Abstract
Objectives: The study aimed to investigate whether placebo interventions that induce expectations of a miotic or a mydriatic drug effect can affect pupil size and accommodation, and in the case of such effects, to investigate underlying autonomic mechanisms.
Methods: Fifty healthy adults were randomly assigned to one of five groups and were administered eye drops into their right eye: Twenty participants received saline or pilocarpine drops in a double-blind design and were informed about the miotic effects of pilocarpine, a further 20 participants received saline or tropicamide drops in a double-blind design and were informed about the mydriatic effects of tropicamide, and ten participants served as controls and knowingly received saline eye drops. Pupil size and amplitude of accommodation as well as blood pressure (systolic, diastolic) and heart rate were assessed immediately prior to the intervention as well as at regular intervals during the 45 minutes following the intervention. Following the experiment, participants were questioned regarding subjective treatment effects.
Results: Neither of the two placebo interventions which aimed to decrease or increase pupil size actually affected pupil diameter or amplitude of accommodation as compared to the control intervention. Similarly, blood pressure and heart rate were not impacted by the two experimental placebo interventions. Subjective symptoms, such as itching of the eye or blurred vision, were more frequently reported by participants in the two experimental placebo groups than by controls.
Conclusion: Results suggest that eye functions controlled by the autonomic nervous system, such as pupil size and accommodation, cannot be influenced by placebo interventions based on expectancy manipulation.
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