On the origin of interindividual susceptibility to motion sickness

According to the otolith asymmetry hypothesis for susceptibility to motion sickness, the interlabyrinthine asymmetry in the otoconial mass, which is normally compensated in the circuity of the vestibular system by appropriate neuronal weighting, becomes disrupted as a result of unfamiliar movement patterns or force environments. Measurements in various species have demonstrated a large scatter in the otoconial mass. As the utricle and saccule have distinct functions in the vestibular system it seems appropriate to deal with these organs separately. Results are presented on mass distribution and lateral preponderance of the otoconial mass in the utricles and saccules of the salmon (Salmo salar) and trout (Salmo irideus). The measurements revealed considerably larger dimensions of the saccular otoconia in these species, amounting to twenty-fold. This substantial difference indicates that different regulatory principles underlie the otoconial generation of each of the organs and is presumably related to their specific functions. The lateral preponderance was found to be normally distributed for both organs in both species, with standard deviations of approx. 4% except for the utricular otoconia of the salmon (13%). In a second set of experiments, fish were exposed to a Coriolis force environment. Their sealed aquarium was subjected to constant vertical axis rotation combined with pendular oscillation around the horizontal axis. The aquarium was illuminated by a light source fixed to project through the top, and a video camera recorded the movements of the fish. During combined rotation and pendular oscillation, one group of fish maintained an active compensatory swimming behaviour, whereas the movements of a second group became uncoordinated and often led to passive behaviour. Analysis of the otoconia of these two groups of fish promises to yield further evidence relevant to the otolith asymmetry hypothesis.