Vestibular-visual conflict sickness in the squirrel monkey.

When combined vestibular and visual (optokinetic) stimuli were given to squirrel monkeys in a direction and phase mismatching sinusoidal mode (in yaw plane), five of six monkeys exhibited actual emesis during three repeated exposures on separate trial days. Two animals vomited on two trial days and three animals, on one day. The incidence took place in 38.9% of the total trial days. When the identical vestibular and visual stimuli were given individually, the effect was negligible, thus confirming each stimulus magnitude was not stressful enough to produce emesis when used separately.