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.
Highlights
Transcranial direct current stimulation, a non-invasive neuromodulation technique, offers the promise of a low-costElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.tool for both research and clinical applications in psychology, psychiatry, and neuroscience (Brunoni et al 2011a; Kuo et al 2014; Mondino et al 2014; Mondino et al 2015a; Tanaka and Watanabe 2009)
Experiment 2 examined whether a generic placebo effect of Transcranial direct current stimulation (tDCS) or temporal duration between runs required for delivering tDCS caused the unexpected performance improvement in orientation discrimination task (ODT) found in experiment 1
Experiment 2 found that ODT performance was significantly improved in sham-tDCS alone and not improved in subject assigned to conditions with no tDCS (2-min delay, 10-min delay)
Summary
TDCS has been shown to alter performance and enhance training effects in a wide range of psychological paradigms (Ditye et al 2012; Martin et al 2014; Ruf et al 2017; Saunders et al 2015; Segrave et al 2014). Findings regarding tDCS are often equivocal and difficult to replicate with a number of recent meta-analyses suggest that tDCS may have small effects if any (Berlim et al 2013; Horvath et al 2015; Medina and Cason 2017; Santos et al 2018). A more fundamental problem is that clinical and cognitive applications studied are complex and effects of tDCS are difficult to predict given its known effects on the basic underlying neurophysiology (Das et al 2016; Giordano et al 2017; Stagg et al 2018)
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