Null Effects on Working Memory and Verbal Fluency Tasks When Applying Anodal tDCS to the Inferior Frontal Gyrus of Healthy Participants

Abstract

Transcranial direct current stimulation (tDCS) is a technique used to modify cognition by modulating underlying cortical excitability via weak electric current applied through the scalp. Although many studies have reported positive effects with tDCS, a number of recent studies highlight that tDCS effects can be small and difficult to reproduce. This is especially the case when attempting to modulate performance using single applications of tDCS in healthy participants. Possible reasons may be that optimal stimulation parameters have yet to be identified, and that individual variation in cortical activity and/or level of ability confound outcomes. To address these points, we carried out a series of experiments in which we attempted to modulate performance in fluency and working memory probe tasks using stimulation parameters which have been associated with positive outcomes: we targeted the left inferior frontal gyrus (LIFG) and compared performance when applying a 1.5 mA anodal current for 25 min and with sham stimulation. There is evidence that LIFG plays a role in these tasks and previous studies have found positive effects of stimulation. We also compared our experimental group (N = 19–20) with a control group receiving no stimulation (n = 24). More importantly, we also considered effects on subgroups subdivided according to memory span as well as to more direct measures of executive function abilities and motivational levels. We found no systematic effect of stimulation. Our findings are in line with a growing body of evidence that tDCS produces unreliable effects. We acknowledge that our findings speak to the conditions we investigated, and that alternative protocols (e.g., multiple sessions, clinical samples, and different stimulation polarities) may be more effective. We encourage further research to explore optimal conditions for tDCS efficacy, given the potential benefits that this technique poses for understanding and enhancing cognition.

Transcranial Direct Current Stimulation (tDCS) over the Intraparietal Sulcus Does Not Influence Working Memory Performance

“…The results of the present study are not the first reporting null effect of tDCS on WM or on other cognitive abilities and are consistent with the meta-analytic studies that have highlighted the problem of mixed and contradictory findings about single session tDCS effects in young and healthy participants (Westwood and Romani, 2018;Lukasik et al, 2018;Meier and Sauter, 2018). In addition, some studies that have shown a beneficial role of tDCS stimulation over the DLPFC, nevertheless also report important individual differences with TDCS-related WM performance increase in some participants, and performance decrease or no effect in other participants (Talsma et al, 2017).…”

Attention neuroenhancement through tDCS or neurofeedback: a randomized, single-blind, controlled trial

“…Some issues could be associated with this null effect in tDCS. First, it is essential to highlight that several other tDCS studies have been finding null effects on attention 53 and in other cognitive functions, such as working memory 54 , or physiological processes, such as substance craving 55,56 . Several studies have been discussing the contradictory effects of tDCS 57 , which could be becoming more common due to the recognition of publishing bias, such as the file drawer effect [58][59][60] ; and also related to complex mechanisms associated with tDCS parameters that can modulate its effects, such as: dose 61,62 , participants' baseline performance 19,20 ; timing of tDCS application, i.e., if before or during a cognitive or behavioral task 63,64 ; anatomical differences between subjects 65 ; or even their arousal level during the neuromodulation 66 .…”

“…(1) P1 between 90 and 160 ms on parieto-occipital electrodes (geodesic net61,62,78,77,67,71,76,75,72); (2) N1 between 160 and 220 ms on the same electrodes of P1; (3) P3cue-between 250 and 400 ms on the same electrodes of P1; (4) P3between 300 and 450 ms on parietal(61,62,78,79,54,55,31,80) and occipital(71,75,76,72,67,77) electrodes. The electrode area for each ERP can be seen in Fig.3.…”

Non-Invasive Brain Stimulation for the Modulation of Aggressive Behavior—A Systematic Review of Randomized Sham-Controlled Studies

“…Despite these promising findings, some papers reported null effects of tDCS [ 163 , 164 , 165 , 166 , 167 ] and rTMS [ 168 , 169 , 170 ], and TBS research is still in its early stage, showing the need for further research and systematic reviews of the literature to consolidate results and address inconsistent findings.…”