Investigating the Effects of tDCS on Visual Orientation Discrimination Task Performance: “the Possible Influence of Placebo”

Abstract

The non-invasive neuromodulation technique tDCS offers the promise of a low-cost tool for both research and clinical applications in psychology, psychiatry, and neuroscience. However, findings regarding its efficacy are often equivocal. A key issue is that the clinical and cognitive applications studied are often complex and thus effects of tDCS are difficult to predict given its known effects on the basic underlying neurophysiology, namely alterations in cortical inhibition-excitation balance. As such, it may be beneficial to assess the effects of tDCS in tasks whose performance has a clear link to cortical inhibition-excitation balance such as the visual orientation discrimination task (ODT). In prior studies in our laboratory, no practice effects were found during 2 consecutive runs of the ODT, thus in the current investigation, to examine the effects of tDCS, subjects received 10 min of 2 mA occipital tDCS (sham, anode, cathode) between a first and second run of ODT. Surprisingly, subjects' performance significantly improved in the second run of ODT compared to the first one regardless of the tDCS stimulation type they received (anodal, cathodal, or sham-tDCS). Possible causes for such an improvement could have been due to either a generic "placebo" effect of tDCS (as all subjects received some form of tDCS) or an increased delay period between the two runs of ODT of the current study compared to our previous work (10-min duration required to administer tDCS as opposed to~2 min in previous studies as a "break"). As such, we tested these two possibilities with a subsequent experiment in which subjects received 2-min or 10-min delay between the 2 runs (with no tDCS) or 10 min of sham-tDCS. Only sham-tDCS resulted in improved performance thus these data add to a growing literature suggesting that tDCS has powerful placebo effect that may occur even in the absence of active cortical modulation.

Effectiveness of noninvasive brain stimulation in the treatment of anxiety disorders: a meta-analysis of sham or behaviour-controlled studies

“…Moreover, the inclusion of a sham or control condition for comparison with the stimulation condition is not standard in research, despite the fact that the placebo effect of noninvasive brain stimulation techniques is well-known in both participants and experimenters, highlighting the importance of applying double-blind procedures. 183,184 Future studies should also move in the direction of coupling noninvasive brain stimulation with behavioural or cognitive interventions, investigating whether combined treatment is more effective than monotherapy. Another crucial point about the efficacy of noninvasive brain stimu lation protocols is based on sex.…”

Placebo Brain Stimulation Affects Subjective but Not Neurocognitive Measures of Error Processing

“…Proponents of brain stimulation techniques argue that more refined stimulation protocols may be necessary, taking into account individual variability in brain structure and function for instance (Antal et al 2015;Opitz et al 2015). Expectancy effects could partly underlie the perceived efficacy of tDCS effects (Dawood et al 2019;Turi et al 2019) and the present study demonstrates that expectations about cognitive enhancement versus impairment clearly exert subjective but not objective effects. In our study design, we kept the accuracy at a constant level by adjusting the time window for responding-but even then the response times turned out to be comparable between the different conditions.…”

Non-invasive brain stimulation effectiveness in anxiety disorder treatment: a meta-analysis on sham/behavior-controlled studies

“…Concerning the studies including a clinical sample, only few protocols investigated NIBS efficacy at a group-level. Moreover, the inclusion of a sham or control condition to be compared to the real stimulation is not the standard in researchers’ procedure, despite the placebo effect of NIBS techniques is well-known, in both participants and experimenters, thus highlighting the importance of applying double blind procedures 181, 182 . Future studies should also move in the direction of coupling NIBS with behavioral / cognitive interventions, investigating whether coupled treatment can be more effective than monotherapies.…”