Visual Reinforcement of Illusory Rotations during Centrifugation: A Novel Habituation Strategy to Motion Sickness
G. Bertolini1, J.E. Bos2, D. Bron3, T. Frett1, E. Groen2, R. Hemmersbach1, F.L. Wuyts4
Background: Artificial gravity (AG) is the only countermeasure providing an “Earth-like” solution to weightless health hazards. Head movements during centrifugation, however, cause motion sickness (Cross-coupling stimulus) due to conflicts between perceived, though illusory, rotations and sensed gravity direction. Existing habituation protocols successfully abate motion sickness. The reduction of the conflict supposedly occurs by decreasing vestibular responses to rotation (as quantified by reflexes), i.e. adapting the semicircular canals contribution to the multi-sensory integration generating self-motion perception. In outer space, however, perception of self-rotation has an important role in self-motion sensing, as astronauts are already deprived of gravity reference. The aim of this research is to evaluate if a different adaptation is possible, abating motion sickness without decreasing response to rotation.
Methods: We tested 19 healthy subjects on the ESA Short Arm Human Centrifuge at DRL, Cologne. The control group (CG: 9 subjects, 3 f) performed a “standard” habituation protocol consisting of 30° clockwise head rolls during centrifugation at 100°/s (1 g at feet). The test group (TG: 10 subjects, 5 f) performed an identical protocol, with the addition of visual stimuli triggered by head movements providing optokinetic stimuli matching the predicted vestibular sensation. Motion sickness was measured using a 1-20 scale, while an eye tracker recorded the vestibulo-ocular reflex (VOR). Measurements were repeated after 24h.
Results: Only 15 subjects (7 CG, 8 TG) completed the experiment. The same reduction of motion sickness from day 1 to day 2 was observed in both groups (median [MAD] CG: -4 ; TG: -4 , p=0.78). The CG had a significantly larger reduction of the VOR duration than the TG (CG: -4  s; TG: -1  s, p=0.05).
Conclusions: Subjects habituate even if illusory self-rotation induced by head tilts is sustained by visual input. Visually reinforced habituation preserves vestibular reflexive responses to rotations and it is therefore a good candidate to achieve an alternative adaptation of multi-sensory integration that preserve rotation sensing but abate motion sickness in AG.