Turning green in microgravity: facial color changes as best physiological indicator of space sickness

ABSTRACT Motion sickness (MS) is commonly hypothesized to arise from sensory conflicts between incongruent sources of sensory information. Different types of sensory conflicts can induce MS, yet it remains unclear whether distinct contexts produce different physiological responses. Moreover, there is a lack of reliable objective predictors of MS, particularly for space motion sickness (SMS), which appears unrelated to motion sickness susceptibility on Earth. This study examined multiple physiological measures as potential objective markers of MS, including heart rate, blood pressure, salivary cortisol, skin conductance, skin surface temperature, and facial skin colorimetry. Subjective motion sickness severity and symptomatology were assessed using standardized questionnaires (SSQ, MSAQ, MSSQ). All measures were collected before and immediately after exposure to two sensory conflict paradigms: virtual reality (visuo-vestibular conflict) and parabolic flight (otolitho-canal conflict). Post-exposure, both paradigms were associated with increased cortisol, skin conductance, and skin greeness. Notably, increased skin greenness was associated with greater MS severity in parabolic flight and strongly correlated with subjective nausea ratings in both paradigms. Skin temperature and systolic blood were affected differently by VR and parabolic flight. No robust new physiological predictors of MS were identified. Overall, our findings suggest that facial skin color -particularly skin greenness- may serve as a simple, non-invasive, and reliable objective indicator of MS severity.