Cognitive Effects of Transcranial Direct Current Stimulation in Healthy and Clinical Populations: An Overview

Efficacy and acceptability of transcranial direct current stimulation (tDCS) for major depressive disorder: An individual patient data meta-analysis

We evaluated the efficacy and acceptability of transcranial direct current stimulation (tDCS) for treating acute depressive episodes using individual patient data that provide more precise estimates than aggregate data meta-analysis. A systematic review of placebo-controlled trials on tDCS as only intervention was conducted until December-2018. Data from each study was collated to estimate odds ratio (OR) and number needed to treat (NNT) of response and remission, and depression improvement. Endpoints were pre-determined. Nine eligible studies (572 participants), presenting moderate/high certainty of evidence, were included. Active tDCS was significantly superior to sham for response (30.9% vs. 18.9% respectively; OR = 1.96, 95%CI [1.30-2.95], NNT = 9), remission (19.9% vs. 11.7%, OR = 1.94 [1.19-3.16], NNT = 13) and depression improvement (effect size of β = 0.31, [0.15-0.47]). Moreover, continuous clinical improvement was observed even after the end of acute tDCS treatment. There were no differences in all-cause discontinuation rates and no predictors of response were identified. To conclude, active tDCS was statistically superior to sham in all outcomes, although its clinical effects were moderate.

Increased leg muscle fatigability during 2 mA and 4 mA transcranial direct current stimulation over the left motor cortex

Transcranial direct current stimulation (tDCS) using intensities ≤ 2 mA on physical and cognitive outcomes has been extensively investigated. Studies comparing the effects of different intensities of tDCS have yielded mixed results and little is known about how higher intensities (> 2 mA) affect outcomes. This study examined the effects of tDCS at 2 mA and 4 mA on leg muscle fatigability. This was a double-blind, randomized, sham-controlled study. Sixteen healthy young adults underwent tDCS at three randomly ordered intensities (sham, 2 mA, 4 mA). Leg muscle fatigability of both legs was assessed via isokinetic fatigue testing (40 maximal reps, 120°/s). Torque- and work-derived fatigue indices (FI-T and FI-W, respectively), as well as total work performed (TW), were calculated. FI-T of the right knee extensors indicated increased fatigability in 2 mA and 4 mA compared with sham (p = 0.01, d = 0.73 and p < 0.001, d = 1.61, respectively). FI-W of the right knee extensors also indicated increased fatigability in 2 mA and 4 mA compared to sham (p = 0.01, d = 0.57 and p < 0.001, d = 1.12, respectively) and 4 mA compared with 2 mA (p = 0.034, d = 0.37). tDCS intensity did not affect TW performed. The 2 mA and 4 mA tDCS intensities increased the fatigability of the right knee extensors in young, healthy participants, potentially from altered motor unit recruitment/discharge rate or cortical hyperexcitability. Despite this increase in fatigability, the TW performed in both these conditions was not different from sham.

The Tolerability and Efficacy of 4 mA Transcranial Direct Current Stimulation on Leg Muscle Fatigability

Transcranial direct current stimulation (tDCS) modulates cortical excitability and affects a variety of outcomes. tDCS at intensities ≤2 mA is well-tolerated, but the tolerability and efficacy of tDCS at intensities >2 mA merits systematic investigation. The study objective was to determine the tolerability and effects of 4 mA tDCS on leg muscle fatigability. Thirty-one young, healthy adults underwent two randomly ordered tDCS conditions (sham, 4 mA) applied before and during an isokinetic fatigue test of the knee extensors and flexors. Subjects reported the severity of the sensations felt from tDCS. Primary outcomes were sensation tolerability and the fatigue index of the knee extensors and flexors. A repeated-measures ANOVA determined statistical significance (p < 0.05). Sensation severity at 4 mA tDCS was not substantially different than sham. However, two subjects reported a moderate–severe headache, which dissipated soon after the stimulation ended. The left knee flexors had significantly greater fatigability with 4 mA tDCS compared with sham (p = 0.018). tDCS at 4 mA was well-tolerated by young, healthy subjects and increased left knee flexor fatigability. Exploration of higher intensity tDCS (>2 mA) to determine the potential benefits of increasing intensity, especially in clinical populations with decreased brain activity/excitability, is warranted.

Exploring the effects of Transcranial Direct Current Stimulation over the prefrontal cortex on working memory: A cluster analysis approach

BACKGROUND

The interest in the use of anodal transcranial direct current stimulation (tDCS) for the enhancement of cognitive functioning has increased significantly in recent years. However, the efficacy of this technique remains to be established.

OBJECTIVE

The current study explored the effects of anodal vs. sham tDCS over the left dorsolateral prefrontal cortex (DLPFC) during the performance of the digit span backwards task.

METHODS

30 healthy participants received 'offline' anodal tDCS (1.5 mA, 15 min) to the left DLPFC in an intra-individual, cross-over, sham-controlled experimental design. Memory span performance was assessed before (baseline), immediately after tDCS administration (T1) and 10 min post-T1 (T2). We applied cluster analysis in order to characterize individual responses to tDCS, and in order to identify naturally occurring subgroups that may be present.

RESULTS

Analysis of all the subjects showed that anodal tDCS failed to improve memory span performance. Cluster analysis revealed the presence of a subgroup of 'responders' that significantly improved their performance after anodal (vs. sham) tDCS in T1 (47%) and T2 (46%). However, there was no significant improvement in performance after anodal tDCS compared to the best baseline performance.

CONCLUSION

Our findings suggest that tDCS does not improve memory span performance and highlights the need for better ways to optimize methodological approaches in order to account for inter-individual variability and accurately assess the evidential value of tDCS-linked cognitive outcomes.

Effects of High-Definition Transcranial Direct Current Stimulation and Theta Burst Stimulation for Modulating the Posterior Parietal Cortex

OBJECTIVES

Noninvasive brain stimulation methods, including high-definition transcranial direct current stimulation (HD-tDCS) and theta burst stimulation (TBS) have emerged as novel tools to modulate and explore brain function. However, the relative efficacy of these newer stimulation approaches for modulating cognitive functioning remains unclear. This study investigated the cognitive effects of HD-tDCS, intermittent TBS (iTBS) and prolonged continuous TBS (ProcTBS) and explored the potential of these approaches for modulating hypothesized functions of the left posterior parietal cortex (PPC).

METHODS

Twenty-two healthy volunteers attended four experimental sessions in a cross-over experimental design. In each session, participants either received HD-tDCS, iTBS, ProcTBS or sham, and completed cognitive tasks, including a divided attention task, a working memory maintenance task and an attention task (emotional Stroop test).

RESULTS

The results showed that compared to sham, HD-tDCS, iTBS and ProcTBS caused significantly faster response times on the emotional Stroop task. The effect size (Cohen's d) was d = .32 for iTBS (p < .001), .21 for ProcTBS (p = .01) and .15 for HD-tDCS (p = .044). However, for the performance on the divided attention and working memory maintenance tasks, no significant effect of stimulation was found.

CONCLUSIONS

The results suggest that repetitive transcranial magnetic stimulation techniques, including TBS, may have greater efficacy for modulating cognition compared with HD-tDCS, and extend existing knowledge about specific functions of the left PPC.

Replacing tDCS with theta tACS provides selective, but not general WM benefits

Working memory (WM) can be improved after repeated training sessions paired with noninvasive neurostimulation techniques. Previously, we reported that WM training paired with tDCS succeeded behaviorally by enhancing anterior-posterior theta phase coherence and reducing alpha power. Here, in two experiments we tested several theta and alpha frequencies and two transcranial alternating current stimulation (tACS) montages in an effort to shortcut WM training while preserving behavioral gains. In Experiment 1, in separate sessions participants received online tACS at two frequencies derived from the previous study with the respective goal of improving and impairing WM performance. We selected the mean group peak value theta (7 Hz) to benefit WM and alpha (11 Hz) to impair WM. Stimulation (tACS) over right frontoparietal sites (F4-P4) during 3-back WM tasks (object, spatial) produced no behavioral consequences. In Experiment 2 we stimulated at a slower theta frequency (4.5 Hz), which was also significant in our prior study, and tested whether frontoparietal or bifrontal montages would be more effective at improving WM. This experiment revealed selectively improved object WM after right frontoparietal tACS alone. In summary, one session of tACS failed to produce the magnitude or breadth of WM gains observed after 4-10 tDCS-WM training sessions. In short, despite looking for loopholes we found little tACS savings.

Transcranial alternating current stimulation entrains single-neuron activity in the primate brain

Spike timing is thought to play a critical role in neural computation and communication. Methods for adjusting spike timing are therefore of great interest to researchers and clinicians alike. Transcranial electrical stimulation (tES) is a noninvasive technique that uses weak electric fields to manipulate brain activity. Early results have suggested that this technique can improve subjects' behavioral performance on a wide range of tasks and ameliorate some clinical conditions. Nevertheless, considerable skepticism remains about its efficacy, especially because the electric fields reaching the brain during tES are small, whereas the likelihood of indirect effects is large. Our understanding of its effects in humans is largely based on extrapolations from simple model systems and indirect measures of neural activity. As a result, fundamental questions remain about whether and how tES can influence neuronal activity in the human brain. Here, we demonstrate that tES, as typically applied to humans, affects the firing patterns of individual neurons in alert nonhuman primates, which are the best available animal model for the human brain. Specifically, tES consistently influences the timing, but not the rate, of spiking activity within the targeted brain region. Such effects are frequency- and location-specific and can reach deep brain structures; control experiments show that they cannot be explained by sensory stimulation or other indirect influences. These data thus provide a strong mechanistic rationale for the use of tES in humans and will help guide the development of future tES applications.

Can Transcranial Electrical Stimulation Localize Brain Function?

Transcranial electrical stimulation (TES) uses constant (TDCS) or alternating currents (TACS) to modulate brain activity. Most TES studies apply low-intensity currents through scalp electrodes (≤2 mA) using bipolar electrode arrangements, producing weak electrical fields in the brain (<1 V/m). Low-intensity TES has been employed in humans to induce changes in task performance during or after stimulation. In analogy to focal transcranial magnetic stimulation, TES-induced behavioral effects have often been taken as evidence for a causal involvement of the brain region underlying one of the two stimulation electrodes, often referred to as the active electrode. Here, we critically review the utility of bipolar low-intensity TES to localize human brain function. We summarize physiological substrates that constitute peripheral targets for TES and may mediate subliminal or overtly perceived peripheral stimulation during TES. We argue that peripheral co-stimulation may contribute to the behavioral effects of TES and should be controlled for by "sham" TES. We discuss biophysical properties of TES, which need to be considered, if one wishes to make realistic assumptions about which brain regions were preferentially targeted by TES. Using results from electric field calculations, we evaluate the validity of different strategies that have been used for selective spatial targeting. Finally, we comment on the challenge of adjusting the dose of TES considering dose-response relationships between the weak tissue currents and the physiological effects in targeted cortical areas. These considerations call for caution when attributing behavioral effects during or after low-intensity TES studies to a specific brain region and may facilitate the selection of best practices for future TES studies.