Measuring Placebo Effects of Transcranial Direct Current Stimulation (tDCS) on Motor Learning

Studies using transcranial direct current stimulation (tDCS) to enhance motor training areoften irreproducible. This may be partly due to differences in stimulation parameters across studies, but it is also plausible that uncontrolled placebo effects may interact with the true ‘treatment’ effect of tDCS. Thus, the purpose of this study was to test whether there was a placebo effect of tDCS on motor training and to identify possible mechanisms of such an effect. Fifty-one participants (age: 22.2 ± 4.16; 26 F) were randomly assigned to one of three groups: active anodal tDCS (n=18), sham tDCS (n=18), or no stimulation control (n=15). Participant expectations about how much tDCS could enhance motor function and their general suggestibility were assessed. Participants then completed 30 trials of functional upper extremity motor training with or without online tDCS. Stimulation (20-min, 2mA) was applied to the right primary motor cortex (C4) in a double-blind, sham-controlled fashion, while the control group was unblinded and not exposed to any stimulation. Following motor training, expectations about how much tDCS could enhance motor function were assessed again for participants in the sham and active tDCS groups only. Results showed no effect of active tDCS on motor training (p=.67). However, there was a significant placebo effect, such that the collapsed sham and active tDCS groups improved more during motor training than the control group (p=.02). This placebo effect was significantly influenced by post-training expectations about tDCS (p=.0004). Thus, this exploratory study showed that there is a measurable placebo effect of tDCS on motor training, likely driven by participants’ perceptions of whether they received stimulation. Future studies should consider placebo effects of tDCS and identify their underlying mechanisms in order to leverage them in clinical care.