In conclusion, rTMS is relatively safe. The majority of studies were conducted in relatively small samples, using few session numbers of rTMS, and two out of six studies did not include a sham condition, making it impossible to rule out placebo or practice effects. Based on the conducted studies so far, there is relatively little evidence that several sessions of rTMS improve ADHD symptoms or cognition, with the exception of one study in adults that used multisession rTMS and stimulated right DLPFC and IFC combined with cognitive training, which needs replication. More multisession sham-controlled RCTs in large patient numbers are needed, in particular in paediatric ADHD, to more thoroughly test TMS effects using different protocols.
2.2.2. Transcranial Direct Current Stimulation (tDCS)
tDCS applies a weak continuous direct electric current to underlying brain regions via scalp electrodes with the electrical current passing between a positively charged anode and a negatively charged cathode. In general, currents induce plasticity by causing subthreshold polarity-dependent increases (anodal stimulation) or decreases (cathodal stimulation) in membrane potentials that modify spontaneous discharge rates and cortical excitability, thus increasing/decreasing cortical function and synaptic strength [
72] (Ashkan et al., 2013). tDCS is much easier to apply and has lower financial costs than TMS. Furthermore, tDCS has the advantage of being less painful than TMS and hence is more suitable for paediatric applications. Side effects are minimal in children (and adults), typically involving transient itching and reddening of the scalp site of stimulation in some participants (Krishnan et al., 2015; Zewdie et al., 2020) [
148,
149].
Combining cognitive training with tDCS over a cortical area that mediates the cognitive function being trained [
134] (Kuo & Nitsche, 2012) has been shown to yield larger and long-lasting functional improvements that modify the impaired system [
136] (Cramer et al., 2011), presumably via a synergistic effect of training-induced and stimulation-induced plasticity [
135] (Ziemann & Siebner, 2008). Combined effects of cognitive training with tDCS in other disorders and healthy subjects have been shown to last up to 6 months [
150,
151] (Boggio et al., 2012; Kuo et al., 2014) and 1 year (Katz et al., 2017) [
125].
Functional neuroimaging studies have furthermore demonstrated modulation not only of the stimulation site but also of functionally interconnected (sub)cortical regions [
152] (Polania et al., 2011), which makes them useful for targeting networks such as fronto-striatal systems in ADHD. Furthermore, relevant to ADHD, striatal dopamine [
153] (Pogarell et al., 2007) and noradrenaline [
131,
154] (Kuo et al., 2017; Mishima et al., 2019) have been implicated in the mechanism of action, both of which are typically reduced in ADHD (Cortese et al., 2018) [
13].
Unlike with rTMS, the majority of tDCS studies (12 out of 18) (see Table 2) have been conducted in children with ADHD, presumably due to the high tolerability and relatively low side effect profile of tDCS, which would make it a good treatment option if efficacious. The majority of studies used very small session numbers and tested cognitive effects only (see Table 2).
Two double-blind, sham-controlled, crossover studies applied single session stimulation over the DLPFC. In 15 adolescents with ADHD, anode-left/cathode-right tDCS over bilateral DLPFC improved WCST completion time, n-back reaction times and Stroop reaction times and commission errors to incongruent trials but had no effect on n-back accuracy or Go/No-Go task performance [
155] (Nejati, Salehinejad et al., 2020). In 10 ADHD adolescents, anodal tDCS over the left dlPFC improved n-back accuracy and reaction times compared to both sham and cathodal tDCS; anodal and cathodal tDCS also improved WCST performance, but anodal tDCS led to greater improvement; cathodal tDCS also improved No-Go accuracy, potentially via interhemispheric inhibition increasing right prefrontal activation [
155] (Nejati, Salehinejad et al., 2020), a region associated with motor response inhibition in children and adults [
156,
157,
158] (Rubia et al., 2013; Rubia et al., 2003; Rubia et al., 2007). This last finding is in line with a single-blind, crossover study in 21 adolescents with ADHD, which found in a subsample of seven participants that, compared to sham, one session of anodal, but not cathodal, tDCS over the right IFC reduced commission errors (trend-level) and reaction time variability in an interference inhibition task (Breitling et al., 2016) [
159].
Two single-blind, sham-controlled crossover studies stimulated left DLPFC or right IFC in 20 high school students with ADHD symptoms that were above cut-off on validated ADHD questionnaires. Single session anodal relative to cathodal tDCS over the left DLPFC improved Go accuracy while cathodal tDCS relative to anodal tDCS and sham improved No-Go accuracy in the Go/No-Go task, but there was no change in Stroop task performance [
160] (Soltaninejad et al., 2019). Anodal tDCS over the rIFC relative to sham improved Go accuracy, but there were no changes in other Go/No-Go or Stroop task measures (Soltaninejad et al., 2015) [
161].
A double-blind, sham-controlled RCT in 50 children with ADHD tested the effects of 15 sessions of 20 min of right IFC anodal tDCS combined with cognitive training in executive function tasks. The study found that both groups improved in clinical symptoms and cognitive functions, but the improvement in the real versus sham tDCS in primary and secondary clinical outcome measures was significantly less pronounced [
162] (Westwood et al., 2021). Groups did not differ in a large battery of executive function cognitive outcome measures [
162] (Westwood et al., 2021) nor in EEG measures within a smaller subsample of data collected from 26 participants only [
163] (Westwood et al., 2021). Furthermore, the real tDCS group had worse adverse effects related to mood, sleep and appetite immediately after stimulation (Westwood et al., 2021) [
163].
A double-blind crossover study applied five daily sessions of anodal or sham tDCS over left DLPFC in 15 adolescents with ADHD, but because of a carry-over and learning effects, only the first sessions were analysed, thus reducing the sample to seven to eight participants per condition [
164] (Soff et al., 2017). Compared to sham, anodal tDCS improved parent-rated inattention and cognitive measures of attention (QbTest; which combines cognitive measures of hyperactivity, impulsiveness and inattention in a hybrid n-back/GNG task) one week but not immediately after the last stimulation session, while cognitive measures of hyperactivity on the QbTest were improved immediately after anodal tDCS and seven days later [
164] (Soff et al., 2017). Analysis of 13 out of the 15 ADHD adolescents after a single session of anodal tDCS relative to sham showed reduced reaction time variability but increased errors on the QbTest, but this analysis included the carryover effect [
165] (Sotnikova et al., 2017).
A double-blind, sham-controlled crossover study found that overnight slow-wave oscillatory anodal tDCS over left and right DLPFC, relative to sham, improved declarative memory in 12 ADHD children [
166] (Prehn-Kristensen et al., 2014), reaction time and its intra-subject variability on Go trials in a Go/No-Go task in 14 ADHD children [
167] (Munz et al., 2015) but had no effects on No-Go accuracy, alertness, digit-span or motor memory.
An open label trial in nine ADHD children found that five daily sessions of anodal tDCS to left DLPFC combined with a picture association cognitive training task reduced errors in attention (omission) and switch-task performance, but did not improve working memory, while parents, with one exception, reported improvements in some of their children’s behaviour [
168] (Bandeira et al., 2016).
In a double-blind crossover study in 14 children and adolescents with ADHD, the right IFC was stimulated with either conventional tDCS, high definition tDCS (HD-tDCS) or sham while performing a working memory task with inhibitory elements which was repeated after stimulation as an outcome measure. HD-tDCS is a 4:1 small electrode array with one electrode encircled by four electrodes of the opposite polarity, which delivers a more spatially restricted and therefore focal stimulation that can reduce side effects from stimulating non-target brain regions. The study found that neither a single session of conventional anodal tDCS nor HD-tDCS over right IFC combined with working memory performance compared to sham had any effect on performance in the n-back task; however, ERP data from 10 participants in ADHD showed elevated N200 and P300 after the two tDCS conditions versus sham and a shift towards the values seen in a healthy control group (Breitling et al., 2020) [
169].
One study applied one session of anodal tDCS over the right inferior (and some superior) parietal lobe in 17 ADHD children in a single-blind crossover study. In line with the role of inferior parietal lobe in orienting attention, anodal relative to sham tDCS improved performance in bottom-up orienting attention but deteriorated selective attention as measured in the Stroop interference reaction time and error effects and had no effect on alerting or top-down executive attention as measured in the shifting attention and Go/No-Go tasks (Salehinejad et al., 2020) [
170].
One recent study tested effects of tDCS on reward-related decision making in ADHD [
171] (Nejati, Sarraj Khorrami, et al., 2020). Twenty children with ADHD received tDCS in three separate sessions with either anodal tDCS over the left DLPFC and cathodal tDCS over right vmPFC; the reversed montage; or sham stimulation. Anodal tDCS over the right vmPFC, coupled with cathodal tDCS over the left DLPFC, reduced risky decision-making in the Balloon Analogue R Task but had no effect on the key impulsiveness outcome measure in the delay discounting task (k mean); it did have an effect on some conditions, but these were not corrected for multiple testing (Nejati, Sarraj Khorrami et al., 2020) [
171].
Another recent study compared the clinical and cognitive effects of tDCS with tRNS in ADHD. Although similar to tDCS in terms of equipment and setup, tRNS applies an alternating current at random frequencies and/or intensities. The mechanisms by which tRNS influences brain activity are less known but are thought to be different than for tDCS [
172] (Fertonani & Miniussi, 2017). The most prevalent explanation for tRNS is stochastic resonance, whereby the introduction of an appropriate level of random noise enhances the output of subthreshold signals; thus, the application of weak electric currents amounts to an introduction of neural noise [
172] (Fertonani & Miniussi, 2017). Information processing at the neuronal level is sensitive to stochastic resonance [
173] (McDonnell & Ward, 2011). The double-blind cross-over study compared five sessions of transcranial random noise stimulation (tRNS) over left DLPFC and right IFC with tDCS of left DLPFC combined with executive function training in 19 children with ADHD. Relative to tDCS, tRNS showed a clinical improvement in ADHD rating scale scores from baseline after treatment and one week later. Cognitively, tRNS compared to tDCS improved working memory, but only processing speed during sustained attention. An exploratory moderation analysis predicted a trend-level larger tRNS effect on the ADHD rating scale for those patients who showed the greatest improvement in working memory. tRNS yielded fewer reports of side effects, in line with the literature on adults showing that tRNS is a more comfortable neurostimulation method than tDCS (Berger et al., 2021) [
174].
Only four studies have been conducted in adults with ADHD. In a double-blind parallel study in 60 adults, anodal tDCS over the left DLPFC compared to sham had no effect in two Go/No-Go tasks or a functional cortical network activity based on EEG recordings in a subsample of 50 patients [
175] (Cosmo et al., 2015). One single-blind crossover study applied a single session of anodal tDCS over the left and right DLPFC in 20 undergraduate students with ADHD, which, compared to sham, improved in hyperactivity measures (i.e., multiple/random responses) in a sustained attention task but had no effect on omission errors or reaction times [
176] (Jacoby et al., 2018). A double-blind crossover study in 37 adults with ADHD administered three sessions of visual working memory training combined with anodal tDCS of the left DLPFC and reported that compared to sham, anodal tDCS reduced commission errors in a sustained attention task immediately but not three days after the last stimulation, while there was no effect on omission errors, reaction times, stop task or visual working memory training performance [
177] (Allenby et al., 2018). [
177] One double-blind parallel study in 17 adults with ADHD found that tDCS of anodal right/cathode-left DLPFC (
n = 9) versus sham (
n = 8) improved inattention but not hyperactivity/impulsive symptoms immediately after five daily sessions of stimulation and at a 2-week follow-up, with total ADHD scores also improving at the 2-week follow-up, although group differences disappeared at the 4-week follow-up (Cachoeira et al., 2017) [
178]. Finally, in a double-blind crossover study in 37 adults with ADHD, participants were asked to perform a Flanker (
n = 18) or a stop task (
n = 19) before and after receiving a single session of anodal tDCS over the left or right DLPFC relative to sham. In the Flanker task, left but not right DLPFC stimulation reduced reaction times on incongruent but not congruent trials compared to sham and right DLPFC stimulation. This was furthermore correlated with increased left and right P300 increase in EEG measures on incongruent trials after left and right DLPFC stimulation compared to sham, respectively and with reduced N200 amplitude after left compared to right DLPFC stimulation. In the stop task, there was no effect in inhibitory measures, but left DLPFC stimulation relative to sham increased Go reaction time, which was correlated with increased P200 amplitude during Go trials (Dubreuil-Vall et al., 2020) [
179].
In conclusion, only 3 out of 17 tDCS studies tested clinical effects. Two studies found that tDCS of left DLPFC improved clinical inattention symptoms while one study foundthat tRNS compared to tDCS improved ADHD symptoms (see Table 2).
With respect to cognition, most studies found effects in the performance of some but not other tasks, with little consistency in findings between studies, and most studies did not correct for multiple testing (see
Table 2). Two meta-analyses tested for consistent findings of tDCS on cognition in ADHD. A meta-analysis of 10 studies (201 children/adults with ADHD) found that one to five sessions of anodal tDCS over mainly left DLPFC significantly improved cognitive performance in inhibitory control measures (Hedges’
g = 0.12) and in n-back reaction times (
g = 0.66) [
180] (Salehinejad et al., 2019). However, effect sizes were small, and the meta-analysis likely overestimated statistical significance by not controlling for interdependency between measures and conflated inhibitory with non-inhibitory cognitive measures [
181] (Westwood et al., 2021). Addressing these and other limitations, a larger meta-analysis of 12 tDCS studies (232 children/adults with ADHD) found that one to five sessions of anodal tDCS over mainly left DLPFC led to small, trend-level significant improvements in cognitive measures of inhibition (
g = 0.21) and of processing speed (
g = 0.14) but not of attention (
g = 0.18) (Westwood et al., 2021) [
181].
In conclusion, the findings of the use of tDCS to improve ADHD symptoms and cognition have been mixed, with some positive results on improving cognition, with, however, very small effects sizes observed in meta-analyses (see also Table 1). However, the comparability of results was hampered by the large heterogeneity in study designs, stimulation parameters and site of anodal and cathodal stimulation. Larger samples and more homogeneously designed studies using a larger number of sessions of localised tDCS with and without cognitive training are needed to more confidently assess clinical and cognitive benefits.
Importantly, for both TMS and tDCS but also tRNS or tACS, systematic testing is needed to identify the optimal stimulation parameters that can elicit reliable clinical or cognitive effects. Parameters that should be tested include optimal stimulation sites, frequency, duration, and superiority of stimulation effects combined with cognitive training. For tDCS, tRNS and tACS, studies should consider if effects depend on age, electrode size and inter-electrode distance, the focality of stimulation and antagonistic effects of cathodal stimulation on the desired effect of the anodal stimulation. Children, for example, have thinner skulls and less corticospinal fluid, which means potentiation of the effects of brain stimulation compared to adults and optimal dosages cannot be easily transferred from adult studies. For example, cathodal tDCS at 1 mA, which has excitability-diminishing effects in adults, has shown to have excitatory effects in children and adolescents when applied over the motor cortex [
182] (Moliadze et al., 2015). Stronger intensity might be needed for deeper regions, such as IFC, as opposed to more superficial regions, such as DLPFC, which might explain the null findings in studies of stimulation of rIFC in ADHD (Salehinejad et al., 2020). Clear and evidenced dosage guidance is therefore paramount for paediatric studies, especially since stimulation intensity and duration are non-linear [
183] (Lefaucheur et al., 2017) and neuroplasticity changes are strongest during childhood development [
184] (Knudsen, 2004). Furthermore, hardly anything is known on the longer-term efficacy of tDCS/tRNS/tACS or TMS protocols in ADHD. In healthy volunteers, up to 1-year longer-term cognitive effects have been observed with tDCS combined with cognitive training [
125] (Katz et al., 2017) and up to 1 month in other psychiatric disorders [
185,
186] (Kekic et al., 2016; Moffa et al., 2018) with evidence for longer-term effects also with TMS in other psychiatric disorders (Janicak & Dokucu, 2015; Mehta et al., 2019) [
138,
139].
Given that tDCS is thought to affect neuroplasticity [
187,
188] (Kim et al., 2014; Nitsche et al., 2008), potential longer-term efficacy could be the real advantage of tDCS over stimulant medication. There is furthermore potential to combine tDCS with pharmacological or non-pharmacological treatments, in particular with cognitive training, as mentioned above.
While direct side effects appear to be minor and transient for non-invasive brain stimulation, such as itching and tingling over the stimulation site [
148,
170] (Krishnan et al., 2015; Salehinejad et al., 2020), there are, however, important neuroethical concerns about potential unknown negative effects of localised brain stimulation on the still-developing brain. It has been suggested that the neural state at the time of stimulation (Silvanto et al., 2008) [
189] or baseline cortical excitation-inhibition levels may influence stimulation effects [
190] (Krause et al., 2013), with those with suboptimal basal neural states likely to benefit more than those who already have an optimal activation pattern. This would suggest that application in psychiatric patient groups like ADHD who have suboptimal activation patterns may be more justified than its application as cognitive enhancer in healthy children and adults [
191] (Cohen-Kadosh et al., 2012). It is also possible that the stimulation of a particular region negatively affects the activation in other regions, which could then impair non-targeted functions.
Inter-individual differences in traits, which may be associated with differences in baseline neural states, have in fact shown to affect the benefits or costs of brain stimulation. For example, subjects with high mathematical anxiety benefited in their reaction time to mathematical tasks with tDCS over DLPFC, while those with low mathematical anxiety had an impairment in reaction time. Moreover, both groups became worse in an interference inhibition task [
192] (Sarkar et al., 2014), which could possibly reflect a negative effect of tDCS of DLPFC on the neighbouring IFC region, which mediates interference inhibition. Similarly, prefrontal stimulation improved automaticity of learning but impaired numerical learning mediated by parietal regions, while parietal stimulation impaired automaticity of learning mediated by prefrontal regions and improved numerical learning [
193] (Iuculano & Kadosh, 2013). Inter-individual differences in brain activation at baseline are hence likely to determine whether patients benefit or not from tDCS over a particular brain region, suggesting that future brain stimulation treatment should be individualised based on baseline brain and cognitive dysfunctions. This is particularly pertinent given that there is heterogeneity in cognitive dysfunction in ADHD (Nigg et al., 2005; Roberts et al., 2017) [
11,
12].
Findings of cognitive costs of tDCS on functions mediated by other brain regions are particularly worrying in paediatric applications where the brain is still developing and more plastic. It will therefore be crucial to assess potential costs on non-targeted cognitive functions which may occur via indirect down-stimulation of other brain regions that are interconnected with the stimulated site and that mediate these non-
targeted functions. This knowledge will be crucial to understand the risk-benefit ratio of localised brain stimulation to the individual patient and to children in particular. These worries of effects on non-targeted brain regions also apply to the neurofeedback studies. Most ethical considerations have concluded that there are no ethical reasons against tDCS in children and adolescents who have a medical condition that is handicapping and where potential side effects can be outweighed by benefits, while use of tDCS as cognitive enhancer in healthy children and adolescents is not advised [
191] (Cohen Kadosh et al., 2012). These benefits and risks, however, will still have to be established in ADHD as well as in other childhood disorders.
Table 2. Clinical and cognitive effects of sham-controlled tDCS studies.
|
Stimulation Protocol |
Outcome Measures (Bold/Underlined = Improvement; Cursive = Impairment) |
Study |
Design |
n |
Mean Age |
Anode/Cathode |
mA |
Sessions |
Timing a |
Duration (mins) |
Clinical |
Cognitive |
Children |
† Bandeira et al., 2016 [168] |
Open label |
9 |
11 |
L DLPFC/R SOA |
2 |
5 |
Online |
28 |
Patient Global Impression of Improvement |
Visual Attention Test (OM); NEPSY-II-inhibition (Switch errors); Digit Span; Corsi Cubes |
Breitling et al., 2016 [169] |
Single-blind, sham-controlled, randomised, crossover |
21 |
14 |
R IFC/L Cheek |
1 |
1 |
Online |
20 |
n/t |
Flanker (Incongruent trials: COM c,d & RTV c) e |
|
|
|
|
L Cheek/R IFC |
1 |
1 |
Online |
20 |
n/t |
Flanker |
Munz et al., 2015 [167] |
Double-blind, sham-controlled, randomised, crossover |
14 |
12 |
L DLPFC/R Cheek; R DLPFC/L Cheek |
0.25 |
1 |
Offline |
25 (5 on, 1 off) |
n/t |
Go/No-Go (Go RT & RTV); Motor memory; Alertness |
Nejati et al., 2020, Exp 1 [171] |
Double-blind, sham-controlled, randomised, crossover |
15 |
10 |
L DLPFC/R DLPFC |
1 |
1 |
Offline |
15 |
n/t |
Go/No-Go; N-back (Acc, RT); Stroop (Incongruent trials: COM & RT); WCST (Completion time) |
Nejati et al., 2020, Exp 2 [171] |
Double-blind, sham-controlled, randomised, crossover |
10 |
9 |
L DLPFC/R SOA |
1 |
1 |
Offline |
15 |
n/t |
Go/No-Go; N-back (Acc c, RT) d; WCST (Total categories completed, total & pers errors) d |
|
|
|
|
R SOA/L DLPFC |
1 |
1 |
Offline |
15 |
n/t |
Go/No-Go (No--Go acc) d; N-back; WCST (Total categories completed, total & pers errors c) d |
Prehn-Kristensen et al., 2014 [166] |
Double-blind, sham-controlled, randomised, parallel |
12 |
12 |
L DLPFC/R Cheek; R DLPFC/L Cheek |
0.25 |
1 |
Offline |
25 (5 on, 1 off) |
n/t |
Declarative Memory (Acc); Alertness; Digit Span |
Soff et al., 2017 [164] |
Double-blind, sham-controlled, randomised, crossover |
15 |
14 |
L DLPFC/Vertex |
1 |
5 |
Online |
20 |
FBB-ADHD(Inattention f) g,h |
QbTest (Inattention f; hyperactivity i) g,h |
Soltaninejad et al., 2019 [161] |
Single-blind, sham-controlled, randomised, crossover |
20 |
16 |
L DLPFC/R SOA |
1.5 |
1 |
Online |
15 |
n/t |
Go/No-Go (Go Acc) c,d; Stroop |
|
|
|
|
R SOA/L DLPFC |
1.5 |
1 |
Online |
15 |
n/t |
Go/No-Go (NoGo Acc) c,j; Stroop |
‡ Soltaninejad et al., 2015 [161] |
Single-blind, sham-controlled, randomised, crossover |
20 |
16 |
rIFC/L SOA |
1 |
1 |
Online |
15 |
n/t |
Go/No-Go (Go Acc); Stroop |
Sotnikova et al., 2017 [165] |
Double-blind, sham-controlled, randomised, crossover |
13 |
14 |
L DLPFC/Vertex |
1 |
1 |
Online |
20 |
n/t |
QbTest (RT, RTV k, OMs, Acc) l |
Breitling et al., 2020 [169] |
Double-blind, sham- and HD-tDCS controlled, randomised, crossover |
ADHD: 15HC: 15 |
13 (10–16) |
R IFC/L SOA |
1 |
3 with CT |
Online |
20 |
n/t |
WM task; ERPs N200; P300 |
Salehinejad et al., 2020 [170] |
Single-blind, sham-controlled, randomised, cross-over |
19 |
9 (8–12) |
1 |
2 |
Online |
23 |
n/t |
ANT (orienting); GNG; SAT; Stroop |
† Westwood et al., 2021 [162] |
Double-blind, sham-controlled, randomised, parallel |
50 |
14 |
R IFC/L SOA |
1 |
15 |
Online |
20 |
ADHD-RS; Conners 3P |
GNG; Stop; Simon; WCST; CPT; MCT; time estimation; NIH WM; Verbal Fluency |
Nejati et al., 2020 [171] |
Double-blind, sham-controlled, randomised, cross-over |
20 |
9 |
L DLPFC/R vmPFC R DLPFC/L vmPFC Sham |
1 |
1 |
Online |
20 |
n/t |
BART; CDDT (k20, k10) |
† Berger et al., 2021 [174] |
Double-blind, active controlled, randomised, cross-over |
19 |
7–12 |
L DLPFC (tDCS)/R SOA L DLPFC/R IFC (tRNS) |
0.75 |
5 |
Online |
5 |
n/t |
ADHD-RS; Working & short-term memory, Moxo-CPT (all improved with tRNS vs. tDCS) |
Adults |
† Allenby et al., 2018 [177] |
Double-blind, sham-controlled, randomised, crossover |
37 |
32 |
L DLPFC/R SOA |
2 |
3 |
Online |
20 |
n/t |
Conners CPT (COM m); Stop Task |
Cachoeira et al., 2017 [178] |
Double-blind, sham-controlled, randomised, parallel |
A: 9 S: 8 |
A: 31 S: 34 |
R DLPFC/L DLPFC |
2 |
5 |
Offline |
20 |
ADHD Checklist (Inattention, Total) n; SDS (after tDCS); ADHD total score 2 weeks |
None |
Cosmo et al., 2015 [175] |
Double-blind, sham-controlled, randomised, parallel |
A: 30 S: 30 |
A: 32 S: 33 |
LDLPFC/R DLPFC |
1 |
1 |
Offline |
20 |
n/t |
Go/No-Go |
Jacoby et al., 2018 [176] |
Single-blind, sham-controlled, randomised, crossover |
20 |
23 |
L&R DLPFC/Cerebellum |
1.8 |
1 |
Offline |
20 |
n/t |
CPT (multi-button presses) |
Dubreuil-Vall et al., 2020 [179] |
Double-blind, sham-controlled, randomised, crossover |
37 |
18–67 |
L DLPFC/R SOA R DLPFC/R SOA |
2 |
1 |
Offline |
30 |
n/t |
Flanker (incongruent RT) n = 18; L P300; L N200. Stop (go RTs); L P200. n = 19 Flanker; Stop |