Uncertainty and Promise: the Effects of Transcranial Direct Current Stimulation on Working Memory

Individual response to transcranial direct current stimulation as a function of working memory capacity and electrode montage

Introduction

Attempts to improve cognitive abilities via transcranial direct current stimulation (tDCS) have led to ambiguous results, likely due to the method’s susceptibility to methodological and inter-individual factors. Conventional tDCS, i.e., using an active electrode over brain areas associated with the targeted cognitive function and a supposedly passive reference, neglects stimulation effects on entire neural networks.

Methods

We investigated the advantage of frontoparietal network stimulation (right prefrontal anode, left posterior parietal cathode) against conventional and sham tDCS in modulating working memory (WM) capacity dependent transfer effects of a single-session distractor inhibition (DIIN) training. Since previous results did not clarify whether electrode montage drives this individual transfer, we here compared conventional to frontoparietal and sham tDCS and reanalyzed data of 124 young, healthy participants in a more robust way using linear mixed effect modeling.

Results

The interaction of electrode montage and WM capacity resulted in systematic differences in transfer effects. While higher performance gains were observed with increasing WM capacity in the frontoparietal stimulation group, low WM capacity individuals benefited more in the sham condition. The conventional stimulation group showed subtle performance gains independent of WM capacity.

Discussion

Our results confirm our previous findings of WM capacity dependent transfer effects on WM by a single-session DIIN training combined with tDCS and additionally highlight the pivotal role of the specific electrode montage. WM capacity dependent differences in frontoparietal network recruitment, especially regarding the parietal involvement, are assumed to underlie this observation.

The effect of tDCS electrode montage on attention and working memory

The effects of transcranial direct current stimulation (tDCS) for improving attention and working memory have been generally mixed and small, potentially due to variability between studies with montages, stimulus parameters and outcome measures. The tDCS montage is an important parameter which determines the degree and intensity of stimulation in targeted brain regions. This study aimed to examine the effects of using three different montages for modulating attention and working memory performance: Bi-frontal, Broad-frontal and Broad-parietal. Ninety-three healthy adults participated in a counterbalanced cross-over study. Participants received both active and sham tDCS with either the Bi-frontal, Broad-frontal or Broad-parietal montage during performance of both a 1- and 2-back task. TDCS montage moderated 2-back working memory reaction time performance, though not accuracy, with faster reaction times observed for active compared to sham tDCS with the Broad-frontal montage only (F (1,90) = 5.26, p = .024, η² = 0.06). TDCS montage did not significantly moderate performance on the 1-back task. The cognitive effects of tDCS varied according to montage, task, and outcome measure. TDCS administered with the cathode placed extracephalically in a Broad-frontal montage may be beneficial for improving working memory.

Predicting Working Memory Training Benefits From Transcranial Direct Current Stimulation Using Resting-State fMRI

The effects of transcranial direct current stimulation (tDCS) on working memory (WM) performance are promising but variable and contested. In particular, designs involving one session of tDCS are prone to variable outcomes with notable effects of individual differences. Some participants benefit, whereas others are impaired by the same tDCS protocol. In contrast, protocols including multiple sessions of tDCS more consistently report WM improvement across participants. The objective of the current project was to test whether differences in resting-state connectivity between stimulation site and two WM-relevant networks [default mode network (DMN) and central executive network (CEN)] could account for initial and longitudinal responses to tDCS. Healthy young adults completed 5 days of visual WM training during sham or anodal right frontal tDCS. The behavioral data showed that only the active tDCS group significantly improved over the visual WM training period. There were no significant correlations between initial response to tDCS and resting-state activity. DMN activity in the anterior cingulate cortex significantly correlated with WM training slope. These data underscore the importance of sampling in studies applying tDCS; homogeneity (e.g., of gender, special population, and WM capacity) may produce more consistent data in a single experiment with limited power, whereas heterogeneity is important in determining the mechanism(s) and potential for tDCS-linked protocols. This issue is a limitation in tDCS findings that continues to hamper its optimization and translational value.

Causal Role of the Right Dorsolateral Prefrontal Cortex in Organizational Fairness Perception: Evidence From a Transcranial Direct Current Stimulation Study

The right dorsolateral prefrontal cortex (rDLPFC) plays an essential role in social decision-making. Although several neural imaging studies have provided evidence that the rDLPFC is correlated with fairness perception, little research has investigated the causal effect of this encephalic region on individuals’ consciousness, particularly perceptions of organizational fairness. The present study explores the causal relationship between the rDLPFC and organizational fairness perception by using brain modulation techniques. Healthy participants received transcranial direct current stimulation (tDCS) and fulfilled the modified ultimatum game (UG) in the sham-controlled experiment. Our results showed that only cathodal stimulation of the rDLPFC resulted in increasing rejection offers compared with the sham stimulation in conditions of disadvantageous inequity. No differences were found between the anodal and sham stimulation in any inequity condition. This study strengthens the main functional effects of the rDLPFC in negative emotional control in relation to organizational fairness perceptions.

Investigating the effects of tDCS on Visual Orientation Discrimination Task Performance: 'The possible influence of placebo'

The non-invasive neuromodulation technique tDCS offers the promise of a low cost tool for both research and clinical applications in psychology, psychiatry and neuroscience. However, findings regarding its efficacy are often equivocal. A key issue is that the clinical and cognitive applications studied are often complex and thus effects of tDCS are difficult to predict given its known effects on the basic underlying neurophysiology, namely alterations in cortical inhibition-excitation balance. As such, it may be beneficial to assess the effects of tDCS in tasks whose performance has a clear link to cortical inhibition-excitation balance such as the visual orientation discrimination task (ODT). In prior studies in our laboratory no practise effects were found during 2 consecutive runs of the ODT, thus in the current investigation, to examine the effects of tDCS, subjects received 10 minutes of 2mA occipital tDCS (sham, anode, cathode) between a first and second run of ODT. Surprisingly, subjects' performance significantly improved in the second run of ODT compared to the first one regardless of the tDCS stimulation type they received (anodal, cathodal, or sham-tDCS). Possible causes for such an improvement could have been due to either a generic 'placebo' effect of tDCS (as all subjects received some form of tDCS) or an increased delay period between the two runs of ODT of the current study compared to our previous work (10 minutes duration required to administer tDCS as opposed to ~2 minutes in previous studies as a 'break'). As such, we tested these two possibilities with a subsequent experiment in which subjects received 2 minutes or 10 minutes delay between the 2 runs (with no tDCS) or 10 minutes of sham-tDCS. Only sham-tDCS resulted in improved performance thus these data add to a growing literature suggesting that tDCS has powerful placebo effect that may occur even in the absence of active cortical modulation.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

SIGNIFICANCE

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

Reward motivation and neurostimulation interact to improve working memory performance in healthy older adults: A simultaneous tDCS-fNIRS study

Several studies have evaluated the effect of anodal transcranial direct current stimulation (tDCS) over the prefrontal cortex (PFC) for the enhancement of working memory (WM) performance in healthy older adults. However, the mixed results obtained so far suggest the need for concurrent brain imaging, in order to more directly examine tDCS effects. The present study adopted a continuous multimodal approach utilizing functional near-infrared spectroscopy (fNIRS) to examine the interactive effects of tDCS combined with manipulations of reward motivation. Twenty-one older adults (mean age = 69.7 years; SD = 5.05) performed an experimental visuo-spatial WM task before, during and after the delivery of 1.5 mA anodal tDCS/sham over the left prefrontal cortex (PFC). During stimulation, participants received performance-contingent reward for every fast and correct response during the WM task. In both sessions, hemodynamic activity of the bilateral frontal, motor and parietal areas was recorded across the entire duration of the WM task. Cognitive functions and reward sensitivity were also assessed with standard measures. Results demonstrated a significant impact of tDCS on both WM performance and hemodynamic activity. Specifically, faster responses in the WM task were observed both during and after anodal tDCS, while no differences were found under sham control conditions. However, these effects emerged only when taking into account individual visuo-spatial WM capacity. Additionally, during and after the anodal tDCS, increased hemodynamic activity relative to sham was observed in the bilateral PFC, while no effects of tDCS were detected in the motor and parietal areas. These results provide the first evidence of tDCS-dependent functional changes in PFC activity in healthy older adults during the execution of a WM task. Moreover, they highlight the utility of combining reward motivation with prefrontal anodal tDCS, as a potential strategy to improve WM efficiency in low performing healthy older adults.

No Effects of Non-invasive Brain Stimulation on Multiple Sessions of Object-Location-Memory Training in Healthy Older Adults

Object-location memory (OLM) is known to decline with normal aging, a process accelerated in pathological conditions like mild cognitive impairment (MCI). In order to maintain cognitive health and to delay the transition from healthy to pathological conditions, novel strategies are being explored. Tentative evidence suggests that combining cognitive training and anodal transcranial direct current stimulation (atDCS), both reported to induce small and often inconsistent behavioral improvements, could generate larger or more consistent improvements or both, compared to each intervention alone. Here, we explored the combined efficacy of these techniques on OLM. In a subject-blind sham-controlled cross-over design 32 healthy older adults underwent a 3-day visuospatial training paired with either anodal (20 min) or sham (30 s) atDCS (1 mA, temporoparietal). Subjects were asked to learn the correct object-location pairings on a street map, shown over five learning blocks on each training day. Acquisition performance was assessed by accuracy on a given learning block in terms of percentage of correct responses. Training success (performance on last training day) and delayed memory after 1-month were analyzed by mixed model analysis and were controlled for gender, age, education, sequence of stimulation and baseline performance. Exploratory analysis of atDCS effects on within-session (online) and between-session (offline) memory performance were conducted. Moreover, transfer effects on similar trained (visuospatial) and less similar (visuo-constructive, verbal) untrained memory tasks were explored, both immediately after training, and on follow-up. We found that atDCS paired with OLM-training did not enhance success in training or performance in 1-month delayed memory or transfer tasks. In sum, this study did not support the notion that the combined atDCS-training approach improves immediate or delayed OLM in older adults. However, specifics of the experimental design, and a non-optimal timing of atDCS between sessions might have masked beneficial effects and should be more systematically addressed in future studies.

Enhancing Working Memory Training with Transcranial Direct Current Stimulation

Working memory (WM) is a fundamental cognitive ability that supports complex thought but is limited in capacity. Thus, WM training interventions have become very popular as a means of potentially improving WM-related skills. Another promising intervention that has gained increasing traction in recent years is transcranial direct current stimulation (tDCS), a noninvasive form of brain stimulation that can modulate cortical excitability and temporarily increase brain plasticity. As such, it has the potential to boost learning and enhance performance on cognitive tasks. This study assessed the efficacy of tDCS to supplement WM training. Sixty-two participants were randomized to receive either right prefrontal, left prefrontal, or sham stimulation with concurrent visuospatial WM training over the course of seven training sessions. Results showed that tDCS enhanced training performance, which was strikingly preserved several months after training completion. Furthermore, we observed stronger effects when tDCS was spaced over a weekend break relative to consecutive daily training, and we also demonstrated selective transfer in the right prefrontal group to nontrained tasks of visual and spatial WM. These findings shed light on how tDCS may be leveraged as a tool to enhance performance on WM-intensive learning tasks.