The effect of antimotion sickness drugs on habituation to motion.

The effect of antimotion sickness drugs on habituation was studied. Subjects were rotated once a day for 5 d to the malaise III end-point after receiving placebo, 1 mg scopolamine, 10 mg d-amphetamine, or the combination of 0.6 mg scopolamine with 5 mg of d-amphetamine. The placebo scores had a Spearman coefficient of correlation of 0.88 with the initial untreated tests. This demonstrated a high reliability for the M-III end-point and that little habituation resulted from the test design. The combination of 0.6 scopolamine with 5 mg amphetamine produced the fastest rate of habituation closely followed by the dose of 1 mg scopolamine. 10 mg of d-amphetamine also produced an increase in habituation over placebo scores. When the medications were discontinued on day 5 a rebound in sensitivity to vestibular stimulation occurred with scopolamine and scopolamine with d-amphetamine. The increased habituation appears to be due to the greater exposure to vestibular stimulation permitted by the medications.