Making the rich richer: Frontoparietal tDCS enhances transfer effects of a single-session distractor inhibition training on working memory in high capacity individuals but reduces them in low capacity individuals

Exploring the intra-individual reliability of tDCS: A registered report

Transcranial direct current stimulation (tDCS), a form of non-invasive brain stimulation, has become an important tool for the study of in-vivo brain function due to its modulatory effects. Over the past two decades, interest in the influence of tDCS on behaviour has increased markedly, resulting in a large body of literature spanning multiple domains. However, the effect of tDCS on human performance often varies, bringing into question the reliability of this approach. While reviews and meta-analyses highlight the contributions of methodological inconsistencies and individual differences, no published studies have directly tested the intra-individual reliability of tDCS effects on behaviour. Here, we conducted a large scale, double-blinded, sham-controlled registered report to assess the reliability of two single-session low-dose tDCS montages, previously found to impact response selection and motor learning operations, across two separate time periods. Our planned analysis found no evidence for either protocol being effective nor reliable. Post-hoc explorative analyses found evidence that tDCS influenced motor learning, but not response selection learning. In addition, the reliability of motor learning performance across trials was shown to be disrupted by tDCS. These findings are amongst the first to shed light specifically on the intra-individual reliability of tDCS effects on behaviour and provide valuable information to the field.

Evaluation of Transcranial Direct Current Stimulation in Motor Function and Neural Rehabilitation

ABSTRACT

Transcranial direct current stimulation (tDCS) is used in neuromodulation to regulate the excitability of the cerebral cortex and induce neural plasticity. It was initially used to rehabilitate patients with neurological diseases. However, with the increasing number of studies involving healthy individuals, this technology is currently used in the field of sports as well. The administration of tDCS to the cerebral cortex, especially over the primary motor cortex (M1), has been found to improve muscle strength, enhance endurance, and promote motor skills in humans. This study mainly summarizes the effects of tDCS on motor function, mainly involving motor promotion of tDCS in healthy athletes and nonathletes, and in patients diagnosed with neurological diseases. The tDCS is a promising and effective tool used to promote motor function by regulating cortical excitability. However, no consensus is available regarding individually appropriate models of tDCS.

Long-term effects of repeated multitarget high-definition transcranial direct current stimulation combined with cognitive training on response inhibition gains

Background

Few studies have investigated the effects of repeated sessions of transcranial direct current stimulation (tDCS) combined with concurrent cognitive training on improving response inhibition, and the findings have been heterogeneous in the limited research. This study investigated the long-lasting and transfer effects of 10 consecutive sessions of multitarget anodal HD-tDCS combined with concurrent cognitive training on improving response inhibition compared with multitarget stimulation or training alone.

Methods

Ninety-four healthy university students aged 18–25 were randomly assigned to undergo different interventions, including real stimulation combined with stop-signal task (SST) training, real stimulation, sham stimulation combined with SST training, and sham stimulation. Each intervention lasted 20 min daily for 10 consecutive days, and the stimulation protocol targeted right inferior frontal gyrus (rIFG) and pre-supplementary motor area (pre-SMA) simultaneously with a total current intensity of 2.5 mA. Performance on SST and possible transfer effects to Stroop task, attention network test, and N-back task were measured before and 1 day and 1 month after completing the intervention course.

Results

The main findings showed that the combined protocol and the stimulation alone significantly reduced stop-signal reaction time (SSRT) in the post-intervention and follow-up tests compared to the pre-intervention test. However, training alone only decreased SSRT in the post-test. The sham control exhibited no changes. Subgroup analysis revealed that the combined protocol and the stimulation alone induced a decrease in the SSRT of the low-performance subgroup at the post-test and follow-up test compared with the pre-test. However, only the combined protocol, but not the stimulation alone, improved the SSRT of the high-performance subgroup. The transfer effects were absent.

Conclusion

This study provides supportive evidence for the synergistic effect of the combined protocol, indicating its superiority over the single intervention method. In addition, the long-term after-effects can persist for up to at least 1 month. Our findings also provide insights into the clinical application and strategy for treating response inhibition deficits.

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.

Training attentive individuation leads to visuo-spatial working memory improvement in low-performing older adults: An online study

Cognitive decrements are typical of physiological aging. Among these age-related cognitive changes, visuo-spatial working memory (vWM) decline has a prominent role due to its effects on other cognitive functions and daily routines. To reinforce vWM in the aging population, several cognitive training interventions have been developed in the past years. Given that vWM functioning depends (at least partially) on the efficiency of attention selection of the relevant objects, in the present study we implemented a short (five sessions), online intervention that primarily trained attentive individuation of target items and tested training effects on a vWM task. Attention training effects were compared with practice (i.e., a group that repeatedly performed the same vWM task) and test-retest effects (i.e., a passive group). After the training, the results showed attention training effects of the same magnitude as practice effects, confirming that the enhancement of attentive individuation has a positive cascade influence on maintaining items in vWM. Moreover, training and practice effects were only evident in low-performing older adults. Thus, interindividual differences at baseline crucially contribute to training outcomes and are a fundamental factor to be accounted for in the implementation of cognitive training protocols.

Boosting working memory: Uncovering the differential effects of tDCS and tACS

Working memory (WM) is essential for reasoning, decision making and problem solving. Recently, there has been an increasing effort in improving WM through non-invasive brain stimulation, especially transcranial direct and alternating current stimulation (tDCS/tACS). Studies suggest that tDCS and tACS can modulate WM performance, but large variability in research approaches hinders identification of optimal stimulation protocols and interpretation of study results. Moreover, it is unclear whether tDCS and tACS differentially affect WM. Here, we summarize and compare studies examining the effects of tDCS and tACS on WM performance in healthy adults. Following PRISMA-selection criteria, our systematic review resulted in 43 studies (29 tDCS, 11 tACS, 3 both) with a total of 1826 adult participants. For tDCS, only 4 out of 23 single-session studies reported effects on WM, while 7 out of 9 multi-session experiments showed positive effects on WM training. For tACS, 10 out of 14 studies demonstrated effects on WM, which were frequency dependent and robust for frontoparietal stimulation. Our review revealed no reliable effect of single-session tDCS on WM, but moderate effects of multi-session tDCS and single-session tACS. We discuss implications of these findings and future directions in the emerging research field of non-invasive brain stimulation and WM.