[Motion sickness in lower vertebrates: studies under conditions of weightlessness and under land conditions]

The review presents literature data and results of the author's studies with the goal to find out whether lower animals are susceptible to motion sickness. In our studies, fish and amphibians were tested for 2 h and more by using a rotating device (f = 0.24Nz, a(centrifugal) = 0.144 g) and a parallel swing (f = 0.2 Nz, a(horizontal) = 0.059 g). The performed studies did not reveal in 4 fish species and in toads any characteristic reactions of motion sickness (sopite syndrome, prodromal preparatory behavior, vomiting). At the same time, in toads characteristic stress reactions appeared (escape response, an increase in the number of urinations, inhibition of appetite), as well as some other reactions not associated with motion sickness (regular head movements, eye retraction). In trout fry this stimulation promoted dividing individuals into groups different by locomotor reaction to stress, as well as revealing individuals with a well-expressed compensatory reaction that we called the otolithotropic reaction. Analysis of results obtained by other authors confirms our conclusion. Thus, lower vertebrates, unlike mammals, are immune to motion sickness either under land conditions or under conditions of weightlessness. On the basis of available experimental data and theoretical concepts of mechanisms of motion sickness development formulated in several hypotheses (mismatch hypothesis, Traisman's hypothesis, resonance hypothesis) a synthetic hypothesis of motion sickness is presented that has the conceptual significance. According to the hypothesis, unusual stimulation producing sensomotor or senso-sensory conflict or action of vestibular and visual stimuli of about 0.2 Hz frequency is perceived by CNS as poisoning and causes corresponding reactions. The motion sickness actually is a byproduct of technical evolution. It is suggested that lower vertebrates, unlike mammals, lack a hypothetical center of subjective "nauseating" sensations; therefore they are immune to motion sickness.