The Role of Expectation and Beliefs on the Effects of Non-Invasive Brain Stimulation

Abstract

Non-invasive brain stimulation (NIBS) techniques are used in clinical and cognitive neuroscience to induce a mild magnetic or electric field in the brain to modulate behavior and cortical activation. Despite the great body of literature demonstrating promising results, unexpected or even paradoxical outcomes are sometimes observed. This might be due either to technical and methodological issues (e.g., stimulation parameters, stimulated brain area), or to participants’ expectations and beliefs before and during the stimulation sessions. In this narrative review, we present some studies showing that placebo and nocebo effects, associated with positive and negative expectations, respectively, could be present in NIBS trials, both in experimental and in clinical settings. The lack of systematic evaluation of subjective expectations and beliefs before and after stimulation could represent a caveat that overshadows the potential contribution of placebo and nocebo effects in the outcome of NIBS trials.

Transcranial Direct Current Stimulation May Reduce Prefrontal Recruitment During Dual Task Walking in Functionally Limited Older Adults – A Pilot Study

“…Thirdly, while blinding efficacy was assessed and achieved in the parent study following the completion of the entire intervention (i.e., 10 stimulation sessions) (Manor et al, 2018), participants' beliefs in the type of stimulation received was not queried after the first stimulation session. Therefore, it is undetermined whether expectancy beliefs influenced the findings in the current results (Braga et al, 2021). Future studies should assess blinding efficacy after each stimulation visit in which outcome measures are taken.…”

Blinding in tDCS Studies: Correct End-of-Study Guess Does Not Moderate the Effects on Associative and Working Memory

“…Rabipour et al [ 38 , 59 ] demonstrated that participants in tDCS studies have variable assumptions and knowledge about the intervention before the experiment—some participants see tDCS as a promising intervention, while others can be skeptical. The relevance of participants’ expectations for tDCS effects has recently been tackled in a narrative review by Braga et al [ 36 ], who argued for a systematic evaluation of participants’ expectations in NIBS research to control their potential confounding effect.…”

“…In tDCS experiments on cognition, if the participants are aware of the received stimulation type, they can engage more (during real stimulation) or less (during sham stimulation) while completing cognitive tasks. On the other hand, expectations may influence the outcomes if participants anticipate the improvement of their performance because of the assumed tDCS benefit [ 36 ]. Moreover, the interaction between awareness and expectations may further influence the results of tDCS experiments.…”

Non-invasive brain stimulation and neuroenhancement

“…Instructional manipulations combined with sham tES could induce cognitive placebo or even nocebo effects in healthy individuals ( Turi et al, 2018 ). Ineffective blinding may not only interfere with attentional processes during tES, but might lead to psychobiological responses through expectations ( Colagiuri et al, 2011 , Oken et al, 2008 , Braga et al, 2021 ). Experimentally manipulating the expectancy of tES outcome, e.g., by placebo-inducing written instructions combined with sham tES can on itself influence cognitive functions in young healthy adults ( Turi et al, 2017 , Turi et al, 2018 ).…”