Effects of HD-tDCS on memory and metamemory for general knowledge questions that vary by difficulty

Brain stimulation over dorsomedial prefrontal cortex causally affects metacognitive bias but not mentalising

Despite the importance of metacognition for everyday decision-making, its neural substrates are far from understood. Recent neuroimaging studies linked metacognitive processes to dorsomedial prefrontal cortex (dmPFC), a region known to be involved in monitoring task difficulty. dmPFC is also thought to be involved in mentalising, consistent with theoretical accounts of metacognition as a self-directed subform of mentalising. However, it is unclear whether, and if so how, dmPFC causally affects metacognitive judgements, and whether this can be explained by a more general role of dmPFC for mentalising. To test this, participants performed two tasks targeting metacognition in perceptual decisions and mentalising whilst undergoing excitatory anodal versus sham dmPFC tDCS. dmPFC tDCS significantly decreased subjective confidence reports while leaving first-level performance in accuracy and reaction times unaffected, suggesting a causal contribution of dmPFC to representing metacognitive bias. Furthermore, we found no effect of dmPFC tDCS on neither metacognitive sensitivity and efficiency nor on mentalising, providing no evidence for an overlap between perceptual metacognition and mentalising in the dmPFC. Together, our findings highlight the dmPFC's causal role in metacognition by representing subjective confidence during evaluations of cognitive performance.

Prefrontal tDCS fails to modulate memory retrieval in younger and older adults

Previous research shows that a single session of anodal transcranial direct current stimulation (tDCS) to the left dorsolateral prefrontal cortex (dlPFC) can improve the accuracy of episodic memory retrieval, but stimulation effects are not always found and may be moderated by time of day. Here, we report the results from a rigorous clinical trial (NCT03723850) designed to replicate these tDCS findings in younger adults and extend them to cognitively normal older adults. We conducted the largest double-blind, between-subjects tDCS study on memory retrieval in younger and older adults to date. 150 younger adults and 91 older adults received anodal tDCS or sham stimulation to the left dlPFC prior to episodic memory retrieval and working memory tasks. We also manipulated when tDCS was administered (time of day: morning vs. afternoon), task difficulty (easy vs. hard), and stimulus format (verbal vs. visual/pictorial) to test the extent that these variables are important for identifying tDCS effects. Contrary to our preregistered predictions, we did not find any effect of tDCS or time of day on younger or older adults' episodic or working memory performance. This outcome was not due to insensitivity of our cognitive tasks, given that we found expected effects of task difficulty and age-related effects on our memory measures. Based on these and prior tDCS results, we conclude that a single dose of tDCS using the typical and often recommended parameters does not reliably improve episodic memory retrieval in either age group.

Lack of effects of online HD-tDCS over the left or right DLPFC in an associative memory and metamemory monitoring task

Neuroimaging studies have shown that activity in the prefrontal cortex correlates with two critical aspects of normal memory functioning: retrieval of episodic memories and subjective "feelings-of-knowing" about our memory. Brain stimulation can be used to test the causal role of the prefrontal cortex in these processes, and whether the role differs for the left versus right prefrontal cortex. We compared the effects of online High-Definition transcranial Direct Current Stimulation (HD-tDCS) over the left or right dorsolateral prefrontal cortex (DLPFC) compared to sham during a proverb-name associative memory and feeling-of-knowing task. There were no significant effects of HD-tDCS on either associative recognition or feeling-of-knowing performance, with Bayesian analyses showing moderate support for the null hypotheses. Despite past work showing effects of HD-tDCS on other memory and feeling-of-knowing tasks, and neuroimaging showing effects with similar tasks, these findings add to the literature of non-significant effects with tDCS. This work highlights the need to better understand factors that determine the effectiveness of tDCS, especially if tDCS is to have a successful future as a clinical intervention.

Neurochemical Predictors of Generalized Learning Induced by Brain Stimulation and Training

Methods of cognitive enhancement for humans are most impactful when they generalize across tasks. However, the extent to which such "transfer" is possible via interventions is widely debated. In addition, the contribution of excitatory and inhibitory processes to such transfer is unknown. Here, in a large-scale neuroimaging individual differences study with humans (both sexes), we paired multitasking training and noninvasive brain stimulation (transcranial direct current stimulation, tDCS) over multiple days and assessed performance across a range of paradigms. In addition, we varied tDCS dosage (1.0 and 2.0 mA), electrode montage (left or right prefrontal regions), and training task (multitasking vs a control task) and assessed GABA and glutamate concentrations via ultrahigh field 7T magnetic resonance spectroscopy. Generalized benefits were observed in spatial attention, indexed by visual search performance, when multitasking training was combined with 1.0 mA stimulation targeting either the left or right prefrontal cortex (PFC). This transfer effect persisted for ∼30 d post intervention. Critically, the transferred benefits associated with right prefrontal tDCS were predicted by pretraining concentrations of glutamate in the PFC. Thus, the effects of this combined stimulation and training protocol appear to be linked predominantly to excitatory brain processes.

Inhibitory control in WM gate-opening: Insights from alpha desynchronization and norepinephrine activity under atDCS stimulation

Our everyday activities require the maintenance and continuous updating of information in working memory (WM). To control this dynamic, WM gating mechanisms have been suggested to be in place, but the neurophysiological mechanisms behind these processes are far from being understood. This is especially the case when it comes to the role of oscillatory neural activity. In the current study we combined EEG recordings, and anodal transcranial direct current stimulation (atDCS) and pupil diameter recordings to triangulate neurophysiology, functional neuroanatomy and neurobiology. The results revealed that atDCS, compared to sham stimulation, affected the WM gate opening mechanism, but not the WM gate closing mechanism. The altered behavioral performance was associated with specific changes in alpha band activities (reflected by alpha desynchronization), indicating a role for inhibitory control during WM gate opening. Functionally, the left superior and inferior parietal cortices, were associated with these processes. The findings are the first to show a causal relevance of alpha desynchronization processes in WM gating processes. Notably, pupil diameter recordings as an indirect index of the norepinephrine (NE) system activity revealed that individuals with stronger inhibitory control (as indexed through alpha desynchronization) showed less pupil dilation, suggesting they needed less NE activity to support WM gate opening. However, when atDCS was applied, this connection disappeared. The study suggests a close link between inhibitory controlled WM gating in parietal cortices, alpha band dynamics and the NE system.

Neural correlates of metacognition: Disentangling the brain circuits underlying prospective and retrospective second-order judgments through noninvasive brain stimulation

Metacognition encompasses the capability to monitor and control one's cognitive processes, with metamemory and metadecision configuring among the most studied higher order functions. Although imaging experiments evaluated the role of disparate brain regions, neural substrates of metacognitive judgments remain undetermined. The aim of this systematic review is to summarize and discuss the available evidence concerning the neural bases of metacognition which has been collected by assessing the effects of noninvasive brain stimulation (NIBS) on human subjects' metacognitive capacities. Based on such literature analysis, our goal is, at first, to verify whether prospective and retrospective second-order judgments are localized within separate brain circuits and, subsequently, to provide compelling clues useful for identifying new targets for future NIBS studies. The search was conducted following the preferred reporting items for systematic reviews and meta-analyses guidelines among PubMed, PsycINFO, PsycARTICLES, PSYNDEX, MEDLINE, and ERIC databases. Overall, 25 studies met the eligibility criteria, yielding a total of 36 experiments employing transcranial magnetic stimulation and 16 ones making use of transcranial electrical stimulation techniques, including transcranial direct current stimulation and transcranial alternating current stimulation. Importantly, we found that both perspective and retrospective judgments about both memory and perceptual decision-making performances depend on the activation of the anterior and lateral portions of the prefrontal cortex, as well as on the activity of more caudal regions such as the premotor cortex and the precuneus. Combining this evidence with results from previous imaging and lesion studies, we advance ventromedial prefrontal cortex as a promising target for future NIBS studies.

HD-tDCS over the left DLPFC increases cued recall and subjective question familiarity rather than other aspects of memory and metamemory

When retrieving information from memory there is an interplay between memory and metamemory processes, and the prefrontal cortex has been implicated in both memory and metamemory. Previous work shown that High Definition transcranial Direct Current Stimulation (HD-tDCS) over the dorsolateral prefrontal cortex (DLPFC) can lead to improvements in memory and metamemory monitoring, but findings are mixed. Our original design targeted metamemory, but because the prefrontal cortex plays a role in both memory and metamemory, we tested for effects of HD-tDCS on multiple memory tasks (e.g., recall, cued recall, and recognition) and multiple aspects of metamemory (e.g., once-knew-it ratings, feeling-of-knowing ratings, metamemory accuracy, and metamemory control). There were HD-tDCS-related improvements in cued recall performance, but not other memory tasks. For metamemory, there were HD-tDCS-related increases in subjective once-knew-it ratings, but not other aspects of metamemory. These results highlight the need to consider the effects of HD-tDCS on memory and metamemory at different timepoints during retrieval, as well as specific conditions that show benefits from HD-tDCS.

Differential effects of remotely supervised transcranial direct current stimulation on recognition memory depending on task order

Background

Prior work has shown positive effects of High Definition transcranial direct current stimulation (HD-tDCS) over the dorsolateral prefrontal cortex (DLPFC) on semantic memory performance and metamemory monitoring accuracy. However, HD-tDCS requires setup by a trained researcher, which is not always feasible. Few studies have used remotely supervised (rs) tDCS in healthy populations, and remote supervision has strong practical benefits.

Objective/hypothesis

The goal of the current study was to test if previously shown effects of HD-tDCS over the left DLPFC on semantic memory performance and metamemory monitoring accuracy extended to conventional rs-tDCS, which is less focal than HD-tDCS, and to episodic memory and metamemory tasks.

Materials and methods

A total of 36 healthy participants completed 6 weeks of rs-tDCS sessions, with either active left or right anodal DLPFC stimulation, or sham. Participants completed semantic and episodic memory and metamemory tasks, which each lasted for three consecutive sessions, and session order was counterbalanced across participants.

Results

Overall, there were no main effects of rs-tDCS on metamemory monitoring accuracy or memory performance for either the semantic or the episodic tasks. However, there were effects of rs-tDCS that depended on the order of completing the episodic and semantic task sessions. When participants completed the semantic task sessions after the episodic task sessions, semantic recognition was greater in the left anodal DLPFC condition. In a parallel effect, when participants completed the episodic task sessions after the semantic task sessions, episodic recognition was greater in the right anodal DLPFC condition.

Conclusion

Prior experience with tDCS is a factor for effects of rs-tDCS on cognition. Additionally, the current experiment provides evidence for the feasibility of fully remotely supervised tDCS in healthy participants.

High-definition trans cranial direct current stimulation and its effects on cognitive function: a systematic review

High-Definition Transcranial Direct Current Stimulation (HD-tDCS) is focal and improves higher mental functions. Due to the lack of published evidence, we conducted this review on the effect of HD-tDCS on cognitive functions in healthy and diseased individuals. We performed an electronic-data and gray-literature search to obtain the relevant studies for the review. The two distinct literature searches obtained a total of 468 studies. Out of these, a total of 12 studies were conducted on higher mental functions, and of these, two were on disordered consciousness, five were on memory, two were on speech, two were on cognition, and one was on execution. We submitted nine studies with control group to methodological quality assessment using the PEDro Scale. Remaining three studies underwent quality assessment by Quality Assessment Tool for Before-After (Pre-Post) Studies with No Control Group. We found that anodal HD-tDCS stimulation is significantly effective in treating disordered consciousness and improving memory, speech, cognition, and execution.

High-Definition Transcranial Stimulation over the Dorsolateral Prefrontal Cortex Alters the Sunk Cost Effect: A Mental Accounting Framework

The sunk cost effect refers to the fact that human decisions are consistently influenced by previous irrecoverable and irrelevant costs. Recent neuroimaging experiments suggest that the dorsolateral prefrontal cortex (dlPFC) plays a pivotal role in the sunk cost effect yet the causal and neurocomputational role of the dlPFC remains elusive. In this study, two cohorts of healthy human male and female adults were recruited to complete a novel two-step decision-making task during the anodal-sham or cathodal-sham high-definition transcranial direct current stimulation (HD-tDCS) over the dlPFC, respectively. Consistent with previous studies, we showed that the sunk cost deterred participants from making further investment and therefore engendered a de-escalation effect. Such behavior can be captured by a weighted mental accounting model with a recalibrated reference point in which the direction and magnitude of the sunk cost effects hinge on the decision weights apportioned to the option values. Interestingly, transcranial stimulation did not influence participants' initial willingness to incur sunk costs but only altered sunk costs' downstream effects. Specifically, anodal stimulation over the right dlPFC amplified the de-escalation effect of sunk costs whereas cathodal stimulation yielded the opposite result. HD-tDCS also changed the decision weights of the mental accounting model, providing a causal and computational link between PFC and sunk cost effects.SIGNIFICANCE STATEMENT Traditional economic theory assumes that decisions only concern the marginal costs and benefits yet human choices are notoriously susceptible to previously-incurred costs (termed the sunk cost effect). In the current study, we showed that direct current stimulation (DCS) of the right dorsolateral prefrontal cortex (dlPFC) altered sunk cost effects in participants' subsequent choices. Such effects can be captured by a mental accounting model where transcranial stimulation modulates the decision weights assigned to different options in the value integration process. These findings help elucidate the computational and causal role of the dlPFC in the context of sunk costs.

Perspectives on Rehabilitation Using Non-invasive Brain Stimulation Based on Second-Person Neuroscience of Teaching-Learning Interactions

Recent advances in second-person neuroscience have allowed the underlying neural mechanisms involved in teaching-learning interactions to be better understood. Teaching is not merely a one-way transfer of information from teacher to student; it is a complex interaction that requires metacognitive and mentalizing skills to understand others’ intentions and integrate information regarding oneself and others. Physiotherapy involving therapists instructing patients on how to improve their motor skills is a clinical field in which teaching-learning interactions play a central role. Accumulating evidence suggests that non-invasive brain stimulation (NIBS) modulates cognitive functions; however, NIBS approaches to teaching-learning interactions are yet to be utilized in rehabilitation. In this review, I evaluate the present research into NIBS and its role in enhancing metacognitive and mentalizing abilities; I then review hyperscanning studies of teaching-learning interactions and explore the potential clinical applications of NIBS in rehabilitation. Dual-brain stimulation using NIBS has been developed based on findings of brain-to-brain synchrony in hyperscanning studies, and it is delivered simultaneously to two individuals to increase inter-brain synchronized oscillations at the stimulated frequency. Artificial induction of brain-to-brain synchrony has the potential to promote instruction-based learning. The brain-to-brain interface, which induces inter-brain synchronization by adjusting the patient’s brain activity, using NIBS, to the therapist’s brain activity, could have a positive effect on both therapist-patient interactions and rehabilitation outcomes. NIBS based on second-person neuroscience has the potential to serve as a useful addition to the current neuroscientific methods used in complementary interventions for rehabilitation.

Brain connectivity alterations during sleep by closed-loop transcranial neurostimulation predict metamemory sensitivity

Metamemory involves the ability to correctly judge the accuracy of our memories. The retrieval of memories can be improved using transcranial electrical stimulation (tES) during sleep, but evidence for improvements to metamemory sensitivity is limited. Applying tES can enhance sleep-dependent memory consolidation, which along with metamemory requires the coordination of activity across distributed neural systems, suggesting that examining functional connectivity is important for understanding these processes. Nevertheless, little research has examined how functional connectivity modulations relate to overnight changes in metamemory sensitivity. Here, we developed a closed-loop short-duration tES method, time-locked to up-states of ongoing slow-wave oscillations, to cue specific memory replays in humans. We measured electroencephalographic (EEG) coherence changes following stimulation pulses, and characterized network alterations with graph theoretic metrics. Using machine learning techniques, we show that pulsed tES elicited network changes in multiple frequency bands, including increased connectivity in the theta band and increased efficiency in the spindle band. Additionally, stimulation-induced changes in beta-band path length were predictive of overnight changes in metamemory sensitivity. These findings add new insights into the growing literature investigating increases in memory performance through brain stimulation during sleep, and highlight the importance of examining functional connectivity to explain its effects.

Numerous studies have demonstrated a clear link between sleep and memory—namely, memories are consolidated during sleep, leading to more stable and long-lasting representations. We have previously shown that tagging episodes with specific patterns of brain stimulation during encoding and replaying those patterns during sleep can enhance this consolidation process to improve confidence and decision-making of memories (metamemory). Here, we extend this work to examine network-level brain changes that occur following stimulation during sleep that predict metamemory improvements. Using graph theoretic and machine-learning methods, we found that stimulation-induced changes in beta-band path length predicted overnight improvements in metamemory. This novel finding sheds new light on the neural mechanisms of memory consolidation and suggests potential applications for improving metamemory.

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

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

Dataset of concurrent EEG, ECG, and behavior with multiple doses of transcranial electrical stimulation

We present a dataset combining human-participant high-density electroencephalography (EEG) with physiological and continuous behavioral metrics during transcranial electrical stimulation (tES). Data include within participant application of nine High-Definition tES (HD-tES) types, targeting three cortical regions (frontal, motor, parietal) with three stimulation waveforms (DC, 5 Hz, 30 Hz); more than 783 total stimulation trials over 62 sessions with EEG, physiological (ECG, EOG), and continuous behavioral vigilance/alertness metrics. Experiment 1 and 2 consisted of participants performing a continuous vigilance/alertness task over three 70-minute and two 70.5-minute sessions, respectively. Demographic data were collected, as well as self-reported wellness questionnaires before and after each session. Participants received all 9 stimulation types in Experiment 1, with each session including three stimulation types, with 4 trials per type. Participants received two stimulation types in Experiment 2, with 20 trials of a given stimulation type per session. Within-participant reliability was tested by repeating select sessions. This unique dataset supports a range of hypothesis testing including interactions of tDCS/tACS location and frequency, brain-state, physiology, fatigue, and cognitive performance.

High-definition transcranial direct current stimulation facilitates emotional face processing in individuals with high autistic traits: A sham-controlled study

The deficit in emotional face processing is a critical impairment for individuals with high autistic traits. The temporalparietal junction(TPJ) is considered to be closely related to emotional face processing. The aim of this study was to examine the effect of highdefinition transcranial direct current stimulation (HD-tDCS) over the right temporal-parietal junction (rTPJ) on facial emotion processing of individuals with high autistic traits using eye-tracking technology. Twenty-nine participants with high autistic traits completed an eyetracking task (including happy, fearful and neutral faces) before and after five consecutive days of stimulation (anodal or sham). Results showed that anodal HD-tDCS significantly increased fixation time and fixation count in the mouth area, but this effect was not found after the sham stimulation. Moreover, this increased effect of mouth recognition with anodal rTPJ HD-tDCS was shown in both happy and fearful faces, but no remarkable difference was found in neutral faces. These findings suggest that right TPJ anodal HD-tDCS can facilitate emotional face processing in individuals with high autistic traits.

Effective connectivity during autobiographical memory search

INTRODUCTION

We used dynamic causal modeling (DCM) to examine effective connectivity during cued autobiographical memory (AM) search in a left-hemispheric network consisting of six major regions within the large network of brain regions recruited during memory retrieval processes.

METHODS

Functional MRI data were acquired while participants were shown verbal cues describing common life events and requested to search for a personal memory associated with the cue. We examined directed couplings between the ventromedial (vmPFC), dorsomedial (dmPFC), and dorsolateral prefrontal cortices (dlPFC), hippocampus, angular gyrus, and the posterior midline cortex (RSC/PCC/Prec).

RESULTS

During AM search, the vmPFC, dlPFC, and RSC/PCC/Prec acted as primary drivers of activity in the rest of the network. Moreover, when AM search completed successfully (Hits), the effective connectivity of the hippocampus on the vmPFC and angular gyrus was up-modulated. Likewise, there was an increase in the influence of the RSC/PCC/Prec in the activity of the dlPFC and dmPFC. Further analysis indicated that the modulation observed during Hits is primarily a distributed phenomenon that relies on the interplay between different brain regions.

CONCLUSION

These results suggest that prefrontal and posterior midline cortical regions together with the dlPFC largely coordinate the processes underlying AM search, setting up the conditions on which the angular gyrus and the hippocampus may act upon when the outcome of the search is successful.

The effects of transcranial direct current stimulation on corticospinal and cortico-cortical excitability and response variability: Conventional versus high-definition montages

Response variability following transcranial direct current stimulation (tDCS) highlights need for exploring different tDCS electrode montages. Corticospinal excitability (CSE), cortico-cortical excitability and intra-individual variability was compared following conventional and high-definition (HD) anodal (a-tDCS) and cathodal (c-tDCS) tDCS. Fifteen healthy males attended four sessions at-least one-week apart: conventional a-tDCS, conventional c-tDCS, HD-a-tDCS, HD-c-tDCS. TDCS was administered (1 mA, 10-minutes) over primary motor cortex (M1), via 6 × 4 cm active and 7 × 5 cm return electrodes (conventional tDCS) and 4 × 1 ring-electrodes 3.5 cm apart over M1 (HD-tDCS). For CSE, twenty-five single-pulse transcranial magnetic stimulation (TMS) peak-to-peak motor evoked potentials (MEP) were recorded at baseline, 0-minutes and 30-minutes post-tDCS. Twenty-five paired-pulse MEPs with 3-millisecond (ms) inter-pulse interval (IPI) and twenty-five at 10 ms assessed short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). MEP standardised z-values standard deviations represented intra-individual variability. No significant changes in CSE from baseline were reported for all four interventions. No significant differences were reported in CSE between conventional and HD a-tDCS, but significant differences between conventional and HD c-tDCS 0-minutes post-tDCS. Conventional tDCS significantly reduced intra-individual variability compared to HD-tDCS for a-tDCS (0-minutes) and c-tDCS (30-minutes). No changes were reported for SICI/ICF. These novel findings of increased intra-individual variability following HD-tDCS, at the current stimulus parameters, highlight need for further nuanced research and refinement to optimise the HD-tDCS dosage-response relationship.

Transcranial Direct Current Stimulation over the Prefrontal Cortex Alters Encoding and Judgments of Learning Based on Fluency

Past research has shown that judgments of learning (JOLs), subjective confidence judgments made at study about later memorability, are inferential in nature and based on cues available during encoding. Participants tend to use fluency as a cue and give higher JOLs to more fluently encoded items, despite having better recognition memory for disfluently encoded items, which leads to poor JOL accuracy. Research has implicated the dorsolateral prefrontal cortex (DLPFC) and anterior prefrontal cortex (aPFC) in JOL and encoding processes, but no studies to date have tested how the roles of these regions vary with the information on which JOLs are based. We used high-definition transcranial direct current stimulation to test the causal roles of DLPFC and aPFC in encoding success, JOL ratings, and JOL accuracy. Participants studied and made JOLs about words that varied in fluency (i.e., frequency and orientation). High-definition transcranial direct current stimulation over the DLPFC impaired encoding, as evidenced by an increase in subsequent false alarms. For words that were less fluently encoded, aPFC stimulation improved JOL accuracy, perhaps making participants more aware of encoding failures under conditions of disfluency. Conversely, DLPFC and aPFC stimulation decreased JOL accuracy for high-frequency words, suggesting the roles of these regions in JOLs vary with the cognitive bases of the judgments. These results contribute to our understanding of the causal roles of prefrontal regions in objective and subjective memory processes and how their contributions to metamemory accuracy vary with information on which subjective assessments are based.

Physics of Transcranial Direct Current Stimulation Devices and Their History

Transcranial direct current stimulation (tDCS) devices apply direct current through electrodes on the scalp with the intention to modulate brain function for experimental or clinical purposes. All tDCS devices include a current controlled stimulator, electrodes that include a disposable electrolyte, and headgear to position the electrodes on the scalp. Transcranial direct current stimulation dose can be defined by the size and position of electrodes and the duration and intensity of current applied across electrodes. Electrode design and preparation are important for reproducibility and tolerability. High-definition tDCS uses smaller electrodes that can be arranged in arrays to optimize brain current flow. When intended to be used at home, tDCS devices require specific device design considerations. Computational models of current flow have been validated and support optimization and hypothesis testing. Consensus on the safety and tolerability of tDCS is protocol specific, but medical-grade tDCS devices minimize risk.

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.

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.