Visual Reinforcement of Illusory Rotations during Centrifugation: A Novel Habituation Strategy to Motion Sickness

Background: Artificial gravity through centrifugation is currently the only countermeasure providing an “Earth-like” solution to weightless health hazards. Every head movement during centrifugation, however, may cause motion sickness due to conflicts between the perceived direction of gravity and the illusory rotations caused by the vestibular activation (Cross-coupling stimulus). Existing habituation protocols are based on repetition of cross-coupling stimuli. Although they successfully abate motion sickness, they also reduce overall responses to rotations. Our aim is to develop a novel habituation strategy to disentangle gravity and rotation perception, reducing motion sickness but retaining 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 normal habituation protocol consisting of repetitive 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 an illusion of rotation (optokinetic stimuli provided through the Oculus Rift) reinforcing the vestibular activation. Motion sickness was measured using a 1-20 scale, while an eye tracker embedded in the Oculus Rift recorded the vestibulo-ocular reflex (VOR). Habituation measurements were obtained repeating the experiment after 24h. Results: Only 15 subjects (7 CG, 8 TG) were able to complete the experiment. No difference between groups was observed in the motion sickness score on day 1 or in the reduction of motion sickness from day 1 to day 2 (median [MAD] CG: -4 [2]; TG: -4 [1], p=0.78). The CG, however, had a significantly larger reduction of the VOR duration than the TG (CG: -4 [1] s; TG: -1 [2] s, p=0.05). Conclusions: Subjects habituate to the cross-coupling stimuli during centrifugation even if illusory self-rotation induced by head tilts is sustained by visually induced perception of self-rotation. Visually reinforced habituation may however induce less reduction of oculomotor response to rotation than classical habituation and may therefore better preserve vestibular reflexive responses.