ON THE ORIGIN OF INTERINDIVIDUAL SUSCEPTIBILITY TO MOTION SICKNESS

According to the otolith asymmetry hypothesis for susceptibility to mo tion 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 u nfamiliar movement patterns or force environments. Measurements in var ious species have demonstrated a large scatter in the otoconial mass. As the utricle and saccule have distinct functions in the vestibular s ystem it seems appropriate to deal with these organs separately. Resul ts are presented on mass distribution and lateral preponderance of the otoconial mass in the utricles and saccules of the salmon (Salmo sala r) and trout (Salmo irideus). The measurements revealed considerably l arger dimensions of the saccular otoconia in these species, amounting to twenty-fold. This substantial difference indicates that different r egulatory principles underlie the otoconial generation of each of the organs and is presumably related to their specific functions. The late ral preponderance was found to be normally distributed for both organs in both species, with standard deviations of approx. 4% except for th e utricular otoconia of the salmon (13%). In a second set of experimen ts, fish were exposed to a Coriolis force environment. Their sealed aq uarium was subjected to constant vertical axis rotation combined with pendular oscillation around the horizontal axis. The aquarium was illu minated by a light source fixed to project through the top, and a vide o camera recorded the movements of the fish. During combined rotation and pendular oscillation, one group of fish maintained an active compe nsatory swimming behaviour, whereas the movements of a second group be came uncoordinated and often led to passive behaviour. Analysis of the otoconia of these two groups of fish promises to yield further eviden ce relevant to the otolith asymmetry hypothesis.