Placebo-Suggestion Modulates Conflict Resolution in the Stroop Task

Abstract

Here, we ask whether placebo-suggestion (without any form of hypnotic induction) can modulate the resolution of cognitive conflict. Naïve participants performed a Stroop Task while wearing an EEG cap described as a “brain wave” machine. In Experiment 1, participants were made to believe that the EEG cap would either enhance or decrease their color perception and performance on the Stroop task. In Experiment 2, participants were explicitly asked to imagine that their color perception and performance would be enhanced or decreased (non-hypnotic imaginative suggestion). We observed effects of placebo-suggestion on Stroop interference on accuracy: interference was decreased with positive suggestion and increased with negative suggestion compared to baseline. Intra-individual variability was also increased under negative suggestion compared to baseline. Compliance with the instruction to imagine a modulation of performance, on the other hand, did not influence accuracy and only had a negative impact on response latencies and on intra-individual variability, especially in the congruent condition of the Stroop Task. Taken together, these results demonstrate that expectations induced by a placebo-suggestion can modulate our ability to resolve cognitive conflict, either facilitating or impairing response accuracy depending on the suggestion’s contents. Our results also demonstrate a dissociation between placebo-suggestion and non-hypnotic imaginative suggestion.

Placebo can enhance creativity

“…Placebo was also found to enhance performance in subconscious cognitive tasks. For example, performance on the Stroop effect, a classical response-time test in cognitive psychology, was improved by a sham EEG[ 21 ] or by verbal suggestion[ 22 ]. More specifically, in the former study, de Gama et al used a within-subjects design that compared performance on the Stroop task at baseline versus -performance during sham EEG said to modulate participants’ visual ability to perceive colors: either to enhance it or to decrease it.…”

I believe, therefore I am: Effects of self-control beliefs on behavioral and electrophysiological markers of inhibitory and emotional attention control

“…Prior to participation, volunteers received an information folder by email to explain the functioning of tDCS, the procedure of the study, and selection criteria. Crucially, they were informed that their prefrontal cortex would be stimulated with the aim to influence self-control processes (da Gama et al, 2013). One group of participants was led to believe that their ability to intentionally exert self-control would be enhanced (More Self-Control group, MSC), while the other group was informed that their intentional self-control capacities would be weakened (Less Self-Control group, LSC) 2 .…”

“…Upon subscription to the experiment via an online platform (Experimetrix), participants received an information folder and a tDCS safety checklist via email (similar to da Gama et al, 2013), to induce the expectancy of real brain stimulation and its capacity to change self-control exertion. Upon arrival in the lab, participants were seated in front of a computer screen, filled out a paper version of the tDCS safety checklist, and were given an information folder containing specific information about the experiment (i.e., whether to expect self-control to increase or decrease).…”

“…However, extensive belief research suggests that this high-level cognitive function is in fact malleable to suggestions. One placebo study employed sham neuro-stimulation to induce the belief that colour perception would either be enhanced or weakened (da Gama et al, 2013). After the bogus neuro-stimulation, participants completed a colour Stroop task, in which they had to name a colour word (e.g., the word "yellow") displayed in another colour (e.g., red font).…”

“…Although placebo and nocebo effects have been described in a wide range of health-related outcomes including pain analgesia (Caldwell & Gritsenko, 2017) and insomnia (Chung, Sharpe, Glozier, Hackett, & Colagiuri, 2017), their influence on higher-order psychological functions such as self-control has received only little attention. While some recent studies have employed placebo or nocebo instructions to manipulate cognitive processes such as memory (Assefi & Garry, 2003;Parker, Garry, Einstein, & McDaniel, 2011;Sinke, Forkmann, Schmidt, Wiech, & Bingel, 2016;Van Oorsouw & Merckelbach, 2007), performance on knowledge tests (Weger & Loughnan, 2013), and implicit learning (Colagiuri, Livesey, & Harris, 2011), so far only few studies examined placebo effects on cognitive control (e.g., da Gama, Slama, Caspar, Gevers, & Cleeremans, 2013;Schwarz & Büchel, 2015). Nonetheless, the research showing effects of belief manipulations on measures of self-control suggests that this high-level cognitive function is in fact permeable to the influence of suggestions.…”