Placebo and nocebo effects on mental fatigue: results from a pilot study

Background. Placebo effects have been demonstrated on fatigue, but existing studies focus on physical fatigue, leaving mental aspects of fatigue unexamined. The goal of the current study was to investigate whether placebo and nocebo effects can also affect mental fatigue, using established methods of verbal suggestion and conditioning. Method. 19 participants followed a conditioning protocol of 4 sessions, with one baseline session, two acquisition sessions, and one evocation session. Participants received a cover story of receiving either fatiguing (nocebo group), energizing (placebo group), or no medication (control group) through a (placebo) nasal spray, and performed a 2-back task to induce fatigue. During the acquisition sessions, task difficulty and length and environmental lighting were surreptitiously manipulated. Changes in fatigue (before to after task) and level of performance (accuracy and response time) were recorded. Results. The placebo group showed lower fatigue than control during acquisition, but this effect was not present during evocation. No differences between control and nocebo were found for any session, and no group differences were detected on performance measures at any point. Discussion. The current procedure was not effective at evoking a placebo or nocebo effect on mental fatigue. While the procedure induced fatigue and the cover story was convincing, a ceiling effect and low number of acquisition trials may have prevented effects from occurring. More naturalistic designs may be preferable for future studies.