Discussion
cognitive impairment, day)Future
ADVERSE EFFECTS
Various tDCS electrode montages targeting either the DLPFC or the fronto-medial regions have been utilized to modulate interference control or response inhibition. However, such montages have not been directly compared in terms of efficacy and adverse effects, even though this approach offers insight into their applicability for specific cognitive targets and could aid future study designs. Here, we chose commonly used stimulation parametersOur findings (supported by both conventional and Bayesian statistical methods) are in line with the incomprehensive results of the literatureA possible issue with the two-electrode montages when both electrodes are placed on the head (such as those we used here) is that the partial contribution of the return electrode in reducing cortical excitability over the area below the electrode cannot be completely ruled out. The use of extracephalic montages (i.e. when the return electrode is not placed on the head) might eliminate the effect of the return electrode on cortical modulationThe parameter space of other stimulation parameters, including currents strength, electrode size, and stimulation timing should also be revisited and systematically tested due to the lack of consensus regarding their effectivenessThe choice of target area has also been a parameter of considerable diversity in the field. Although the asymmetric stimulation of the left DLPFC is fairly popular, and the role of the DLPFC is also supported by evidence from neuroimaging and non-invasive brain stimulation studiesSeveral other brain areas have been proposed as targets for neuromodulation. A neuroimaging study has indicated that the left anterior insula is a region involved in both response inhibition and cognitive control, making this area another potential target for non-invasive brain stimulation in future studiesWith the advancement of computational modeling, the simulation of electric fields has become more accessible. Recent evidence contradicts the notion that tDCS has the most pronounced effect directly beneath the electrodesIt is also worth noting that interference resolution and response inhibition have been operationalized in numerous ways in the literature including the Stroop, flanker, Simon, and anti-saccade, as well as the stop-signal and the Go/No-Go tasks, respectivelyIn the present study, we chose a combined task in order to measure both interference control and response inhibition. This task has been found comparable to the flanker and Go/No-Go tasks with a moderate correlation of convergence validityDespite the additional cognitive load, the current task design might not be sensitive enough to detect subtle changes or electrophysiological changes that do not reach the behavioral level. This might be especially true considering accuracy which was consistently high in all participants and trial types in the present study. The ceiling effect might be related to null results in healthy subjects as it might prevent tDCS effects from manifesting. By applying both anodal and cathodal stimulation, the latter classically intended to inhibit the target area temporarily, we aimed to disentangle the effects of current polarity and capture the potential performance deteriorating effects of tDCS as opposed to the improvements that might reach an upper limit. Importantly, our results derived from healthy adults (with performance potentially reaching a ceiling effect) cannot necessarily be expanded to other potential groups of participants. Our null results may be attributed to the possibility of compensatory activations occurring in both the stimulated areas and their contralateral counterparts, or within functional networks. It has been proposed that tDCS is rather a tool to improve deficient cognitive processes, and indeed, encouraging results exist indicating that tDCS can reverse the abnormal activity of several networks in mild cognitive impairment and that this change in brain activation was associated with improved performanceFinally, another source of inconsistencies of tDCS effects has been attributed not only to methodological differences but also subject variables (such as age, sex, certain cognitive status, and genetics), neurophysiological (e.g. cortical excitability), and other factors (time of day)Future lines of research should address these speculations by systematically studying individualized parameters and individual predictors of tDCS efficacy on large samples of participants. Provided that some conventional tDCS methods keep yielding inconclusive results, novel electrode montages might also be considered. Studies with such scope could not only significantly contribute to the understanding of the mechanisms of tDCS and its clinical applicability but also the understanding of the neural background of cognitive control and response inhibition.