Sensorimotor Impairment Related to Vestibular Adaptation to Altered Gravity

Exposure to altered gravity results in acute sensori-motor impairment, including motion sickness, disorientation, and inhibited posture and locomotion, thought to be related to vestibular maladaptation. These symptoms cause severe discomfort at best, and life-threatening risks at worst. Given the high prevalence of these symptoms and the associated risks, we must strive to understand the early stages of vestibular adaptation to altered gravity environments. To further this aim, we explored the effect of altered gravity on dynamic balance and motion sickness, as well as the relationship between these impairments and orientation perception. Thirteen subjects participated in two consecutive days of centrifugation-induced hyper-gravity (1.5g) exposure along their longitudinal body axis, with each “Hyper-gravity” segment lasting nominally one hour. Before and immediately after each centrifugation session, subjects conducted eyes-closed tandem walk (TW) tasks to measure dynamic balance. Every 3 minutes, subjects verbally reported their motion sickness via the Misery Index Scale (MISC) and their perception of tilt during passive (i.e., operator-controlled) head-on-neck tilts via a Subjective Visual Vertical (SVV) task. On each day of testing, 5 (38%) subjects were unable to complete the entire hour of centrifugation due to motion sickness, though median MISC scores never exceeded 2 (on a scale of 0–10). Motion sickness results showed trends of slower progression on day 2 relative to day 1. Subjects also had significant TW performance decrements following centrifugation (relative to baseline) on both day 1 (W=42, p=0.020) and day 2 (W=28, p=0.016) via Wilcoxon Signed Rank tests, though there was no significant difference between days in the decrements (W=18, p=0.211). When examining the relationship between the sensorimotor impairments observed, two sets of metrics showed evidence of a significant, positive correlation: overestimation during Hyper-gravity was correlated with overestimation during 1g Readaptation (r=0.67, p=0.013), and final motion sickness (MISC) scores were correlated with the delta between baseline and post-centrifugation TW scores (r=0.77, p=0.003). Taken together, the motion sickness and dynamic balance metrics captured here indicate that adaptation to altered gravity results in severe sensorimotor impairments. As can already be seen in these data, these metrics are likely related, warranting further exploration in a longer duration of exposure to altered gravity. This work contributes to the body of data that quantifies sensorimotor impairments driven by adaptation to altered gravity by exploring motion sickness and dynamic balance in a controlled sensory environment, offering insight into sensorimotor impairment during the early hours of the adaptation period.