Abstract
Several recent studies have reported non-linear effects of transcranial direct current stimulation (tDCS), which has been attributed to an interaction between the stimulation parameters (e.g., current strength, duration) and the neural state of the cortex being stimulated (e.g., indexed by baseline performance ability, age) (see Fertonani and Miniussi, 2016). We have recently described one such non-linear interaction between current strength and baseline performance on a visuospatial attention (landmark) task (Benwell et al., 2015). In this previous study, we induced a small overall rightward shift of spatial attention across 38 participants using bi-hemispheric tDCS applied for 20 min (concurrent left posterior parietal (P5) anode and right posterior parietal (P6) cathode) relative to a sham protocol. Importantly, this shift in bias was driven by a state-dependent interaction between current intensity and the discrimination sensitivity of the participant at baseline (pre-stimulation) for the landmark task. Individuals with high discrimination sensitivity (HDS) shifted rightward in response to low- (1 mA) but not high-intensity (2 mA) tDCS, whereas individuals with low discrimination sensitivity (LDS) shifted rightward with high- but not low-intensity stimulation. However, in Benwell et al. (2015) current strength was applied as a between-groups factor, where half of the participants received 1 mA and half received 2 mA tDCS, thus we were unable to compare high and low-intensity tDCS directly within each individual. Here we aimed to replicate these findings using a within-group design. Thirty young adults received 15 min of 1 and 2 mA tDCS, and a sham protocol, each on different days, to test the concept of an interaction between baseline performance and current strength. We found no overall rightward shift of spatial attention with either current strength, and no interaction between performance and current strength. These results provide further evidence of low replicability of non-invasive brain stimulation protocols, and the need for further attempts to replicate the key experimental findings within this field.
Highlights
Transcranial direct current stimulation has been used as a non-invasive method of modulating neuronal excitability in both healthy and clinical groups for over 15 years (Nitsche and Paulus, 2000)
Alertness generally reduced throughout the experiment (3 × 2 ANOVA: Transcranial direct current stimulation (tDCS)-intensity (1ma, 2mA, sham) × time: effect of time [F(2, 26) = 46.73, p < 0.001)] but there was no difference between the three stimulation conditions
A ttest identified no difference in point of subjective equality (PSE) shifts between the 1 and 2 mA tDCS conditions during the online stimulation period [t(28) = 1.61, p = 0.12] relative to baseline, nor was there any difference in PSE shifts between the two tDCS intensities over the course of the overall experiment [t(28) = 0.69, p = 0.5]. This is in contrast to Benwell et al where we identified a small overall rightward shift of spatial bias in response to the left-anode right-cathode (LA/RC) montage relative to sham [a one sample t-test compared to zero, collapsed over both current strengths t(37) = 2.003, p = 0.052]
Summary
Transcranial direct current stimulation (tDCS) has been used as a non-invasive method of modulating neuronal excitability in both healthy and clinical groups for over 15 years (Nitsche and Paulus, 2000). A series of meta-analyses of the tDCS literature has found no strong evidence of efficacy within any cognitive domain probed (Horvath et al, 2015a), no benefits of single-session tDCS in healthy adults (Horvath et al, 2015b) and only a small reliable effect of tDCS in the modulation of motor-evoked potentials (Horvath et al, 2015c, see Horvath et al, 2016a for a failed replication of this small effect using behavioral outcome measures) These points of contention, coupled with a high probability of publication bias in favor of positive study outcomes, have generated debate within the electrical stimulation research community regarding the overall efficacy of this technique (see Antal et al, 2015; Horvath, 2015; Price and Hamilton, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
