Expectancy and conditioning are underlying mechanisms of placebo and nocebo responses. In previous studies with motion sickness, we could induce nocebo responses by both methods, but no placebo responses.
In Experiment 1, 64 volunteers (50% women, mean age = 23.5 years) were evaluated to determine the degree they realized speed changes in nauseogenic rotation. For Experiment 2, 32 volunteers (50% women, mean age = 26.0 years) were exposed to fast rotation (15 rounds per minute, or rpm) on Day 1. On Day 2, they either received a drink with a presumed effective antiemetic (actually placebo) or were told they belonged to the control group. Rotation was surreptitiously reduced (to 10 rpm). On Day 3, they were tested with the initial rotation speed. Outcome variables in both experiments were symptom ratings; additionally in Experiment 2, the number of nauseogenic head movements, tolerated rotation time, and electrogastrogram were analyzed for changes between Days 1 and 2 (expectancy plus speed reduction) and Days 1 and 3 (expectancy plus conditioning).
In Experiment 1, a dose-response function was established for different rotation speeds, with the smallest perceived difference between 10 and 15 rpm. In Experiment 2, placebo application induced better maximal symptom rating, head movement, and rotation time at Day 2 (F = 3.097, p = .043) and Day 3 (F = 3.401, p = .031). Electrogastrogram was unaffected.
Verbal suggestions combined with a conditioning procedure are effective in reducing symptoms of motion sickness.