Manipulating expectancies in optometry practice: ocular accommodation and stereoacuity are sensitive to placebo and nocebo effects

Abstract

Introduction: There is scientific evidence that an individual's beliefs and/or ex-pectations play a role in the behavioural and physiological response to a giventreatment. This study aimed to assess whether the dynamics of the accommoda-tive response and stereoacuity are sensitive to experimentally induced placeboand nocebo effects.Methods: Nineteen healthy university students performed three experimental ses-sions (placebo, nocebo and control) in randomised order, with the dynamics of theaccommodative response (magnitude and variability), stereoacuity and subjectivemeasures being assessed in all sessions. For the experimental manipulation, par-ticipants ingested an inert capsule that was alleged to have positive (white capsule,placebo condition) or negative (yellow capsule, nocebo conditions) effects on thehuman physiology. In the control condition, participants did not ingest a capsule.Results: The data revealed that the variability of accommodation was sensitiveto experimentally induced placebo and nocebo effects, showing a more stableaccommodative response for the placebo compared with the nocebo condition(corrected p-value = 0.04, Cohen's d = 0.60). In addition, better stereoacuity wasfound with the placebo, compared with the nocebo (corrected p-value = 0.01,Cohen's d = 0.69) and control (corrected p-value = 0.03, Cohen's d = 0.59) condi-tions. Successful experimental manipulation was confirmed by the analysis of sub-jective perceptions.Conclusions: These findings provide evidence that manipulating expectationsabout the efficacy of an inert treatment affect the dynamics of the accommoda-tive response (variability of accommodation) and stereoacuity. The results haveimportant applications in both clinical and research outcomes, where individuals´beliefs/expectations could modulate the visual function.