Accommodation and Stereoacuity Affected by Placebo, Nocebo Treatments

When expectations concerning the efficacy of an inactive treatment change, accommodative response and stereoacuity also change, according to a study published in Ophthalmic & Physiological Optics.

Researchers enrolled 19 healthy university students (mean age, 22.6±3.5 years) in an analysis and administered placebo, nocebo, and control treatments in a random order. Experimental manipulation consisted of participants ingesting an inert capsule that purported to have positive or negative effects on human physiology. Participants in the control group did not receive an inert capsule. Patients underwent accommodative response and stereoacuity measurements and subjectively rated their level of activation (10-point visual analogue scale: 0, low activation, 10, high activation) after ingesting the capsule or being informed of their status as a control participant. The students rotated groups during follow-up visits and spent time as a participant in each group over the course of the study.  

Overall, the variability of accommodation was influenced by placebo and nocebo effects. There was a more stable accommodative response in participants taking the placebo versus those taking the nocebo (P = 0.04). Placebo group participants experienced enhanced stereoacuity compared with students ingesting the nocebo capsule (P = 0.01) and control group individuals (P = 0.03). Participants taking the placebo capsule also reported higher levels of activation than control group participants and individuals ingesting the nocebo capsule (8.16±0.83; 7.68±1.20; 6.79±1.40, respectively). 

The investigators also note that the lag and variability of the accommodative response is mediated by the accommodative demand, demonstrating an increased lag and variability at closer viewing distances.

Researchers acknowledge placebo and nocebo effects on treatment outcomes stating, “manipulating expectations regarding the efficacy of an inert treatment can influence visual functioning in the short term, which may be of relevance in both clinical and laboratory settings.”

Study limitations include the strict inclusion of young and healthy individuals and small sample size.