A two-factor model of rotation-induced motion sickness syndrome in squirrel monkeys.

Nonrestrained squirrel monkeys were rotated repeatedly in the horizontal plane once per day for preset times or until a vomiting response occurred. Emesis latencies and frequencies were recorded. Some subjects were fed fresh banana immediately before and after each rotation. The amount consumed provided a measure of conditioned food aversion or feeding suppression. A two-factor model of the motion-sickness syndrome was formulated to account for the data. According to the model, appropriate stimulation of the semicircular canals accompanied by visual and proprioceptive stimulation generates two independent physiologic processes or states: an objective emesis and a subjective nausea process. The emesis process is revealed by vomiting responses and the nausea process is inferred from the appearance of conditioned food aversion. Susceptible monkeys (type I) and resistant monkeys (type II) have different emesis processes, but both have similar nausea processes. Refractory monkeys have no well-developed emesis or nausea processes. Time and order characteristics of the motion-sickness syndrome depend on specific interactions of emesis and nausea processes.