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

Abstract

The aim of this preregistered EEG study was to show how expectations about enhanced or impaired performance through transcranial stimulation affect feelings of agency and error processing. Using a single-blind experimental design, participants (N = 57) were attached to a transcranial direct current stimulation (tDCS) device, and in different blocks, they were verbally instructed to expect enhanced or impaired cognitive performance, or no effects of the brain stimulation. In all cases, but unbeknownst to the participants, we used an inert sham tDCS protocol. Subsequently, we measured their response to errors on a cognitive control task. Our expectancy manipulation was successful: participants reported improved subjective performance in the enhancement compared with the impairment condition—even though objective performance was kept at a constant level across conditions. Participants reported the highest feelings of agency over their task performance in the control condition, and lowest feelings of agency in the impairment condition. The expectancy manipulation did not affect the error-related negativity (ERN) in association with incorrect responses. During the induction phase, expecting impaired versus enhanced performance increased frontal theta power, potentially reflecting a process of increased cognitive control allocation. Our findings show that verbally induced manipulations can affect subjective performance on a cognitive control task, but that stronger manipulations (e.g., through conditioning) are necessary to induce top-down effects on neural error processing.

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

“…One of the plausible reasons for this may be the nature of outcome measures examined in each study, i.e., whether subjective or objective outcome measures were used. Namely, there is evidence that the placebo-like effect is particularly evident when it comes to subjective self-report measures, such as participants’ estimation of their own performance [ 56 , 57 ]. For instance, in a study by van Elk et al [ 56 ], even though participants reported enhanced subjective performance from sham tDCS in the cognitive control task, this did not affect objective neurocognitive measures.…”

“…Namely, there is evidence that the placebo-like effect is particularly evident when it comes to subjective self-report measures, such as participants’ estimation of their own performance [ 56 , 57 ]. For instance, in a study by van Elk et al [ 56 ], even though participants reported enhanced subjective performance from sham tDCS in the cognitive control task, this did not affect objective neurocognitive measures. Other studies which reported the placebo-like effect of tDCS investigated rather subjectively rated or self-reported measures, such as mind-wandering [ 30 ], food-craving [ 39 ], or depression [ 54 , 58 ].…”

“…In addition, previous research that examined the placebo effect in tDCS research often involved prior manipulation of participants’ expectations about this intervention [ 38 , 56 , 59 ]. For instance, researchers provided participants with information sheets that described supposed positive or negative outcomes they can expect from tDCS.…”

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

“…The impact of the nocebo effect in tDCS trials has been investigated in a recent study [ 56 ]. Here, the focus was on how negative expectations about tDCS could affect feelings of agency and error processing.…”

Self-reported Outcome Expectations of Non-invasive Brain Stimulation Are Malleable: a Registered Report that Replicates and Extends Rabipour et al. (2017)

“…Some studies have found NIBS to be an effective tool for cognitive enhancement (Au et al, 2016;Dockery et al, 2009;Katz et al, 2017;Morales-Quezada et al, 2015;Parasuraman & McKinley, 2014;Richmond et al, 2014;Southworth, 1999;Vodyanyk et al, 2021). Other studies have found detrimental effects (Brunyé et al, 2018;Pyke et al, 2020) or no reliable effects (Horne et al, 2020;Horvath et al, 2015aHorvath et al, , 2015bMancuso et al, 2016;Medina & Cason, 2017;Nilsson et al, 2017;Rabipour et al 2018bRabipour et al , 2019Talsma et al, 2017;van Elk et al, 2020). These conflicting findings demonstrate how difficult it is to determine whether NIBS can reliably achieve cognitive enhancement.…”