Threshold-based vestibular adaptation to cross-coupled canal stimulation

Prior experiments have demonstrated that people are able to adapt to cross-coupled accelerations associated with head movements while spinning at high rotation rates (e.g., 23 rpm or 138 degrees/s). However, while adapting, subjects commonly experience serious side effects, such as motion sickness, non-compensatory eye movements, and strong and potentially disorienting illusory body tilt or tumbling sensations. In the present study, we investigated the feasibility of adaptation using a threshold-based method, which ensured that the illusory tilt sensations remained imperceptible or just barely noticeable. This was achieved by incrementally increasing the angular velocity of the horizontal centrifuge while supine subjects made repeated consistent yaw head turns. Incremental adaptation phases started at centrifugation speeds of 3 rpm. Centrifuge speed was slowly increased in steps of 1.5 rpm until a light illusory tilt was experienced. At the end of the incremental procedure, subjects were able to make head turns while rotating 14 rpm without experiencing illusory tilt. Moreover, motion sickness symptoms could be avoided and a limited carry over of the adaptive state to stronger stimulation at 23 rpm was found. The results are compared to prior studies which adapted subjects to super-threshold stimuli.