrode distribution of HD-tDCS during risky tasks and examine synchronous brain activity to show the neural basis.

Concurrent measurement of cerebral hemodynamics and electroencephalography during transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) is currently being used for research and treatment of some neurological and neuropsychiatric disorders, as well as for improvement of cognitive functions. In order to better understand cerebral response to the stimulation and to redefine protocols and dosage, its effects must be monitored. To this end, we have used functional diffuse correlation spectroscopy (fDCS) and time-resolved functional near-infrared spectroscopy (TR-fNIRS) together with electroencephalography (EEG) during and after stimulation of the frontal cortex. Twenty subjects participated in two sessions of stimulation with two different polarity montages and twelve also underwent a sham session. Cerebral blood flow and oxyhemoglobin concentration increased during and after active stimulation in the region under the stimulation electrode while deoxyhemoglobin concentration decreased. The EEG spectrum displayed statistically significant power changes across different stimulation sessions in delta (2 to 4 Hz), theta (4 to 8 Hz), and beta (12 to 18 Hz) bands. Results suggest that fDCS and TR-fNIRS can be employed as neuromonitors of the effects of transcranial electrical stimulation and can be used together with EEG.

[Cognitive neuroscience of aging. Contributions and challenges]

The cognitive neuroscience of aging is a young discipline that has emerged as a result of the combination of: A) the theoretical and explanatory frameworks proposed by the cognitive psychology perspective throughout the second half of the twentieth century; B) the designs and methodological procedures arising from experimental psychology and the need to test the hypotheses proposed from the cognitive psychology perspective; C) the contributions of the computer sciences to the explanation of brain functions; and D) the development and use of neuroimaging techniques that have enabled the recording of brain activity in humans while tasks that test some cognitive process or function are performed. An analysis on the impact of research conducted from this perspective over the last 3decades has been carried out, including its shortcomings, as well as the potential directions and usefulness that will advantageously continue to drive this discipline in its description and explanation of the process es of cerebral and cognitive aging.

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

INTRODUCTION

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

AIMS

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

RESULTS

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

CONCLUSIONS

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

Transcranial Direct Current Stimulation (tDCS): A Beginner's Guide for Design and Implementation

Transcranial direct current stimulation (tDCS) is a popular brain stimulation method that is used to modulate cortical excitability, producing facilitatory or inhibitory effects upon a variety of behaviors. There is, however, a current lack of consensus between studies, with many results suggesting that polarity-specific effects are difficult to obtain. This article explores some of these differences and highlights the experimental parameters that may underlie their occurrence. We provide a general, practical snapshot of tDCS methodology, including what it is used for, how to use it, and considerations for designing an effective and safe experiment. Our aim is to equip researchers who are new to tDCS with the essential knowledge so that they can make informed and well-rounded decisions when designing and running successful experiments. By summarizing the varied approaches, stimulation parameters, and outcomes, this article should help inform future tDCS research in a variety of fields.

Modulating affective experience and emotional intelligence with loving kindness meditation and transcranial direct current stimulation: A pilot study

Positive emotional perceptions and healthy emotional intelligence (EI) are important for social functioning. In this study, we investigated whether loving kindness meditation (LKM) combined with anodal transcranial direct current stimulation (tDCS) would facilitate improvements in EI and changes in affective experience of visual stimuli. LKM has been shown to increase positive emotional experiences and we hypothesized that tDCS could enhance these effects. Eighty-seven undergraduates were randomly assigned to 30 minutes of LKM or a relaxation control recording with anodal tDCS applied to the left dorsolateral prefrontal cortex (left dlPFC) or right temporoparietal junction (right TPJ) at 0.1 or 2.0 milliamps. The primary outcomes were self-reported affect ratings of images from the International Affective Picture System and EI as measured by the Mayer, Salovey and Caruso Emotional Intelligence Test. Results indicated no effects of training on EI, and no main effects of LKM, electrode placement, or tDCS current strength on affect ratings. There was a significant interaction of electrode placement by meditation condition (p = 0.001), such that those assigned to LKM and right TPJ tDCS, regardless of current strength, rated neutral and positive images more positively after training. Results suggest that LKM may enhance positive affective experience.

Therapeutic effect of transcranial direct current stimulation on neuropsychological symptoms of an elderly patient: A case report

ABSTRACT Although growing evidence points to the potential therapeutic effects of transcranial Direct Current Stimulation (tDCS), there is still no consensus on the most appropriate protocol to be used in specific neurological and neuropsychological symptoms. This case report evaluated the neuromodulatory therapeutic effects of two 15-day courses of tDCS on an elderly female patient, aged 78 years with mild neurocognitive disorder, chronic pain and depression-related symptoms. Results indicated an overall significant improvement of cognitive and executive functions, as well as reduction in both depression and chronic pain symptoms. These results highlight the potential of tDCS as a safe and useful neuromodulatory clinical tool in the rehabilitation of elderly patients.

Stimulation of Dorsolateral Prefrontal Cortex Enhances Adaptive Cognitive Control: A High-Definition Transcranial Direct Current Stimulation Study

Conflict adaptation is a hallmark effect of adaptive cognitive control and refers to the adjustment of control to the level of previously experienced conflict. Conflict monitoring theory assumes that the dorsolateral prefrontal cortex (DLPFC) is causally involved in this adjustment. However, to date, evidence in humans is predominantly correlational, and heterogeneous with respect to the lateralization of control in the DLPFC. We used high-definition transcranial direct current stimulation (HD-tDCS), which allows for more focal current delivery than conventional tDCS, to clarify the causal involvement of the DLPFC in conflict adaptation. Specifically, we investigated the regional specificity and lateralization of potential beneficial stimulation effects on conflict adaptation during a visual flanker task. One hundred twenty healthy participants were assigned to four HD-tDCS conditions: left or right DLPFC or left or right primary motor cortex (M1). Each group underwent both active and sham HD-tDCS in crossover, double-blind designs. We obtained a sizeable conflict adaptation effect (measured as the modulation of the flanker effect as a function of previous response conflict) in all groups and conditions. However, this effect was larger under active HD-tDCS than under sham stimulation in both DLPFC groups. In contrast, active stimulation had no effect on conflict adaptation in the M1 groups. In sum, the present results indicate that the DLPFC plays a causal role in adaptive cognitive control, but that the involvement of DLPFC in control is not restricted to the left or right hemisphere. Moreover, our study confirms the potential of HD-tDCS to modulate cognition in a regionally specific manner.

SIGNIFICANCE STATEMENT

Conflict adaptation is a hallmark effect of adaptive cognitive control. While animal studies have suggested causal involvement of the DLPFC in this phenomenon, such evidence is currently lacking in humans. The present study used high-definition transcranial direct current stimulation (HD-tDCS) to demonstrate that the DLPFC is causally involved in conflict adaptation in humans. Our study confirms a central claim of conflict monitoring theory, which up to now has predominantly relied on correlational studies. Our results further indicate an equal involvement of the left and right DLPFC in adaptive control, whereas stimulation of a control region-the primary motor cortex-had no effect on adaptive control. The study thus confirms the potential of HD-tDCS to modulate cognition in a regionally specific manner.

Effects of High-Definition Anodal Transcranial Direct Current Stimulation Applied Simultaneously to Both Primary Motor Cortices on Bimanual Sensorimotor Performance

Many daily activities, such as tying one’s shoe laces, opening a jar of jam or performing a free throw in basketball, require the skillful coordinated use of both hands. Even though the non-invasive method of transcranial direct current stimulation (tDCS) has been repeatedly shown to improve unimanual motor performance, little is known about its effects on bimanual motor performance. More knowledge about how tDCS may improve bimanual behavior would be relevant to motor recovery, e.g., in persons with bilateral impairment of hand function. We therefore examined the impact of high-definition anodal tDCS (HD-atDCS) on the performance of a bimanual sequential sensorimotor task. Thirty-two volunteers (age M = 24.25; SD = 2.75; 14 females) participated in this double-blind study and performed sport stacking in six experimental sessions. In sport stacking, 12 specially designed cups must be stacked (stacked up) and dismantled (stacked down) in predefined patterns as fast as possible. During a pretest, posttest and follow-up test, two sport stacking formations (3-6-3 stack and 1-10-1 stack) were performed. Between the pretest and posttest, all participants were trained in sport stacking with concurrent brain stimulation for three consecutive days. The experimental group (STIM-M1) received HD-atDCS over both primary motor cortices (M1), while the control group received a sham stimulation (SHAM). Three-way analysis of variance (ANOVA) revealed a significant main effect of TIME and a significant interaction of TIME × GROUP. No significant effects were found for GROUP, nor for the three-way interaction of TIME × GROUP × FORMATION. Further two-way ANOVAs showed a significant main effect of TIME and a non-significant main effect for GROUP in both sport stacking formations. A significant interaction between TIME × GROUP was found only for the 3-6-3 formation, indicating superior performance gains for the experimental group (STIM-M1). To account and control for baseline influences on the outcome measurements, ANCOVAs treating pretest scores as covariates revealed a significant effect of the stimulation. From this, we conclude that bilateral HD-atDCS over both M1 improves motor performance in a bimanual sequential sensorimotor task. These results may indicate a beneficial use of tDCS for learning and recovery of bimanual motor skills.

Transcranial direct current stimulation can enhance working memory in Huntington's disease

Transcranial direct current stimulation (tDCS) combined with a cognitive task can enhance targeted aspects of cognitive functioning in clinical populations. The movement disorder Huntington's disease (HD) is associated with progressive cognitive impairment. Deficits in working memory (WM) can be apparent early in the disease and impact functional capacity. We investigated whether tDCS combined with cognitive training could improve WM in patients with HD, and if baseline clinical or cognitive measures may predict efficacy. Twenty participants with HD completed this crossover trial, undergoing 1.5mA anodal tDCS over left dorsolateral prefrontal cortex and sham stimulation on separate visits. Participants and assessor were blinded to condition order, which was randomised across participants. All participants completed baseline clinical and cognitive assessments. Pre- and post-stimulation tasks included digit reordering, computerised n-back tests and a Stroop task. During 15min of tDCS/sham stimulation, participants practiced 1- and 2-back WM tasks. Participants exhibited an increase in WM span on the digit re-ordering span task from pre- to post-stimulation after tDCS, but not after sham stimulation. Gains in WM were positively related to motor symptom ratings and negatively associated with verbal fluency scores. Patients with more severe motor symptoms showed greatest improvement, suggesting that motor symptom ratings may help identify patients who are most likely to benefit from tDCS.

CONCLUSIONS

Dorsolateral prefrontal tDCS appears well tolerated in HD and enhances WM span compared to sham stimulation. Our findings strongly encourage further investigation of the extent to which tDCS combined with cognitive training could enhance everyday function in HD. ClinicalTrials.gov; NCT02216474 Brain stimulation in Movement Disorders; https://clinicaltrials.gov/ct2/show/NCT02216474.

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.

Transcranial direct current stimulation may modulate extinction memory in posttraumatic stress disorder

BACKGROUND

Abnormalities in fear extinction and recall are core components of posttraumatic stress disorder (PTSD). Data from animal and human studies point to a role of the ventromedial prefrontal cortex (vmPFC) in extinction learning and subsequent retention of extinction memories. Given the increasing interest in developing noninvasive brain stimulation protocols for psychopathology treatment, we piloted whether transcranial direct current stimulation (tDCS) during extinction learning, vs. during consolidation of extinction learning, might improve extinction recall in veterans with warzone-related PTSD.

METHODS

Twenty-eight veterans with PTSD completed a 2-day Pavlovian fear conditioning, extinction, and recall paradigm. Participants received one 10-min session of 2 mA anodal tDCS over AF3, intended to target the vmPFC. Fourteen received tDCS that started simultaneously with extinction learning onset, and the remaining 14 participants received tDCS during extinction consolidation. Normalized skin conductance reactivity (SCR) was the primary outcome measure. Linear mixed effects models were used to test for effects of tDCS on late extinction and early extinction recall 24 hr later.

RESULTS

During early recall, veterans who received tDCS during extinction consolidation showed slightly lower SCR in response to previously extinguished stimuli as compared to veterans who received tDCS simultaneous with extinction learning (p = .08), generating a medium effect size (Cohen's d = .38). There was no significant effect of tDCS on SCR during late extinction.

CONCLUSIONS

These preliminary findings suggest that testing the effects of tDCS during consolidation of fear extinction may have promise as a way of enhancing extinction recall.

Prefrontal cortex stimulation does not affect emotional bias, but may slow emotion identification

Transcranial direct current stimulation (tDCS) has recently garnered attention as a putative depression treatment. However, the cognitive mechanisms by which it exerts an antidepressant effect are unclear: tDCS may directly alter 'hot' emotional processing biases, or alleviate depression through changes in 'cold' (non-emotional) cognitive function. Here, 75 healthy participants performed a facial emotion identification task during 20 minutes of anodal or sham tDCS over the left dorsolateral prefrontal cortex (DLPFC) in a double-blind, within-subject crossover design. A subset of 31 participants additionally completed a task measuring attentional distraction during stimulation. Compared to sham stimulation, anodal tDCS of the left DLPFC resulted in an increase in response latency across all emotional conditions. Bayesian analysis showed definitively that tDCS exerted no emotion-dependent effect on behaviour. Thus, we demonstrate that anodal tDCS produces a general, rather than an emotion-specific, effect. We also report a preliminary finding in the subset of participants who completed the distractibility task: increased distractibility during active stimulation correlated significantly with the degree to which tDCS slowed emotion identification. Our results provide insight into the possible mechanisms by which DLPFC tDCS may treat symptoms of depression, suggesting that it may not alter emotional biases, but instead may affect 'cold' cognitive processes.

Cerebellar tDCS Does Not Enhance Performance in an Implicit Categorization Learning Task

Background: Transcranial Direct Current Stimulation (tDCS) is a form of non-invasive electrical stimulation that changes neuronal excitability in a polarity and site-specific manner. In cognitive tasks related to prefrontal and cerebellar learning, cortical tDCS arguably facilitates learning, but the few studies investigating cerebellar tDCS, however, are inconsistent.

Objective: We investigate the effect of cerebellar tDCS on performance of an implicit categorization learning task.

Methods: Forty participants performed a computerized version of an implicit categorization learning task where squares had to be sorted into two categories, according to an unknown but fixed rule that integrated both the size and luminance of the square. Participants did one round of categorization to familiarize themselves with the task and to provide a baseline of performance. After that, 20 participants received anodal tDCS (20 min, 1.5 mA) over the right cerebellum, and 19 participants received sham stimulation and simultaneously started a second session of the categorization task using a new rule.

Results: As expected, subjects performed better in the second session than in the first, baseline session, showing increased accuracy scores and reduced reaction times. Over trials, participants learned the categorization rule, improving their accuracy and reaction times. However, we observed no effect of anodal tDCS stimulation on overall performance or on learning, compared to sham stimulation.

Conclusion: These results suggest that cerebellar tDCS does not modulate performance and learning on an implicit categorization task.

Anodal transcranial direct current stimulation over the left dorsolateral prefrontal cortex modulates attention and pain in fibromyalgia: randomized clinical trial

Cognitive dysfunction in fibromyalgia patients has been reported, especially when increased attentional demands are required. Transcranial direct current stimulation (tDCS) over the dorsolateral prefrontal cortex (DLPFC) has been effective in modulating attention. We tested the effects of a single session of tDCS coupled with a Go/No-go task in modulating three distinct attentional networks: alertness, orienting and executive control. Secondarily, the effect on pain measures was evaluated. Forty females with fibromyalgia were randomized to receive active or sham tDCS. Anodal stimulation (1 mA, 20 min) was applied over the DLPFC. Attention indices were assessed using the Attention Network Test (ANT). Heat pain threshold (HPTh) and tolerance (HPTo) were measured. Active compared to sham tDCS led to increased performance in the orienting (mean difference [MD] = 14.63) and executive (MD = 21.00) attention networks. There was no effect on alertness. Active tDCS increased HPTh as compared to sham (MD = 1.93) and HPTo (MD = 1.52). Regression analysis showed the effect on executive attention is mostly independent of the effect on pain. DLPFC may be an important target for neurostimulation therapies in addition to the primary motor cortex for patients who do not respond adequately to neurostimulation therapies.

Can Neuromodulation also Enhance Social Inequality? Some Possible Indirect Interventions of the State

There is evidence that noninvasive brain stimulation (NIBS), and especially transcranial direct current stimulation (tDCS), can improve some cognitive functions, at least temporarily. However, as the improvement only applies to some “lucky” people, it may raise ethical, social and legal issues related to fairness in selective contexts (exams, competitions, job interviews). In this regard, an important element tends to be overlooked: the variability in individual response to tDCS in particular. If intensive study or practice and massive doses of chemical enhancers can have slightly different effects over different people, tDCS can sometimes be completely ineffective. The variability in individual response, if tDCS were widely used, could add to the already present natural inequalities between people, or even create new ones, leaving some in a disadvantaged condition. The discussion of the various ethical, social and legal consequences of different individual responses to tDCS might also address a potential indirect intervention by the State. In fact, if NIBS were to be widespread in competitive contexts, those who do not benefit from tDCS would be disadvantaged compared to those able to enhance their skills thanks to neuromodulation technologies. The most disadvantaged people for their lower response to tDCS could then acquire the right to receive and use free and safe cognitive enhancing drugs or other forms of bettering cognitive skills and functions, so as to reduce the gap between them and those who respond well to tDCS, in the light of the principle of equal opportunity.

Effects of prefrontal bipolar and high-definition transcranial direct current stimulation on cortical reactivity and working memory in healthy adults

Transcranial direct current stimulation (tDCS) is a well-recognised neuromodulatory technology which has been shown to induce short-lasting changes in motor-cortical excitability. The recent and rapid expansion of tDCS into the cognitive domain, however, necessitates deeper mechanistic understanding of its neurophysiological effects over non-motor brain regions. The present study utilised transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) to probe the immediate and longer-term effects of both a bipolar (BP-tDCS) and more focal 4×1 High-Definition tDCS (HD-tDCS) montage applied over the left DLPFC on TMS-evoked potentials (TEPs) and oscillations in 19 healthy adult participants. 2-back working memory (WM) performance was also assessed as a marker of cognitive function. Region of interest (ROI) analyses taken from the F1 electrode directly adjacent to the stimulation site revealed increased P60 TEP amplitudes at this location 5min following BP-tDCS and 30min following HD-tDCS. Further global cluster based analyses of all scalp electrodes revealed widespread neuromodulatory changes following HD-tDCS, but not BP-tDCS, both five and 30min after stimulation, with reductions also detected in both beta and gamma oscillatory power over parieto-occipital channels 30min after stimulation. No significant changes in WM performance were observed following either HD-tDCS or BP-tDCS. This study highlights the capacity for single-session prefrontal anodal tDCS montages to modulate neurophysiological processes, as assessed with TMS-EEG.

High-definition transcranial direct current stimulation to both primary motor cortices improves unimanual and bimanual dexterity

While most research on brain stimulation with transcranial direct current stimulation (tDCS) targets unimanual motor tasks, little is known about its effects on bimanual motor performance. This study aims to investigate the effects of tDCS on unimanual as well as bimanual motor dexterity. We examined the effects of bihemispheric anodal high-definition tDCS (HD-atDCS) on both primary motor cortices (M1) applied concurrent with unimanual and bimanual motor training. We then measured the effects with the Purdue Pegboard Test (PPT) and compared them to a sham stimulation. Between a pretest and posttest, 31 healthy, right-handed participants practiced the PPT on three consecutive days and received - simultaneous to motor practice - either HD-atDCS over the left and right M1 (STIM, n=16) or a sham stimulation (SHAM, n=15). Five to seven days after the posttest, a follow-up test was conducted. Two-way ANOVAs with repeated measures showed significantly increased performance for all PPT-scores (p<0.001) in both groups. The scores for the right hand, both hands, and overall showed significant TIME x GROUP interactions (p<.05) with more improved performance for the STIM group, while left hand performance was not significantly altered. These effects were most pronounced in the follow-up test. Thus, we can conclude that a bihemispheric HD-atDCS of both M1's improves performance of unimanual and bimanual dexterity. The strength of the effects, however, depends on which hand is used in the unimanual task and the type of bimanual task performed.

Mechanisms and Effects of Transcranial Direct Current Stimulation

The US Air Force Office of Scientific Research convened a meeting of researchers in the fields of neuroscience, psychology, engineering, and medicine to discuss most pressing issues facing ongoing research in the field of transcranial direct current stimulation (tDCS) and related techniques. In this study, we present opinions prepared by participants of the meeting, focusing on the most promising areas of research, immediate and future goals for the field, and the potential for hormesis theory to inform tDCS research. Scientific, medical, and ethical considerations support the ongoing testing of tDCS in healthy and clinical populations, provided best protocols are used to maximize safety. Notwithstanding the need for ongoing research, promising applications include enhancing vigilance/attention in healthy volunteers, which can accelerate training and support learning. Commonly, tDCS is used as an adjunct to training/rehabilitation tasks with the goal of leftward shift in the learning/treatment effect curves. Although trials are encouraging, elucidating the basic mechanisms of tDCS will accelerate validation and adoption. To this end, biomarkers (eg, clinical neuroimaging and findings from animal models) can support hypotheses linking neurobiological mechanisms and behavioral effects. Dosage can be optimized using computational models of current flow and understanding dose–response. Both biomarkers and dosimetry should guide individualized interventions with the goal of reducing variability. Insights from other applied energy domains, including ionizing radiation, transcranial magnetic stimulation, and low-level laser (light) therapy, can be prudently leveraged.

Rest Is Still Best

OBJECTIVE

We examined the impact task interruptions of differing qualitative and quantitative load have on visuospatial vigilance sensitivity.

BACKGROUND

The vigilance decrement and attempts to develop countermeasures to the decrement is one of the most important human factors issues. There is an ongoing debate between those who interpret the increase in the rate of failures to detect signals over time as being due to objective task monotony or task underload and those who interpret this increased failure proneness as being predominately due to cognitive-resource depletion and task overload.

METHOD

Participants were assigned at random to one of six interruptions: Participants were given a complete rest (rest); participants completed a 1-back verbal working-memory (WM) task, a 3-back verbal WM task, a 1-back spatial WM task, or a 3-back spatial WM task; or participants performed the primary vigilance task (continuous).

RESULTS

Postinterruption performance was best for rest and worst for continuous. A resource theory perspective led us to make two possible predictions of relative interruption effect orders of the six conditions out of 720 possible orderings. We found one of the two orders.

CONCLUSION

Overall, the vigilance sensitivity decrement appears to be due to the recurring use of particular cognitive resources, and resource theorists should explore this more extensively in the future.

APPLICATION

Countermeasures for the vigilance decrement should be based on clear cognitive-resource considerations. Rest is the best countermeasure. Intervening tasks should be chosen that minimize resource-demand overlap with the vigilance task.

tDCS for Memory Enhancement: Analysis of the Speculative Aspects of Ethical Issues

Transcranial direct current stimulation (tDCS) is a promising technology to enhance cognitive and physical performance. One of the major areas of interest is the enhancement of memory function in healthy individuals. The early arrival of tDCS on the market for lifestyle uses and cognitive enhancement purposes lead to the voicing of some important ethical concerns, especially because, to date, there are no official guidelines or evaluation procedures to tackle these issues. The aim of this article is to review ethical issues related to uses of tDCS for memory enhancement found in the ethics and neuroscience literature and to evaluate how realistic and scientifically well-founded these concerns are? In order to evaluate how plausible or speculative each issue is, we applied the methodological framework described by Racine et al. (2014) for “informed and reflective” speculation in bioethics. This framework could be succinctly presented as requiring: (1) the explicit acknowledgment of factual assumptions and identification of the value attributed to them; (2) the validation of these assumptions with interdisciplinary literature; and (3) the adoption of a broad perspective to support more comprehensive reflection on normative issues. We identified four major considerations associated with the development of tDCS for memory enhancement: safety, autonomy, justice and authenticity. In order to assess the seriousness and likelihood of harm related to each of these concerns, we analyzed the assumptions underlying the ethical issues, and the level of evidence for each of them. We identified seven distinct assumptions: prevalence, social acceptance, efficacy, ideological stance (bioconservative vs. libertarian), potential for misuse, long term side effects, and the delivery of complete and clear information. We conclude that ethical discussion about memory enhancement via tDCS sometimes involves undue speculation, and closer attention to scientific and social facts would bring a more nuanced analysis. At this time, the most realistic concerns are related to safety and violation of users’ autonomy by a breach of informed consent, as potential immediate and long-term health risks to private users remain unknown or not well defined. Clear and complete information about these risks must be provided to research participants and consumers of tDCS products or related services. Broader public education initiatives and warnings would also be worthwhile to reach those who are constructing their own tDCS devices.

Transcranial Direct Current Stimulation: Considerations for Research in Adolescent Depression

Adolescent depression is a prevalent disorder with substantial morbidity and mortality. Current treatment interventions do not target relevant pathophysiology and are frequently ineffective, thereby leading to a substantial burden for individuals, families, and society. During adolescence, the prefrontal cortex undergoes extensive structural and functional changes. Recent work suggests that frontolimbic development in depressed adolescents is delayed or aberrant. The judicious application of non-invasive brain stimulation techniques to the prefrontal cortex may present a promising opportunity for durable interventions in adolescent depression. Transcranial direct current stimulation (tDCS) applies a low-intensity, continuous current that alters cortical excitability. While this modality does not elicit action potentials, it is thought to manipulate neuronal activity and neuroplasticity. Specifically, tDCS may modulate N-methyl-d-aspartate receptors and L-type voltage-gated calcium channels and effect changes through long-term potentiation or long-term depression-like mechanisms. This mini-review considers the neurobiological rationale for developing tDCS protocols in adolescent depression, reviews existing work in adult mood disorders, surveys the existing tDCS literature in adolescent populations, reviews safety studies, and discusses distinct ethical considerations in work with adolescents.

Effects of Anodal Transcranial Direct Current Stimulation and Serotonergic Enhancement on Memory Performance in Young and Older Adults

In the absence of effective therapies for dementia and its precursors, enhancing neuroplasticity by means of non-invasive brain stimulation such as anodal transcranial direct current stimulation (atDCS) might be a promising approach to counteract or delay the onset of cognitive decline, but effect sizes have been moderate so far. Previous reports indicate that increasing serotonin levels may enhance atDCS-induced neuroplasticity. However, evidence for serotonergic modulation of atDCS effects on memory is still lacking. Here, we conducted a double-blind, randomized, sham-/placebo-controlled trial to investigate the impact of a selective serotonin reuptake inhibitor (SSRI; single dose of 20 mg citalopram) and atDCS over the right temporoparietal cortex (1 mA, 20 min) on memory formation. Twenty young and 20 older subjects completed an object-location learning task in each of the four conditions: sham+placebo, sham+SSRI, atDCS+placebo, and atDCS+SSRI. Outcome measures were performance in immediate (primary outcome) and delayed cued recall. While we found an SSRI effect, but no statistically significant effect of atDCS on immediate recall scores, young and older adults benefited most from the combined application (comparisons: atDCS+SSRI>atDCS+placebo and atDCS+SSRI>sham+placebo). Thus, our data provide evidence that atDCS improves memory formation if serotonergic neurotransmission is enhanced simultaneously. Further studies are needed to assess whether these findings extend to clinical populations with memory impairment and translate into clinically relevant improvements after long-term serotonergic enhancement and repeated stimulation.