Transcranial electrical stimulation improves cognitive training effects in healthy elderly adults with low cognitive performance

Tripod transcranial alternating current stimulation at 5-Hz to alleviate anxiety symptoms: A preliminary report

INTRODUCTION

One of the most common applications of transcranial electrical stimulation (tES) at low current intensity is to induce a relaxed state or reduce anxiety. With technical advancement, different waveforms, montages, and parameters can be incorporated into the treatment regimen. We developed a novel protocol to treat individuals with anxiety disorders by transcranial alternating current stimulation (tACS).

METHODS

A total of 27 individuals with anxiety disorders underwent tACS treatment for 12 sessions, with each session lasting 25 min. tACS at 5 Hz was applied to F4 (1.0 mA), P4 (1.0 mA), and T8 (2.0 mA) EEG lead positions (tripod), with sinewave oscillation between T8 and F4/P4. We evaluated the primary and secondary outcomes using the Beck Anxiety Inventory (BAI) and neuropsychological assessments.

RESULTS

Of the 27 patients, 19 (70.4 %) experienced a reduction in symptom severity >50 %, with an average reduction of BAI 58.5 %. All reported side effects were mild, with itching or tingling being the most common complaint. No significant differences were noted in attention, linguistic working memory, visuospatial working memory, or long-term memory in neuropsychological assessments.

CONCLUSION

The results suggest the potential of this novel tripod tACS design as a rapid anxiety alleviator and the importance of a clinical trial to verify its efficacy.

Advances in Non-Invasive Neuromodulation Techniques for Improving Cognitive Function: A Review

Non-invasive neuromodulation techniques are widely utilized to study and improve cognitive function, with the aim of modulating different cognitive processes. For workers performing high-intensity mental and physical tasks, extreme fatigue may not only affect their working efficiency but may also lead to cognitive decline or cognitive impairment, which, in turn, poses a serious threat to their physical health. The use of non-invasive neuromodulation techniques has important research value for improving and enhancing cognitive function. In this paper, we review the research status, existing problems, and future prospects of transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), transcranial magnetic stimulation (TMS), and transcutaneous acupoint stimulation (TAS), which are the most studied physical methods in non-invasive neuromodulation techniques to improve and enhance cognition. The findings presented in this paper will be of great reference value for the in-depth study of non-invasive neuromodulation techniques in the field of cognition.

The effect of transcranial direct current stimulation associated with video game training on the postural balance of older women in the community: A blind, randomized, clinical trial

BACKGROUND

Falls are frequent in older adults and can cause trauma, injury, and death. Fall prevention with virtual reality presents good results in improving postural control. Transcranial Direct Current Stimulation (tDCS) has been used with the same aim; however, the combination of the two techniques has still been little studied.

PURPOSE

To assess whether tDCS can enhance the effect of video game training (VGT) on improving the postural balance of healthy older women.

METHOD

A blinded, randomized, controlled clinical trial was conducted with 57 older women who were randomized to three balance training groups: Control Group (VGT), Anodal Group (VGT combined with anodic tDCS-atDCS), and Sham Group (VGT combined with sham tDCS-stDCS). Balance training was performed twice a week for four weeks, totalizing eight 20-min sessions using VGT associated with tDCS. Postural balance was assessed pre-and post-training and 30 days after the end of the eight sessions using the Mini-Balance Evaluation Systems Test.

RESULTS

Compared to pre-intervention the Mini BEST test increased similarly in the three groups in post-intervention (control: pre 23.7 ± 2.8 to post 27.0 ± 2.2; anodal: pre 24.4 ± 1 to post 27.7 ± 0.8 and sham: pre 24.2 ± 1.9 to post 26.5 ± 1.6; p < 0.001) and follow-up (control: pre 23.7 ± 2.8 to follow-up 26.8 ± 2.3; anodal: pre 24.4 ± 1 to follow-up 27.3 ± 1.4 and sham: pre 24.2 ± 1.9 to follow-up 26.8 ± 1.5; p < 0.001).

CONCLUSION

There was an improvement in the postural balance of the three training groups that were independent of tDCS.

DISCUSSION

Some studies have shown the positive tDCS effects associated with other tasks to improve balance. However, these results convey the effects of only anodic-tDCS compared to sham-tDCS. Possibly, the effect of VGT surpassed the tDCS effects, promoting a ceiling effect from the combination of these two therapies. However, studies with other therapies combined with tDCS for older adults deserve to be investigated, as well as in frail older people.

Electrical stimulation of the cerebellum facilitates automatic but not controlled word retrieval

Recent research has indicated that the cerebellum is engaged in language functions, yet the role of the cerebellum in lexical-semantic memory is poorly understood. In a double-blind randomized controlled experiment, we therefore targeted the cerebellum by transcranial direct current stimulation (tDCS) to assess and compare the contribution of the cerebellar processing to automatic and controlled retrieval of words in healthy adults (n = 136). Anodal cerebellar tDCS facilitated retrieval of semantically related words in free-associative chains, which was not due to a non-specific acceleration of processing speed. The stimulation had no influence on controlled word retrieval that employed inhibition or switching. The effect of cathodal tDCS was opposite to the anodal stimulation, but statistically non-significant. Our data show that the cerebellum is engaged extracting associative information from the system of semantic representations, established and strengthened/automated by learning, and indicates a domain-general role of this structure in automation of behavior, cognition and language.

A Systematic Review of tACS Effects on Cognitive Functioning in Older Adults Across the Healthy to Dementia Spectrum

Transcranial alternating current stimulation (tACS) is a form of noninvasive brain stimulation that has experienced rapid growth within the aging population over the past decade due to its potential for modulating cognitive functioning across the "intact" to dementia spectrum. For this reason, we performed a systematic review of the literature to evaluate the efficacy of tACS on cognitive functioning in older adults, including those with cognitive impairment. Our review was completed in June 2023 using Psych INFO, Embase, PubMed, and Cochrane databases. Out of 479 screened articles, 21 met inclusion criteria and were organized according to clinical diagnoses. Seven out of nine studies targeted cognitively intact older adults and showed some type of cognitive improvement after stimulation, whereas nine out of twelve studies targeted clinical diagnoses and showed improved cognitive performance to varying degrees. Studies showed considerable heterogeneity in methodology, stimulation parameters, participant characteristics, choice of cognitive task, and analytic strategy, all of which reinforce the need for standardized reporting of tACS methods. Through this heterogeneity, multiple patterns are described, such as disease progression influencing tACS effects and the need for individualized tailoring. For clinical translation, it is imperative that the field (a) better understand the physiological effects of tACS in these populations, especially in respect to biomarkers, (b) document a causal relationship between tACS delivery and neurophysiological/cognitive effects, and (c) systematically establish dosing parameters (e.g., amplitude, stimulation frequency, number and duration of sessions, need for booster/maintenance sessions).

Effect of transcranial direct current stimulation on learning in older adults with and without Parkinson's disease: A systematic review with meta-analysis

Older adults with and without Parkinson's disease show impaired retention after training of motor or cognitive skills. This systematic review with meta-analysis aims to investigate whether adding transcranial direct current stimulation (tDCS) to motor or cognitive training versus placebo boosts motor sequence and working memory training. The effects of interest were estimated between three time points, i.e. pre-training, post-training and follow-up. This review was conducted according to the PRISMA guidelines (PROSPERO: CRD42022348885). Electronic databases were searched from conception to March 2023. Following initial screening, 24 studies were eligible for inclusion in the qualitative synthesis and 20 could be included in the meta-analysis, of which 5 studies concerned motor sequence learning (total n = 186) and 15 working memory training (total n = 650). Results were pooled using an inverse variance random effects meta-analysis. The findings showed no statistically significant additional effects of tDCS over placebo on motor sequence learning outcomes. However, there was a strong trend showing that tDCS boosted working memory training, although methodological limitations and some heterogeneity were also apparent. In conclusion, the present findings do not support wide implementation of tDCS as an add-on to motor sequence training at the moment, but the promising results on cognitive training warrant further investigations.

The moderating effects of sex, age, and education on the outcome of combined cognitive training and transcranial electrical stimulation in older adults

Computerized cognitive training (CCT) has been shown to improve cognition in older adults via targeted exercises for single or multiple cognitive domains. Combining CCT with non-invasive brain stimulation is thought to be even more effective due to synergistic effects in the targeted brain areas and networks. However, little is known about the moderating effects of sex, age, and education on cognitive outcomes. Here, we investigated these factors in a randomized, double-blind study in which we administered CCT either combined with transcranial direct (tDCS), alternating (tACS) current stimulation or sham stimulation. 59 healthy older participants (mean age 71.7 ± 6.1) received either tDCS (2 mA), tACS (5 Hz), or sham stimulation over the left dorsolateral prefrontal cortex during the first 20 min of a CCT (10 sessions, 50 min, twice weekly). Before and after the complete cognitive intervention, a neuropsychological assessment was performed, and the test scores were summarized in a composite score. Our results showed a significant three-way interaction between age, years of education, and stimulation technique (F(6,52) = 5.53, p = 0.007), indicating that the oldest participants with more years of education particularly benefitted from tDCS compared to the sham group, while in the tACS group the youngest participants with less years of education benefit more from the stimulation. These results emphasize the importance of further investigating and taking into account sex, age, and education as moderating factors in the development of individualized stimulation protocols.

Clinical Trial Registration

ClinicalTrials.gov, identifier NCT03475446.

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.

Identifying factors influencing cognitive outcomes after anodal transcranial direct current stimulation in older adults with and without cognitive impairment: A systematic review

Anodal transcranial direct current stimulation (tDCS) can improve cognition in healthy older adults, those with Alzheimer's disease (AD) and mild cognitive impairment (MCI), albeit with considerable variability in response. This systematic review identifies interindividual factors that may influence tDCS outcomes in older individuals with or without cognitive impairment. Peer-reviewed articles were included if they assessed whether cognitive outcomes (memory or global cognition) after tDCS were associated with pre-intervention factors in healthy older adults or individuals with AD/MCI. We identified eight factors that may affect cognitive outcomes after tDCS. Improved tDCS outcomes were predicted by lower baseline cognitive function when tDCS was combined with a co-intervention (but not when used alone). Preserved brain structure and better baseline functional connectivity, genetic polymorphisms, and the use of concomitant medications may predict better tDCS outcomes, but further research is warranted. tDCS outcomes were not consistently associated with age, cognitive reserve, sex, and AD risk factors. Accounting for individual differences in baseline cognition, particularly for combined interventions, may thus maximize the therapeutic potential of tDCS.

Working memory ımprovement after transcranial direct current stimulation paired with working memory training ın diabetic peripheral neuropathy

Association of cognitive deficits and diabetic peripheral neuropathy (DPN) is frequent. Working memory (WM) deficits result in impairment of daily activities, diminished functionality, and treatment compliance. Mounting evidence suggests that transcranial Direct Current Stimulation (tDCS) with concurrent working memory training (WMT) ameliorates cognitive deficits. Emboldening results of tDCS were shown in DPN. The study aimed to evaluate the efficacy of anodal tDCS over the left dorsolateral prefrontal cortex (DLPFC) coupled with cathodal right DLPFC with concurrent WMT in DPN for the first time. The present randomized triple-blind parallel-group sham-controlled study evaluated the efficacy of 5 sessions of tDCS over the DLPFC concurrent with WMT in 28 individuals with painful DPN on cognitive (primary) and pain-related, psychiatric outcome measures before, immediately after, and 1-month after treatment protocol. tDCS enhanced the efficacy of WMT on working memory and yielded lower anxiety levels than sham tDCS but efficacy was not superior to sham on other cognitive domains, pain severity, quality of life, and depression. tDCS with concurrent WMT enhanced WM and ameliorated anxiety in DPN without affecting other cognitive and pain-related outcomes. Further research scrutinizing the short/long-term efficacy with larger samples is accredited.

Broadened assessments, health education and cognitive aids in the remote memory clinic

The prevalence of dementia is increasing and poses a health challenge for individuals and society. Despite the desire to know their risks and the importance of initiating early therapeutic options, large parts of the population do not get access to memory clinic-based assessments. Remote memory clinics facilitate low-level access to cognitive assessments by eschewing the need for face-to-face meetings. At the same time, patients with detected impairment or increased risk can receive non-pharmacological treatment remotely. Sensor technology can evaluate the efficiency of this remote treatment and identify cognitive decline. With remote and (partly) automatized technology the process of cognitive decline can be monitored but more importantly also modified by guiding early interventions and a dementia preventative lifestyle. We highlight how sensor technology aids the expansion of assessments beyond cognition and to other domains, e.g., depression. We also illustrate applications for aiding remote treatment and describe how remote tools can facilitate health education which is the cornerstone for long-lasting lifestyle changes. Tools such as transcranial electric stimulation or sleep-based interventions have currently mostly been used in a face-to-face context but have the potential of remote deployment-a step already taken with memory training apps. Many of the presented methods are readily scalable and of low costs and there is a range of target populations, from the worried well to late-stage dementia.

Transcranial alternating current stimulation and its effects on cognition and the treatment of psychiatric disorders: a systematic review and meta-analysis

BACKGROUND

Transcranial alternating current stimulation (TACS) is a non-invasive method of brain stimulation that is hypothesised to alter cortical excitability and brain electrical activity, modulating functional connectivity within the brain. Several trials have demonstrated its potential in treating psychiatric disorders such as depression and schizophrenia.

OBJECTIVES

To study the efficacy of TACS in ameliorating symptoms of depression and schizophrenia in patients and its effects on cognition in patients and healthy subjects compared to sham stimulation.

DESIGN

Systematic review with meta-analysis.

DATA SOURCES AND METHODS

This PROSPERO-registered systematic review (CRD42022331149) is reported according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. PubMed, EMBASE, CENTRAL and PsycINFO were searched from inception to March 2022. Only randomised-controlled trials were included.

RESULTS

A total of 12 randomised-controlled trials are reviewed for meta-analysis, with three randomised-controlled trials reporting only effects on cognition in psychiatric and cognitively impaired patients, three trials on cognition in healthy subjects, one trial on cognition in both patients and healthy subjects, one trial on only depression, two on both cognition and depression in patients and two on schizophrenia symptoms. No studies were at significant risk of bias. For cognition, TACS showed significant improvement [positive standardised mean differences (SMD) denoting improvement] over sham stimulation in those with psychiatric disorders with an SMD of 0.60 (95% confidence interval [CI]: 0.14, 1.06). Similarly, among patients with depression, an SMD of 1.14 (95% CI: 0.10, 2.18) was found significantly favouring TACS over sham stimulation. Two studies assessed the effect of TACS on schizophrenia symptoms with mixed results.

CONCLUSION

TACS has shown promise in ameliorating symptoms of both schizophrenia and depression in patients. TACS also improves cognition in both patients and healthy subjects. However, these findings are limited by the sample size of included studies, and future studies may be required to better our understanding of the potential of TACS.

REGISTRATION

PROSPERO (CRD42022331149).

The effects of aerobic exercise and transcranial direct current stimulation on cognitive function in older adults with and without cognitive impairment: A systematic review and meta-analysis

BACKGROUND

Aerobic exercise (AE) may slow age-related cognitive decline. However, such cognition-sparing effects are not uniform across cognitive domains and studies. Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation and is also emerging as a potential alternative to pharmaceutical therapies. Like AE, the effectiveness of tDCS is also inconsistent for reducing cognitive impairment in ageing. The unexplored possibility exists that pairing AE and tDCS could produce synergistic effects and reciprocally augment cognition-improving effects in older individuals with and without cognitive impairments. Previous research found such synergistic effects on cognition when cognitive training is paired with tDCS in older individuals with and without mild cognitive impairment (MCI) or dementia.

AIM

The purpose of this systematic review with meta-analysis was to explore if pairing AE with tDCS could augment singular effects of AE and tDCS on global cognition (GC), working memory (WM) and executive function (EF) in older individuals with or without MCI and dementia.

METHODS

Using a PRISMA-based systematic review, we compiled studies that examined the effects of AE alone, tDCS alone, and AE and tDCS combined on cognitive function in older individuals with and without mild cognitive impairment (MCI) or dementia. Using a PICOS approach, we systematically searched PubMed, Scopus and Web of Science searches up to December 2021, we focused on 'MoCA', 'MMSE', 'Mini-Cog' (measures) and 'cognition', 'cognitive function', 'cognitive', 'cognitive performance', 'executive function', 'executive process', 'attention', 'memory', 'memory performance' (outcome terms). We included only randomized controlled trials (RTC) in humans if available in English full text over the past 20 years, with participants' age over 60. We assessed the methodological quality of the included studies (RTC) by the Physiotherapy Evidence Database (PEDro) scale.

RESULTS

Overall, 68 studies were included in the meta-analyses. AE (ES = 0.56 [95% CI: 0.28-0.83], p = 0.01) and tDCS (ES = 0.69 [95% CI: 0.12-1.26], p = 0.02) improved GC in all three groups of older adults combined (healthy, MCI, demented). In healthy population, AE improved GC (ES = 0.46 [95% CI: 0.22-0.69], p = 0.01) and EF (ES = 0.27 [95% CI: 0.05-0.49], p = 0.02). AE improved GC in older adults with MCI (ES = 0.76 [95% CI: 0.21-1.32], p = 0.01). tDCS improved GC (ES = 0.69 [90% CI: 0.12-1.26], p = 0.02), all three cognitive function (GC, WM and EF) combined in older adults with dementia (ES = 1.12 [95% CI: 0.04-2.19], p = 0.04) and improved cognitive function in older adults overall (ES = 0.69 [95% CI: 0.20-1,18], p = 0.01).

CONCLUSION

Our systematic review with meta-analysis provided evidence that beyond the cardiovascular and fitness benefits of AE, pairing AE with tDCS may have the potential to slow symptom progression of cognitive decline in MCI and dementia. Future studies will examine the hypothesis of this present review that a potentiating effect would incrementally improve cognition with increasing severity of cognitive impairment.

Older adults with lower working memory capacity benefit from transcranial direct current stimulation when combined with working memory training: A preliminary study

Aging is a very diverse process: successful agers retain most cognitive functioning, while others experience mild to severe cognitive decline. This decline may eventually negatively impact one’s everyday activities. Therefore, scientists must develop approaches to counteract or, at least, slow down the negative change in cognitive performance of aging individuals. Combining cognitive training and transcranial direct current stimulation (tDCS) is a promising approach that capitalizes on the plasticity of brain networks. However, the efficacy of combined methods depends on individual characteristics, such as the cognitive and emotional state of the individual entering the training program. In this report, we explored the effectiveness of working memory training, combined with tDCS to the right dorsolateral prefrontal cortex (DLPFC), to manipulate working memory performance in older individuals. We hypothesized that individuals with lower working memory capacity would benefit the most from the combined regimen. Thirty older adults took part in a 5-day combined regimen. Before and after the training, we evaluated participants’ working memory performance with five working memory tasks. We found that individual characteristics influenced the outcome of combined cognitive training and tDCS regimens, with the intervention selectively benefiting old-old adults with lower working memory capacity. Future work should consider developing individualized treatments by considering individual differences in cognitive profiles.

Strategies to Promote Cognitive Health in Aging: Recent Evidence and Innovations

PURPOSE OF REVIEW

We review recent work on applications of non-pharmacologic strategies to promote cognitive health in older adulthood and discuss potential network mechanisms, limitations, and considerations for improving intervention uptake and efficacy.

RECENT FINDINGS

In healthy older adults and patients with mild cognitive impairment, cognitive training produces global and domain-specific cognitive gains, though effect sizes tend to be modest and transfer is variable. Non-invasive brain stimulation has shown moderate success in enhancing cognitive function, though the optimum approach, parameters, and cortical targets require further investigation. Physical activity improves cognitive functions in late life, with emerging trials highlighting key intervention components that may maximize treatment outcomes. Multimodal interventions may be superior to single-component interventions in conferring cognitive gains, although interpretation is limited by modest sample sizes and variability in training components and parameters. Across modalities, individual differences in patient characteristics predict therapeutic response. These interventions may advance cognitive health by modulating functional networks that support core cognitive abilities including the default mode, executive control, and salience networks. Effectiveness of cognitive enhancement strategies may be increased with clinician-led coaching, booster sessions, gamification, integration of multiple intervention modalities, and concrete applications to everyday functioning. Future trials involving rigorous comparisons of training components, parameters, and delivery formats will be essential in establishing the precise approaches needed to maximize cognitive outcomes. Novel studies using patient-level clinical and neuroimaging features to predict individual differences in training gains may inform the development of personalized intervention prescriptions to optimize cognitive health in late life.

Can a serious game-based cognitive training attenuate cognitive decline related to Alzheimer's disease? Protocol for a randomized controlled trial

BACKGROUND

Alzheimer's disease (AD) is a major public health issue. Cognitive interventions such as computerized cognitive trainings (CCT) are effective in attenuating cognitive decline in AD. However, in those at risk of dementia related to AD, results are heterogeneous. Efficacy and feasibility of CCT needs to be explored in depth. Moreover, underlying mechanisms of CCT effects on the three cognitive domains typically affected by AD (episodic memory, semantic memory and spatial abilities) remain poorly understood.

METHODS

In this bi-centric, randomized controlled trial (RCT) with parallel groups, participants (planned N = 162, aged 60-85 years) at risk for AD and with at least subjective cognitive decline will be randomized to one of three groups. We will compare serious game-based CCT against a passive wait list control condition and an active control condition (watching documentaries). Training will consist of daily at-home sessions for 10 weeks (50 sessions) and weekly on-site group meetings. Subsequently, the CCT group will continue at-home training for an additional twenty-weeks including monthly on-site booster sessions. Investigators conducting the cognitive assessments will be blinded. Group leaders will be aware of participants' group allocations. Primarily, we will evaluate change using a compound value derived from the comprehensive cognitive assessment for each of three cognitive domains. Secondary, longitudinal functional and structural magnetic resonance imaging (MRI) and evaluation of blood-based biomarkers will serve to investigate neuronal underpinnings of expected training benefits.

DISCUSSION

The present study will address several shortcomings of previous CCT studies. This entails a comparison of serious game-based CCT with both a passive and an active control condition while including social elements crucial for training success and adherence, the combination of at-home and on-site training, inclusion of booster sessions and assessment of physiological markers. Study outcomes will provide information on feasibility and efficacy of serious game-based CCT in older adults at risk for AD and will potentially generalize to treatment guidelines. Moreover, we set out to investigate physiological underpinnings of CCT induced neuronal changes to form the grounds for future individually tailored interventions and neuro-biologically informed trainings.

TRIAL REGISTRATION

This RCT was registered 1st of July 2020 at clinicaltrials.gov (Identifier NCT04452864).

Non-invasive brain stimulation and neuroenhancement

Attempts to enhance human memory and learning ability have a long tradition in science. This topic has recently gained substantial attention because of the increasing percentage of older individuals worldwide and the predicted rise of age-associated cognitive decline in brain functions. Transcranial brain stimulation methods, such as transcranial magnetic (TMS) and transcranial electric (tES) stimulation, have been extensively used in an effort to improve cognitive functions in humans. Here we summarize the available data on low-intensity tES for this purpose, in comparison to repetitive TMS and some pharmacological agents, such as caffeine and nicotine. There is no single area in the brain stimulation field in which only positive outcomes have been reported. For self-directed tES devices, how to restrict variability with regard to efficacy is an essential aspect of device design and function. As with any technique, reproducible outcomes depend on the equipment and how well this is matched to the experience and skill of the operator. For self-administered non-invasive brain stimulation, this requires device designs that rigorously incorporate human operator factors. The wide parameter space of non-invasive brain stimulation, including dose (e.g., duration, intensity (current density), number of repetitions), inclusion/exclusion (e.g., subject's age), and homeostatic effects, administration of tasks before and during stimulation, and, most importantly, placebo or nocebo effects, have to be taken into account. The outcomes of stimulation are expected to depend on these parameters and should be strictly controlled. The consensus among experts is that low-intensity tES is safe as long as tested and accepted protocols (including, for example, dose, inclusion/exclusion) are followed and devices are used which follow established engineering risk-management procedures. Devices and protocols that allow stimulation outside these parameters cannot claim to be "safe" where they are applying stimulation beyond that examined in published studies that also investigated potential side effects. Brain stimulation devices marketed for consumer use are distinct from medical devices because they do not make medical claims and are therefore not necessarily subject to the same level of regulation as medical devices (i.e., by government agencies tasked with regulating medical devices). Manufacturers must follow ethical and best practices in marketing tES stimulators, including not misleading users by referencing effects from human trials using devices and protocols not similar to theirs.

Effects of Multisession Prefrontal Transcranial Direct Current Stimulation on Long-term Memory and Working Memory in Older Adults

Transcranial direct current stimulation (tDCS) is a noninvasive form of electrical brain stimulation popularly used to augment the effects of working memory (WM) training. Although success has been mixed, some studies report enhancements in WM performance persisting days, weeks, or even months that are actually more reminiscent of consolidation effects typically observed in the long-term memory (LTM) domain, rather than WM improvements per se. Although tDCS has been often reported to enhance both WM and LTM, these effects have never been directly compared within the same study. However, given their considerable neural and behavioral overlap, this is a timely comparison to make. This study reports results from a multisession intervention in older adults comparing active and sham tDCS over the left dorsolateral pFC during training on both an n-back WM task and a word learning LTM task. We found strong and robust effects on LTM, but mixed effects on WM that only emerged for those with lower baseline ability. Importantly, mediation analyses showed an indirect effect of tDCS on WM that was mediated by improvements in consolidation. We conclude that tDCS over the left dorsolateral pFC can be used as an effective intervention to foster long-term learning and memory consolidation in aging, which can manifest in performance improvements across multiple memory domains.

Event-Related Potentials as Markers of Efficacy for Combined Working Memory Training and Transcranial Direct Current Stimulation Regimens: A Proof-of-Concept Study

Our brains are often under pressure to process a continuous flow of information in a short time, therefore facing a constantly increasing demand for cognitive resources. Recent studies have highlighted that a lasting improvement of cognitive functions may be achieved by exploiting plasticity, i.e., the brain’s ability to adapt to the ever-changing cognitive demands imposed by the environment. Transcranial direct current stimulation (tDCS), when combined with cognitive training, can promote plasticity, amplify training gains and their maintenance over time. The availability of low-cost wearable devices has made these approaches more feasible, albeit the effectiveness of combined training regimens is still unclear. To quantify the effectiveness of such protocols, many researchers have focused on behavioral measures such as accuracy or reaction time. These variables only return a global, non-specific picture of the underlying cognitive process. Electrophysiology instead has the finer grained resolution required to shed new light on the time course of the events underpinning processes critical to cognitive control, and if and how these processes are modulated by concurrent tDCS. To the best of our knowledge, research in this direction is still very limited. We investigate the electrophysiological correlates of combined 3-day working memory training and non-invasive brain stimulation in young adults. We focus on event-related potentials (ERPs), instead of other features such as oscillations or connectivity, because components can be measured on as little as one electrode. ERP components are, therefore, well suited for use with home devices, usually equipped with a limited number of recording channels. We consider short-, mid-, and long-latency components typically elicited by working memory tasks and assess if and how the amplitude of these components are modulated by the combined training regimen. We found no significant effects of tDCS either behaviorally or in brain activity, as measured by ERPs. We concluded that either tDCS was ineffective (because of the specific protocol or the sample under consideration, i.e., young adults) or brain-related changes, if present, were too subtle. Therefore, we suggest that other measures of brain activity may be more appropriate/sensitive to training- and/or tDCS-induced modulations, such as network connectivity, especially in young adults.

Working Memory Training Coupled With Transcranial Direct Current Stimulation in Older Adults: A Randomized Controlled Experiment

Background

Transcranial direct current stimulation (tDCS) has been employed to boost working memory training (WMT) effects. Nevertheless, there is limited evidence on the efficacy of this combination in older adults. The present study is aimed to assess the delayed transfer effects of tDCS coupled with WMT in older adults in a 15-day follow-up. We explored if general cognitive ability, age, and educational level predicted the effects.

Methods

In this single-center, double-blind randomized sham-controlled experiment, 54 older adults were randomized into three groups: anodal-tDCS (atDCS)+WMT, sham-tDCS (stDCS)+WMT, and double-sham. Five sessions of tDCS (2 mA) were applied over the left dorsolateral prefrontal cortex (DLPFC). Far transfer was measured by Raven’s Advanced Progressive Matrices (RAPM), while the near transfer effects were assessed through Digit Span. A frequentist linear mixed model (LMM) was complemented by a Bayesian approach in data analysis.

Results

Working memory training improved dual n-back performance in both groups submitted to this intervention but only the group that received atDCS+WMT displayed a significant improvement from pretest to follow-up in transfer measures of reasoning (RAPM) and short-term memory (forward Digit Span). Near transfer improvements predicted gains in far transfer, demonstrating that the far transfer is due to an improvement in the trained construct of working memory. Age, formal education, and vocabulary score seem to predict the gains in reasoning. However, Bayesian results do not provide substantial evidence to support this claim.

Conclusion

This study will help to consolidate the incipient but auspicious field of cognitive training coupled with tDCS in healthy older adults. Our findings demonstrated that atDCS may potentialize WMT by promoting transfer effects in short-term memory and reasoning in older adults, which are observed especially at follow-up.

The Effects of Electroencephalogram Feature-Based Transcranial Alternating Current Stimulation on Working Memory and Electrophysiology

Transcranial alternating current stimulation (tACS) can influence cognitive functions by modulating brain oscillations. However, results regarding the effectiveness of tACS in regulating cognitive performance have been inconsistent. In the present study, we aimed to find electroencephalogram (EEG) characteristics associated with the improvements in working memory performance, to select tACS stimulus targets and frequency based on this feature, and to explore effects of selected stimulus on verbal working memory. To achieve this goal, we first investigated the EEG characteristics associated with improvements in working memory performance with the aid of EEG analyses and machine learning techniques. These analyses suggested that 8 Hz activity in the prefrontal region was related to accuracy in the verbal working memory task. The tACS stimulus target and pattern were then selected based on the EEG feature. Finally, the selected tACS frequency (8 Hz tACS in the prefrontal region) was applied to modulate working memory. Such modulation resulted significantly greater improvements, compared with 40 Hz and sham modulations (especially for participants with weak verbal working memory). In conclusion, using EEG features related to positive behavioral changes to select brain regions and stimulation patterns for tACS is an effective intervention for improving working memory. Our results contribute to the groundwork for future tACS closed-loop interventions for cognitive deterioration.