Motion sickness: an evolutionary hypothesis.

Since the occurrence of vomiting as a response to motion is both widespread and apparently disadvantageous, it presents a problem for evolutionary theory. An hypothesis is proposed suggesting that motion sickness is triggered by difficulties which arise in the programming of movements of the eyes or head when the relations between the spatial frameworks defined by the visual, vestibular, or proprioceptive inputs are repeatedly and unpredictably perturbed. Such perturbations may be produced by certain types of motion, or by disturbances in sensory input or motor control produced by ingested toxins. The last would be the important cause in nature, the main function of the emesis being to rid the individual of ingested neurotoxins. Its occurrence in response to motion would be an accidental by product of this system.