The influence of informational priming on motor expectancy in transcranial direct current stimulation (tDCS)

Transcranial direct current stimulation (tDCS) has shown mixed results in cognitive and motor functions, raising questions about its efficacy and/or the potential influence of psychological factors. Expectations about treatment efficacy (a placebo-like mechanism) might explain this mixed tDCS success. Recent research has begun to examine the expectations of tDCS on performance. However, it is unclear how malleable individual tDCS expectations are within the motor domain. This study investigated how informational priming influences participants' expectations of tDCS on motor function and whether prior tDCS knowledge influences this effect. One hundred ninety-eight participants were recruited and assigned into high or low expectation priming groups. A modified expectation assessment scale, which measured expected tDCS-related motor performance, was administered before and after receiving information about tDCS efficacy. The high-expectation priming group was exposed to evidence of such efficacy, while the low-expectation priming group received information emphasizing the lack of evidence of tDCS efficacy. Expectations of tDCS efficacy for all motor domains significantly increased in the high-expectation priming group and decreased in the low-expectation priming group. Furthermore, after priming, the high-expectation priming group had significantly higher expectations than the low-expectation priming group. Interestingly, participants in the high-expectation priming group with prior knowledge of tDCS showed no change in expectations. Notably, these findings demonstrate that expectations about tDCS's efficacy on motor performance are malleable depending on the information type and prior tDCS knowledge. Future studies are encouraged to control or assess participants' expectations of tDCS to optimize its effect on motor outcomes.