Facilitation of visuo-motor learning by transcranial direct current stimulation of the motor and extrastriate visual areas in humans

Early optimization in finger dexterity of skilled pianists: implication of transcranial stimulation

BACKGROUND

It has been shown that non-invasive transcranial direct current stimulation (tDCS) facilitates motor functions in healthy adults and stroke patients. However, little is known about neuroplastic changes induced by tDCS in highly-trained individuals. Here we addressed this issue by assessing the effect of tDCS on dexterity of finger movements in healthy adult pianists. Twelve pianists practiced bimanual keystrokes in an in-phase manner while bilateral tDCS (left anodal/right cathodal or vice versa) of the primary motor cortex was performed. Before and after stimulation, each participant was asked to perform the trained successive keystrokes, and to repetitively strike a key with each of the fingers as fast and accurate as possible while keeping the remaining fingers immobilized voluntarily.

RESULTS

In contrast to previous findings in untrained individuals, tDCS yielded overall no apparent improvement of fine control of finger movements in the professional pianists. However, for some movement features, pianists who commenced training at later age demonstrated larger improvements of fine motor control following tDCS.

CONCLUSIONS

These findings, in combination with lack of any correlation between the age at which pianists commenced the training and motor improvements for sham stimulation conditions, supports the idea that selectively late-started players benefit from tDCS, which we interpret as early optimization of neuroplasticity of the motor system.

Review of transcranial direct current stimulation in poststroke recovery

Motor impairment, dysphagia, aphasia, and visual impairment are common disabling residual deficits experienced by stroke survivors. Recently, many novel rehabilitative modalities have been investigated for their potential to ameliorate such deficits and to improve functional outcomes. Noninvasive brain stimulation techniques, such as transcranial direct current stimulation (tDCS), have emerged as a promising tool to facilitate stroke recovery. tDCS can alter cortical excitability to induce brain plasticity by modulating the lesioned, contralesional, or bilateral hemispheres with various stimulation modalities. Along with peripheral therapies, tDCS can lead to subsequent sustained behavioral and clinical gains in patients with stroke. In this review, we summarize characteristics of tDCS (method of stimulation, safety profile, and mechanism) and its application in the treatment of various stroke-related deficits, and we highlight future directions for tDCS in this capacity.

Physiological and modeling evidence for focal transcranial electrical brain stimulation in humans: a basis for high-definition tDCS

Transcranial Direct Current Stimulation (tDCS) is a non-invasive, low-cost, well-tolerated technique producing lasting modulation of cortical excitability. Behavioral and therapeutic outcomes of tDCS are linked to the targeted brain regions, but there is little evidence that current reaches the brain as intended. We aimed to: (1) validate a computational model for estimating cortical electric fields in human transcranial stimulation, and (2) assess the magnitude and spread of cortical electric field with a novel High-Definition tDCS (HD-tDCS) scalp montage using a 4 × 1-Ring electrode configuration. In three healthy adults, Transcranial Electrical Stimulation (TES) over primary motor cortex (M1) was delivered using the 4 × 1 montage (4 × cathode, surrounding a single central anode; montage radius ~3 cm) with sufficient intensity to elicit a discrete muscle twitch in the hand. The estimated current distribution in M1 was calculated using the individualized MRI-based model, and compared with the observed motor response across subjects. The response magnitude was quantified with stimulation over motor cortex as well as anterior and posterior to motor cortex. In each case the model data were consistent with the motor response across subjects. The estimated cortical electric fields with the 4 × 1 montage were compared (area, magnitude, direction) for TES and tDCS in each subject. We provide direct evidence in humans that TES with a 4 × 1-Ring configuration can activate motor cortex and that current does not substantially spread outside the stimulation area. Computational models predict that both TES and tDCS waveforms using the 4 × 1-Ring configuration generate electric fields in cortex with comparable gross current distribution, and preferentially directed normal (inward) currents. The agreement of modeling and experimental data for both current delivery and focality support the use of the HD-tDCS 4 × 1-Ring montage for cortically targeted neuromodulation.

Language learning without control: the role of the PFC

Learning takes place throughout lifetime but differs in infants and adults because of the development of the PFC, a brain region responsible for cognitive control. To test this hypothesis, adults were investigated in a language learning paradigm under inhibitory, cathodal transcranial direct current stimulation over PFC. The experiment included a learning session interspersed with test phases and a test-only session. The stimulus material required the learning of grammatical dependencies between two elements in a novel language. In a parallel design, cathodal transcranial direct current stimulation over the left PFC, right PFC, or sham stimulation was applied during the learning session but not during the test-only session. Event-related brain potentials (ERPs) were recorded during both sessions. Whereas no ERP learning effects were observed during the learning session, different ERP learning effects as a function of prior stimulation type were found during the test-only session, although behavioral learning success was equal across conditions. With sham stimulation, the ERP learning effect was reflected in a centro-parietal N400-like negativity indicating lexical processes. Inhibitory stimulation over the left PFC, but not over the right PFC, led to a late positivity similar to that previously observed in prelinguistic infants indicating associative learning. The present data demonstrate that adults can learn with and without cognitive control using different learning mechanisms. In the presence of cognitive control, adult language learning is lexically guided, whereas it appears to be associative in nature when PFC control is downregulated.

Enhancement of object detection with transcranial direct current stimulation is associated with increased attention

BACKGROUND

We previously found that Transcranial Direct Current Stimulation (tDCS) improves learning and performance in a task where subjects learn to detect potential threats indicated by small target objects hidden in a complex virtual environment. In the present study, we examined the hypothesis that these effects on learning and performance are related to changes in attention. The effects of tDCS were tested for three forms of attention (alerting, orienting, and executive attention) using the Attention Network Task (ANT), which were compared with performance on the object-learning task.

RESULTS

Participants received either 0.1 mA (N = 10) or 2.0 mA (N = 9) tDCS during training and were tested for performance in object-identification before training (baseline-test) and again immediately after training (immediate test). Participants next performed the Attention Networks Task (ANT), and were later tested for object-identification performance a final time (delayed test). Alerting, but not orienting or executive attention, was significantly higher for participants receiving 2.0 mA compared with 0.1 mA tDCS (p < 0.02). Furthermore, alerting scores were significantly correlated with the proportion of hits (p < 0.01) for participants receiving 2.0 mA.

CONCLUSIONS

These results indicate that tDCS enhancement of performance in this task may be related in part to the enhancement of alerting attention, which may benefit the initial identification, learning and/or subsequent recognition of target objects indicating potential threats.

Can tDCS enhance treatment of aphasia after stroke?

BACKGROUND: Recent advances in the application of transcranial direct current stimulation (tDCS) in healthy populations have led to the exploration of the technique as an adjuvant method to traditional speech therapies in patients with post-stroke aphasia. AIMS: THE PURPOSE OF THE REVIEW IS: (i) to review the features of tDCS that make it an attractive tool for research and potential future use in clinical contexts; (ii) to describe recent studies exploring the facilitation of language performance using tDCS in post-stroke aphasia; (iii) to explore methodological considerations of tDCS that may be key to understanding tDCS in treatment of aphasia post stroke; and (iv) to highlight several caveats and outstanding questions that need to be addressed in future work. MAIN CONTRIBUTION: This review aims to highlight our current understanding of the methodological and theoretical issues surrounding the use of tDCS as an adjuvant tool in the treatment of language difficulties after stroke. CONCLUSIONS: Preliminary evidence shows that tDCS may be a useful tool to complement treatment of aphasia, particularly for speech production in chronic stroke patients. To build on this exciting work, further systematic research is needed to understand the mechanisms of tDCS-induced effects, its application to current models of aphasia recovery, and the complex interactions between different stimulation parameters and language rehabilitation techniques. The potential of tDCS is to optimise language rehabilitation techniques and promote long-term recovery of language. A stimulating future for aphasia rehabilitation!

Multi-session transcranial direct current stimulation (tDCS) elicits inflammatory and regenerative processes in the rat brain

Transcranial direct current stimulation (tDCS) is increasingly being used in human studies as an adjuvant tool to promote recovery of function after stroke. However, its neurobiological effects are still largely unknown. Electric fields are known to influence the migration of various cell types in vitro, but effects in vivo remain to be shown. Hypothesizing that tDCS might elicit the recruitment of cells to the cortex, we here studied the effects of tDCS in the rat brain in vivo. Adult Wistar rats (n = 16) were randomized to either anodal or cathodal stimulation for either 5 or 10 consecutive days (500 µA, 15 min). Bromodeoxyuridine (BrdU) was given systemically to label dividing cells throughout the experiment. Immunohistochemical analyses ex vivo included stainings for activated microglia and endogenous neural stem cells (NSC). Multi-session tDCS with the chosen parameters did not cause a cortical lesion. An innate immune response with early upregulation of Iba1-positive activated microglia occurred after both cathodal and anodal tDCS. The involvement of adaptive immunity as assessed by ICAM1-immunoreactivity was less pronounced. Most interestingly, only cathodal tDCS increased the number of endogenous NSC in the stimulated cortex. After 10 days of cathodal stimulation, proliferating NSC increased by ∼60%, with a significant effect of both polarity and number of tDCS sessions on the recruitment of NSC. We demonstrate a pro-inflammatory effect of both cathodal and anodal tDCS, and a polarity-specific migratory effect on endogenous NSC in vivo. Our data suggest that tDCS in human stroke patients might also elicit NSC activation and modulate neuroinflammation.

The pharmacology of neuroplasticity induced by non-invasive brain stimulation: building models for the clinical use of CNS active drugs

The term neuroplasticity encompasses structural and functional modifications of neuronal connectivity. Abnormal neuroplasticity is involved in various neuropsychiatric diseases, such as dystonia, epilepsy, migraine, Alzheimer's disease, fronto-temporal degeneration, schizophrenia, and post cerebral stroke. Drugs affecting neuroplasticity are increasingly used as therapeutics in these conditions. Neuroplasticity was first discovered and explored in animal experimentation. However, non-invasive brain stimulation (NIBS) has enabled researchers recently to induce and study similar processes in the intact human brain. Plasticity induced by NIBS can be modulated by pharmacological interventions, targeting ion channels, or neurotransmitters. Importantly, abnormalities of plasticity as studied by NIBS are directly related to clinical symptoms in neuropsychiatric diseases. Therefore, a core theme of this review is the hypothesis that NIBS-induced plasticity can explore and potentially predict the therapeutic efficacy of CNS-acting drugs in neuropsychiatric diseases. We will (a) review the basics of neuroplasticity, as explored in animal experimentation, and relate these to our knowledge about neuroplasticity induced in humans by NIBS techniques. We will then (b) discuss pharmacological modulation of plasticity in animals and humans. Finally, we will (c) review abnormalities of plasticity in neuropsychiatric diseases, and discuss how the combination of NIBS with pharmacological intervention may improve our understanding of the pathophysiology of abnormal plasticity in these diseases and their purposeful pharmacological treatment.

Desynchronizing effect of high-frequency stimulation in a generic cortical network model

Transcranial electrical stimulation (TCES) and deep brain stimulation are two different applications of electrical current to the brain used in different areas of medicine. Both have a similar frequency dependence of their efficiency, with the most pronounced effects around 100 Hz. We apply superthreshold electrical stimulation, specifically depolarizing DC current, interrupted at different frequencies, to a simple model of a population of cortical neurons which uses phenomenological descriptions of neurons by Izhikevich and synaptic connections on a similar level of sophistication. With this model, we are able to reproduce the optimal desynchronization around 100 Hz, as well as to predict the full frequency dependence of the efficiency of desynchronization, and thereby to give a possible explanation for the action mechanism of TCES.

Effects of transcranial electrical stimulation on cognition

Alterations of cortical excitability, oscillatory as well as non-oscillatory, are physiological derivates of cognitive processes, such as perception, working memory, learning, and long-term memory formation. Since noninvasive electrical brain stimulation is capable of inducing alterations in the human brain, these stimulation approaches might be attractive tools to modulate cognition. Transcranial direct current stimulation (tDCS) alters spontaneous cortical activity, while transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS) are presumed to induce or interfere with oscillations of cortical networks. Via these mechanisms, the respective stimulation techniques have indeed been shown to modulate cognitive processes in a multitude of studies conducted during the last years. In this review, we will gather knowledge about the potential of noninvasive electrical brain stimulation to study and modify cognitive processes in healthy humans and discuss directions of future research.

Can noninvasive brain stimulation enhance cognition in neuropsychiatric disorders?

Cognitive impairment is a core symptom of many neuropsychiatric diseases and a key contributor to the patient's quality of life. However, an effective therapeutic strategy has yet to be developed. Noninvasive brain stimulation techniques, namely transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), are promising techniques that are under investigation for a variety of otherwise treatment-resistant neuropsychiatric diseases. Notably, these tools can induce alterations in neural networks subserving cognitive operations and thus may provide a means for cognitive restoration. The purpose of this article is to review the available evidence concerning cognitive enhancing properties of noninvasive brain stimulation in neuropsychiatry. We specifically focus on major depression, Alzheimer's disease, schizophrenia, autism and attention deficit hyperactivity disorder (ADHD), where cognitive dysfunction is a major symptom and some studies have been completed with promising results. We provide a critical assessment of the available research and suggestions to guide future efforts. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Impact of tDCS on performance and learning of target detection: interaction with stimulus characteristics and experimental design

We have previously found that transcranial direct current stimulation (tDCS) over right inferior frontal cortex (RIFC) enhances performance during learning of a difficult visual target detection task (Clark et al., 2012). In order to examine the cognitive mechanisms of tDCS that lead to enhanced performance, here we analyzed its differential effects on responses to stimuli that varied by repetition and target presence, differences related to expectancy by comparing performance in single- and double-blind task designs, and individual differences in skin stimulation and mood. Participants were trained for 1h to detect target objects hidden in a complex virtual environment, while anodal tDCS was applied over RIFC at 0.1 mA or 2.0 mA for the first 30 min. Participants were tested immediately before and after training and again 1h later. Higher tDCS current was associated with increased performance for all test stimuli, but was greatest for repeated test stimuli with the presence of hidden-targets. This finding was replicated in a second set of subjects using a double-blind task design. Accuracy for target detection discrimination sensitivity (d'; Z(hits)-Z(false alarms)) was greater for 2.0 mA current (1.77) compared with 0.1 mA (0.95), with no differences in response bias (β). Taken together, these findings indicate that the enhancement of performance with tDCS is sensitive to stimulus repetition and target presence, but not to changes in expectancy, mood, or type of blinded task design. The implications of these findings for understanding the cognitive mechanisms of tDCS are discussed.

Enhanced locomotor adaptation aftereffect in the "broken escalator" phenomenon using anodal tDCS

The everyday experience of stepping onto a stationary escalator causes a stumble, despite our full awareness that the escalator is broken. In the laboratory, this "broken escalator" phenomenon is reproduced when subjects step onto an obviously stationary platform (AFTER trials) that was previously experienced as moving (MOVING trials) and attests to a process of motor adaptation. Given the critical role of M1 in upper limb motor adaptation and the potential for transcranial direct current stimulation (tDCS) to increase cortical excitability, we hypothesized that anodal tDCS over leg M1 and premotor cortices would increase the size and duration of the locomotor aftereffect. Thirty healthy volunteers received either sham or real tDCS (anodal bihemispheric tDCS; 2 mA for 15 min at rest) to induce excitatory effects over the primary motor and premotor cortex before walking onto the moving platform. The real tDCS group, compared with sham, displayed larger trunk sway and increased gait velocity in the first AFTER trial and a persistence of the trunk sway aftereffect into the second AFTER trial. We also used transcranial magnetic stimulation to probe changes in cortical leg excitability using different electrode montages and eyeblink conditioning, before and after tDCS, as well as simulating the current flow of tDCS on the human brain using a computational model of these different tDCS montages. Our data show that anodal tDCS induces excitability changes in lower limb motor cortex with resultant enhancement of locomotor adaptation aftereffects. These findings might encourage the use of tDCS over leg motor and premotor regions to improve locomotor control in patients with neurological gait disorders.

TDCS guided using fMRI significantly accelerates learning to identify concealed objects

The accurate identification of obscured and concealed objects in complex environments was an important skill required for survival during human evolution, and is required today for many forms of expertise. Here we used transcranial direct current stimulation (tDCS) guided using neuroimaging to increase learning rate in a novel, minimally guided discovery-learning paradigm. Ninety-six subjects identified threat-related objects concealed in naturalistic virtual surroundings used in real-world training. A variety of brain networks were found using functional magnetic resonance imaging (fMRI) data collected at different stages of learning, with two of these networks focused in right inferior frontal and right parietal cortex. Anodal 2.0 mA tDCS performed for 30 min over these regions in a series of single-blind, randomized studies resulted in significant improvements in learning and performance compared with 0.1 mA tDCS. This difference in performance increased to a factor of two after a one-hour delay. A dose-response effect of current strength on learning was also found. Taken together, these brain imaging and stimulation studies suggest that right frontal and parietal cortex are involved in learning to identify concealed objects in naturalistic surroundings. Furthermore, they suggest that the application of anodal tDCS over these regions can greatly increase learning, resulting in one of the largest effects on learning yet reported. The methods developed here may be useful to decrease the time required to attain expertise in a variety of settings.

Enhancing motor skill learning with transcranial direct current stimulation - a concise review with applications to stroke

In the past few years, there has been a rapid increase in the application of non-invasive brain stimulation to study brain-behavior relations in an effort to potentially increase the effectiveness of neuro-rehabilitation. Transcranial direct current stimulation (tDCS), an emerging technique of non-invasive brain stimulation, has shown to produce beneficial neural effects in consequence with improvements in motor behavior. tDCS has gained popularity as it is economical, simple to use, portable, and increases corticospinal excitability without producing any serious side effects. As tDCS has been increasingly investigated as an effective tool for various disorders, numerous improvements, and developments have been proposed with respect to this technique. tDCS has been widely used to identify the functional relevance of particular brain regions in motor skill learning and also to facilitate activity in specific cortical areas involved in motor learning, in turn improving motor function. Understanding the interaction between tDCS and motor learning can lead to important implications for developing various rehabilitation approaches. This paper provides a concise overview of tDCS as a neuromodulatory technique and its interaction with motor learning. The paper further briefly goes through the application of this priming technique in the stroke population.

Brain-machine interfaces and transcranial stimulation: future implications for directing functional movement and improving function after spinal injury in humans

Since its origins, the primary goal of transforming thought into action and sensation into perception has been to improve the quality of life for the physically impaired. Brain-machine interfaces (BMI) aim to improve the quality of life for large numbers of neurological patients. In particular, this novel technology is meant to play a major role in the near future as a serious contribution to spinal cord rehabilitation. During the last decade we have witnessed a dramatic increase in BMI research with impressive demonstrations of rodents, nonhuman primates, and humans controlling robots, wheelchairs, and graphical cursors in real time through signals collected from the brain. In this chapter we first review the different techniques used in the field of BMI, including electroencephalography (EEG), electrocorticography (ECoG), magnetoencephalography (MEG), and chronic multielectrode recordings. In addition we review the use of transcranial magnetic stimulation (TMS) for noninvasive modulation of excitability in relatively focal cortical areas. The chapter concludes with a discussion on the future implications of BMIs for directing functional movement and improving function after spinal injury in humans.

Transcranial direct current stimulation modulates human color discrimination in a pathway-specific manner

Previous research showed that transcranial direct current stimulation (tDCS) can modulate visual cortex excitability. However, there is no experiment on the effects of tDCS on color perception to date. The present study aimed to investigate the effects of tDCS on color discrimination tasks. Fifteen healthy subjects (mean age of 25.6 ± 4.4 years) were tested with Cambridge Color Test 2.0 (Trivector and ellipses protocols) and a Forced-choice Spatial Color Contrast Sensitivity task (vertical red-green sinusoidal grating) while receiving tDCS. Anodal, cathodal, and sham tDCS were delivered at Oz for 22 min using two square electrodes (25 cm(2) with a current of 1.5 mA) in sessions separated by 7 days. Anodal tDCS significantly increased tritan sensitivity (p < 0.01) and had no significant effect on protan, deutan, or red-green grating discrimination. The effects on the tritan discrimination returned to baseline after 15 min (p < 0.01). Cathodal tDCS reduced the sensitivity in the deutan axis and increased sensitivity in the tritan axis (p < 0.05). The lack of anodal tDCS effects in the protan, deutan, and red-green grating sensitivities could be explained by a "ceiling effect" since adults in this age range tend to have optimal color discrimination performance for these hues. The differential effects of cathodal tDCS on tritan and deutan sensitivities and the absence of the proposed ceiling effects for the tritan axes might be explained by Parvocellular (P) and Koniocellular (K) systems with regard to their functional, physiological, and anatomical differences. The results also support the existence of a systematic segregation of P and K color-coding cells in V1. Future research and possible clinical implications are discussed.

Random noise stimulation improves neuroplasticity in perceptual learning

Perceptual learning is considered a manifestation of neural plasticity in the human brain. We investigated brain plasticity mechanisms in a learning task using noninvasive transcranial electrical stimulation (tES). We hypothesized that different types of tES would have varying actions on the nervous system, which would result in different efficacies of neural plasticity modulation. Thus, the principal goal of the present study was to verify the possibility of inducing differential plasticity effects using two tES approaches [i.e., direct current stimulation (tDCS) and random noise stimulation (tRNS)] during the execution of a visual perceptual learning task.

Restoration of vision after optic nerve lesions with noninvasive transorbital alternating current stimulation: a clinical observational study

BACKGROUND

Non-invasive current stimulation can induce neuroplastic changes in the normal brain, including visual system structures. Because it is not known if such plasticity is of clinical value, we wished to learn if vision restoration can be induced after optic nerve damage.

METHODS

In an open-label, clinical observational study 446 patients with optic nerve lesions were treated with non-invasive repetitive transorbital alternating current stimulation (rtACS). Current bursts (<1000 μA, 5-20 Hz) were applied to induce phosphenes for one or two 10-day stimulation periods. Efficacy was assessed by monocular measurements of visual acuity and visual field (VF) size. EEG recordings at rest (n = 68) were made before and after treatment and global power spectra changes were analyzed.

RESULTS

rtACS improved VF size in the right and left eye by 7.1% and 9.3% (p < 0.001), respectively. VF enlargements were present in 40.4% of right and 49.5% of left eyes. Visual acuity (VA) significantly increased in both eyes (right = 0.02, left = 0.015; p < 0.001). A second 10-day course was conducted 6 months in a subset of 62 patients and resulted in additional significant improvements of VA. Analysis of EEG power spectra revealed that VA and VF improvements were associated with increased alpha power. Increased theta power was observed in patients that had only VF enlargements but no VA change. In contrast, non-responders had increased delta power spectra in frontal and occipital areas.

CONCLUSIONS

rtACS leads to long-lasting improvements in VA and VF size and after-effects in EEG power spectra. Because physiological and clinical parameters are correlated we hypothesize that rtACS enhances plasticity by inducing synchronization in different cortical regions, but the precise mechanisms needs further clarification. These encouraging results require confirmation by controlled clinical trials.

Modulation of motor learning and memory formation by non-invasive cortical stimulation of the primary motor cortex

Transcranial magnetic (TMS) and direct current (tDCS) stimulation are non-invasive brain stimulation techniques that allow researchers to purposefully modulate cortical excitability in focal areas of the brain. Recent work has provided preclinical evidence indicating that TMS and tDCS can facilitate motor performance, motor memory formation, and motor skill learning in healthy subjects and possibly in patients with brain lesions. Although the optimal stimulation parameters to accomplish these goals remain to be determined, and controlled multicentre clinical studies are lacking, these findings suggest that cortical stimulation techniques could become in the future adjuvant strategies in the rehabilitation of motor deficits. The aim of this article is to critically review these findings and to discuss future directions regarding the possibility of combining these techniques with other interventions in neurorehabilitation.

Polarity-dependent transcranial direct current stimulation effects on central auditory processing

Given the polarity dependent effects of transcranial direct current stimulation (tDCS) in facilitating or inhibiting neuronal processing, and tDCS effects on pitch perception, we tested the effects of tDCS on temporal aspects of auditory processing. We aimed to change baseline activity of the auditory cortex using tDCS as to modulate temporal aspects of auditory processing in healthy subjects without hearing impairment. Eleven subjects received 2mA bilateral anodal, cathodal and sham tDCS over auditory cortex in a randomized and counterbalanced order. Subjects were evaluated by the Random Gap Detection Test (RGDT), a test measuring temporal processing abilities in the auditory domain, before and during the stimulation. Statistical analysis revealed a significant interaction effect of time vs. tDCS condition for 4000 Hz and for clicks. Post-hoc tests showed significant differences according to stimulation polarity on RGDT performance: anodal improved 22.5% and cathodal decreased 54.5% subjects' performance, as compared to baseline. For clicks, anodal also increased performance in 29.4% when compared to baseline. tDCS presented polarity-dependent effects on the activity of the auditory cortex, which results in a positive or negative impact in a temporal resolution task performance. These results encourage further studies exploring tDCS in central auditory processing disorders.

Transcranial direct current stimulation influences probabilistic association learning in schizophrenia

Schizophrenia is associated with heterogeneity in symptoms, cognition and treatment response. Probabilistic association learning, involving a gradual learning of cue-outcome associations, activates a frontal-striatal network in healthy adults. Studies of probabilistic association learning in schizophrenia have shown frontal-striatal dysfunction although considerable heterogeneity in performance has also been reported. Anodal transcranial direct current stimulation (tDCS) to the dorsolateral prefrontal cortex has been shown to improve probabilistic association learning in healthy adults. The aim of the current study was to determine the extent to which anodal tDCS to the left dorsolateral prefrontal cortex would reverse probabilistic association learning deficits in schizophrenia. Prior to tDCS, 20 people with schizophrenia performed an initial baseline assessment without stimulation. Anodal tDCS was administered continuously for 20 min at an intensity of 2.0 mA to the left dorsolateral prefrontal cortex in a single-blind, counterbalanced, sham-controlled, cross-over design while participants performed 150 trials of a probabilistic association learning test. Although anodal tDCS failed to improve probabilistic association learning based on the whole sample performance, greater variance in the active relative to the sham conditions suggested a subset of people may respond to treatment. Further correlation, regression and cluster analyses revealed differential effects of baseline performance on active tDCS and sham treatment and that there was a subset of people with schizophrenia who displayed improvement with tDCS suggesting that anodal tDCS to the dorsolateral prefrontal cortex may facilitate access to existing prefrontal cortex neural reserves in people with schizophrenia who show adequate capacity to learn at baseline.

Transcranial Direct Current Stimulation Improves Word Retrieval in Healthy and Nonfluent Aphasic Subjects

A number of studies have shown that modulating cortical activity by means of transcranial direct current stimulation (tDCS) affects performances of both healthy and brain-damaged subjects. In this study, we investigated the potential of tDCS to enhance associative verbal learning in 10 healthy individuals and to improve word retrieval deficits in three patients with stroke-induced aphasia. In healthy individuals, tDCS (20 min, 1 mA) was applied over Wernicke's area (position CP5 of the International 10–20 EEG System) while they learned 20 new “words” (legal nonwords arbitrarily assigned to 20 different pictures). The healthy subjects participated in a randomized counterbalanced double-blind procedure in which they were subjected to one session of anodic tDCS over left Wernicke's area, one sham session over this location and one session of anodic tDCS stimulating the right occipito-parietal area. Each experimental session was performed during a different week (over three consecutive weeks) with 6 days of intersession interval. Over 2 weeks, three aphasic subjects participated in a randomized double-blind experiment involving intensive language training for their anomic difficulties in two tDCS conditions. Each subject participated in five consecutive daily sessions of anodic tDCS (20 min, 1 mA) and sham stimulation over Wernicke's area while they performed a picture-naming task. By the end of each week, anodic tDCS had significantly improved their accuracy on the picture-naming task. Both normal subjects and aphasic patients also had shorter naming latencies during anodic tDCS than during sham condition. At two follow-ups (1 and 3 weeks after the end of treatment), performed only in two aphasic subjects, response accuracy and reaction times were still significantly better in the anodic than in the sham condition, suggesting a long-term effect on recovery of their anomic disturbances.

Modulating the brain at work using noninvasive transcranial stimulation

This paper proposes a shift in the way researchers currently view and use transcranial brain stimulation technologies. From a neuroscience perspective, the standard application of both transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) has been mainly to explore the function of various brain regions. These tools allow for noninvasive and painless modulation of cortical tissue. In the course of studying the function of an area, many studies often report enhanced performance of a task during or following the stimulation. However, little follow-up research is typically done to further explore these effects. Approaching this growing pool of cognitive neuroscience literature with a neuroergonomics mindset (i.e., studying the brain at work), the possibilities of using these stimulation techniques for more than simply investigating the function of cortical areas become evident. In this paper, we discuss how cognitive neuroscience brain stimulation studies may complement neuroergonomics research on human performance optimization. And, through this discussion, we hope to shift the mindset of viewing transcranial stimulation techniques as solely investigatory basic science tools or possible clinical therapeutic devices to viewing transcranial stimulation techniques as interventional tools to be incorporated in applied science research and systems for the augmentation and enhancement of human operator performance.

Transcranial electrical stimulation (tES - tDCS; tRNS, tACS) methods

Weak transcranial direct current stimulation (tDCS) with a homogenous DC field at intensities of around 1 mA induces long-lasting changes in the brain. tDCS can be used to manipulate brain excitability via membrane polarisation: cathodal stimulation hyperpolarises, while anodal stimulation depolarises the resting membrane potential, whereby the induced after-effects depend on polarity, duration and intensity of the stimulation. A variety of other parameters influence tDCS effects; co-application of neuropharmacologically active drugs may most impressively prolong or even reverse stimulation effects. Transcranial alternating stimulation (tACS) and random noise stimulation (tRNS) are used to interfere with ongoing neuronal oscillations and also finally produce neuroplastic effects if applied with appropriate parameters.

Non-invasive brain stimulation improves object-location learning in the elderly

Remembering the location of objects, an integral part of everyday life, is known to decline with advancing age and early in the course of neurodegenerative dementia. Here, we aimed to test if object-location learning and its retention could be modified by noninvasive brain stimulation. In a group of 20 elderly (mean age 62.1 years) right-handed individuals, we applied transcranial direct current stimulation (tDCS; 20 minutes, 1 mA) over the right temporoparietal cortex, while subjects acquired the correct position of buildings on a street map using an associative learning paradigm. Each subject participated in a randomized and balanced order in 1 session of anodal tDCS and 1 session of sham stimulation, in a double-blind design with 2 parallel versions of the task. Outcome measures were learning success at the end of each session, and immediate as well as delayed (1 week) free recall. We found that subjects performed comparably in the learning task in the 2 conditions, but showed improved recall 1 week after learning with anodal tDCS compared with learning with sham stimulation. In conclusion, retention of object-location learning in the elderly may be modulated by noninvasive brain stimulation, a finding of potential relevance not only for normal aging but also for memory deficits in pathological aging.

Transcranial direct current stimulation (tDCS) produces localized and specific alterations in neurochemistry: a ¹H magnetic resonance spectroscopy study

Transcranial direct current stimulation (tDCS) has been found to produce significant changes in behavior, including a large increase of learning and performance for a difficult visual perceptual task (Clark et al., NeuroImage 2010). The mechanisms by which tDCS produces these behavioral effects are currently uncertain. One hypothesis is that anodal tDCS leads to increased metabolic activity in the brain, which enhances cognitive and memory processes. Here we examined the neuronal mechanisms by which tDCS influences learning by measuring changes in brain metabolite concentrations using proton magnetic resonance spectroscopy (¹H MRS). As perception and learning can also influence neurochemistry, here we applied tDCS during rest. MRS data was obtained before and after 2.0 mA of anodal tDCS was applied for 30 min over electrode site P4, with the cathode placed on the contralateral arm. MRS data were acquired from the right parietal lobe beneath the anodal tDCS electrode, and from the homologous regions of the left hemisphere once before and once after tDCS. Significantly higher combined glutamate and glutamine levels were found in right parietal cortex, beneath the stimulating electrode, with non-significant increases in homologous regions of the opposite hemisphere. In addition, a significant interaction between hemispheres was found for tDCS effects on tNAA. These results suggest that changes in glutamatergic activity and tNAA may be related to the mechanisms by which tDCS influences learning and behavior.

Probing for hemispheric specialization for motor skill learning: a transcranial direct current stimulation study

Convergent findings point to a left-sided specialization for the representation of learned actions in right-handed humans, but it is unknown whether analogous hemispheric specialization exists for motor skill learning. In the present study, we explored this question by comparing the effects of anodal transcranial direct current stimulation (tDCS) over either left or right motor cortex (M1) on motor skill learning in either hand, using a tDCS montage to better isolate stimulation to one hemisphere. Results were compared with those previously found with a montage more commonly used in the field. Six groups trained for three sessions on a visually guided sequential pinch force modulation task with their right or left hand and received right M1, left M1, or sham tDCS. A linear mixed-model analysis for motor skill showed a significant main effect for stimulation group (left M1, right M1, sham) but not for hand (right, left) or their interaction. Left M1 tDCS induced significantly greater skill learning than sham when hand data were combined, a result consistent not only with the hypothesized left hemisphere specialization for motor skill learning but also with possible increased left M1 responsiveness to tDCS. The unihemispheric montage effect size was one-half that of the more common montage, and subsequent power analysis indicated that 75 subjects per group would be needed to detect differences seen with only 12 subjects with the customary bihemispheric montage.

Altered bidirectional plasticity and reduced implicit motor learning in concussed athletes

Persistent motor/cognitive alterations and increased prevalence of Alzheimer's disease are known consequences of recurrent sports concussions, the most prevalent cause of mild traumatic brain injury (TBI) among youth. Animal models of TBI demonstrated that impaired learning was related to persistent synaptic plasticity suppression in the form of long-term potentiation (LTP) and depression (LTD). In humans, single and repeated concussive injuries lead to lifelong and cumulative enhancements of gamma-aminobutyric acid (GABA)-mediated inhibition, which is known to suppress LTP/LTD plasticity. To test the hypothesis that increased GABAergic inhibition after repeated concussions suppresses LTP/LTD and contributes to learning impairments, we used a paired associative stimulation (PAS) protocol to induce LTP/LTD-like effects in primary motor cortex (M1) jointly with an implicit motor learning task (serial reaction time task, SRTT). Our results indicate that repeated concussions induced persistent elevations of GABA(B)-mediated intracortical inhibition in M1, which was associated with suppressed PAS-induced LTP/LTD-like synaptic plasticity. This synaptic plasticity suppression was related to reduced implicit motor learning on the SRTT task relative to normal LTP/LTD-like synaptic plasticity in unconcussed teammates. These findings identify GABA neurotransmission alterations after repeated concussions and suggest that impaired learning after multiple concussions could at least partly be related to compromised GABA-dependent LTP/LTD synaptic plasticity.

Modulation of human trigeminal and extracranial nociceptive processing by transcranial direct current stimulation of the motor cortex

Objective: The study was conducted to investigate the after-effect of transcranial direct current stimulation (tDCS) applied over the human primary motor cortex (M1) on trigeminal and extracranial nociceptive processing.

Basic procedures: Nineteen healthy volunteers were stimulated using cathodal, anodal (both 1 mA) or sham tDCS for 20 minutes. Pain processing was assessed by recording trigeminal and extracranial pain-related evoked potentials (PREPs) following electrical stimulation of the contralateral forehead and hand at baseline, 0, 20 and 50 minutes post-tDCS.

Main findings: Cathodal tDCS resulted in decreased peak-to-peak amplitudes (PPAs) by 18% while anodal tDCS lead to increased PPAs of PREPs by 35% ( p < .05).

Principal conclusions: The decreased PPAs suggest an inhibition and the increased PPAs of PREPs suggest an excitation of trigeminal and extracranial pain processing induced by tDCS of the M1. These results may provide evidence for the effectiveness of tDCS as a therapeutic instrument in treating headache disorders.

Transcranial direct current stimulation of the prefrontal cortex modulates working memory performance: combined behavioural and electrophysiological evidence

BACKGROUND

Transcranial direct current stimulation (tDCS) is a technique that can systematically modify behaviour by inducing changes in the underlying brain function. In order to better understand the neuromodulatory effect of tDCS, the present study examined the impact of tDCS on performance in a working memory (WM) task and its underlying neural activity. In two experimental sessions, participants performed a letter two-back WM task after sham and either anodal or cathodal tDCS over the left dorsolateral prefrontal cortex (DLPFC).

RESULTS

Results showed that tDCS modulated WM performance by altering the underlying oscillatory brain activity in a polarity-specific way. We observed an increase in WM performance and amplified oscillatory power in the theta and alpha bands after anodal tDCS whereas cathodal tDCS interfered with WM performance and decreased oscillatory power in the theta and alpha bands under posterior electrode sides.

CONCLUSIONS

The present study demonstrates that tDCS can alter WM performance by modulating the underlying neural oscillations. This result can be considered an important step towards a better understanding of the mechanisms involved in tDCS-induced modulations of WM performance, which is of particular importance, given the proposal to use electrical brain stimulation for the therapeutic treatment of memory deficits in clinical settings.

Contribution of the Premotor Cortex to Consolidation of Motor Sequence Learning in Humans During Sleep

Motor learning and memory consolidation require the contribution of different cortices. For motor sequence learning, the primary motor cortex is involved primarily in its acquisition. Premotor areas might be important for consolidation. In accordance, modulation of cortical excitability via transcranial DC stimulation (tDCS) during learning affects performance when applied to the primary motor cortex, but not premotor cortex. We aimed to explore whether premotor tDCS influences task performance during motor memory consolidation. The impact of excitability-enhancing, -diminishing, or placebo premotor tDCS during rapid eye movement (REM) sleep on recall in the serial reaction time task (SRTT) was explored in healthy humans. The motor task was learned in the evening. Recall was performed immediately after tDCS or the following morning. In two separate control experiments, excitability-enhancing premotor tDCS was performed 4 h after task learning during daytime or immediately before conduction of a simple reaction time task. Excitability-enhancing tDCS performed during REM sleep increased recall of the learned movement sequences, when tested immediately after stimulation. REM density was enhanced by excitability-increasing tDCS and reduced by inhibitory tDCS, but did not correlate with task performance. In the control experiments, tDCS did not improve performance. We conclude that the premotor cortex is involved in motor memory consolidation during REM sleep.

Modulating functional connectivity patterns and topological functional organization of the human brain with transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that alters cortical excitability and activity in a polarity-dependent way. Stimulation for few minutes has been shown to induce plastic alterations of cortical excitability and to improve cognitive performance. These effects might be caused by stimulation-induced alterations of functional cortical network connectivity. We aimed to investigate the impact of tDCS on cortical network function through functional connectivity and graph theoretical analysis. Single recordings in healthy volunteers with 62 electroencephalography channels were acquired before and after 10 min of facilitatory anodal tDCS over the primary motor cortex (M1), combined with inhibitory cathodal tDCS of the contralateral frontopolar cortex, in resting state and during voluntary hand movements. Correlation matrices containing all 62 pairwise electrode combinations were calculated with the synchronization likelihood (SL) method and thresholded to construct undirected graphs for the θ, α, β, low-γ and high-γ frequency bands. SL matrices and undirected graphs were compared before and after tDCS. Functional connectivity patterns significantly increased within premotor, motor, and sensorimotor areas of the stimulated hemisphere during motor activity in the 60-90 Hz frequency range. Additionally, tDCS-induced significant intrahemispheric and interhemispheric connectivity changes in all the studied frequency bands. In summary, we show for the first time evidence for tDCS-induced changes in brain synchronization and topological functional organization.

Task-Dependent Modulation of Inputs to Proximal Upper Limb Following Transcranial Direct Current Stimulation of Primary Motor Cortex

Cathodal transcranial DC stimulation (c-tDCS) suppresses excitability of primary motor cortex (M1) controlling contralateral hand muscles. This study assessed whether c-tDCS would have similar effects on ipsi- and contralateral M1 projections to a proximal upper limb muscle. Transcranial magnetic stimulation (TMS) of left M1 was used to elicit motor evoked potentials (MEPs) in the left and right infraspinatus (INF) muscle immediately before and after c-tDCS of left M1, and at 20 and 40 min, post-c-tDCS. TMS was delivered as participants preactivated each INF in isolation (left, right) or both INF together (bilateral). After c-tDCS, ipsilateral MEPs in left INF and contralateral MEPs in right INF were suppressed in the left task but not in the bilateral or right tasks, indicative of task-dependent modulation. Ipsilateral silent period duration in the left INF was reduced after c-tDCS, indicative of altered transcallosal inhibition. These findings may have implications for the use of tDCS as an adjunct to therapy for the proximal upper limb after stroke.

Cognitive enhancement: methods, ethics, regulatory challenges

Cognitive enhancement takes many and diverse forms. Various methods of cognitive enhancement have implications for the near future. At the same time, these technologies raise a range of ethical issues. For example, they interact with notions of authenticity, the good life, and the role of medicine in our lives. Present and anticipated methods for cognitive enhancement also create challenges for public policy and regulation.

Enhancement of planning ability by transcranial direct current stimulation

The functional neuroanatomy of executive function critically involves the dorsolateral prefrontal cortex. Transcranial direct current stimulation (tDCS) has been established as a noninvasive tool for transient modulation of cortical function. Here, we examined the effects of tDCS of the left dorsolateral prefrontal cortex on planning function by using the Tower of London task to evaluate performance during and after anodal, cathodal (1 mA, 15 min), and sham tDCS in 24 healthy volunteers. The key finding was a double dissociation of polarity and training phase: improved performance was found with cathodal tDCS applied during acquisition and early consolidation, when preceding anodal tDCS, but not in the later training session. In contrast, anodal tDCS enhanced performance when applied in the later sessions following cathodal tDCS. Our results indicate that both anodal and cathodal tDCS can improve planning performance as quantified by the Tower of London test. Most importantly, these data demonstrate training-phase-specific effects of tDCS. We propose that excitability decreasing cathodal tDCS mediates its early beneficial effect through noise reduction of neuronal activity, whereas a further adaptive configuration of specific neuronal connections is supported by excitability enhancing anodal tDCS in the later training phase by enhanced efficacy of active connections. This gain of function was sustained in a follow-up 6 and 12 months after training. In conclusion, the specific coupling of stimulation and training phase interventions may support the treatment of cognitive disorders involving frontal lobe functions.

Brain polarization enhances the formation and retention of motor memories

One of the first steps in the acquisition of a new motor skill is the formation of motor memories. Here we tested the capacity of transcranial DC stimulation (tDCS) applied over the motor cortex during motor practice to increase motor memory formation and retention. Nine healthy individuals underwent a crossover transcranial magnetic stimulation (TMS) study designed to test motor memory formation resulting from training. Anodal tDCS elicited an increase in the magnitude and duration of motor memories in a polarity-specific manner, as reflected by changes in the kinematic characteristics of TMS-evoked movements after anodal, but not cathodal or sham stimulation. This effect was present only when training and stimulation were associated and mediated by a differential modulation of corticomotor excitability of the involved muscles. These results indicate that anodal brain polarization can enhance the initial formation and retention of a new motor memory resulting from training. These processes may be the underlying mechanisms by which tDCS enhances motor learning.