Visual and vestibular reweighting after cyber- and space-sickness.

Sensory conflicts are widely recognized as the primary drivers of motion sickness (MS), though the underlying integrative processes remain poorly understood. This study investigated sensory reweighting following exposure to two different sensory conflict paradigms. Visual and vestibular reflexes were assessed before and after sensory conflict. In the first paradigm, participants were exposed to a visuo-vestibular conflict using visually induced illusory motion (vection) in two environments in immersive virtual reality. In the second paradigm, vestibular conflict was induced by gravitational changes in parabolic flight. Semi-circular canal integration was measured via the vestibulo-ocular reflex (VOR) suppression task, while visual weight was assessed through optokinetic nystagmus (OKN). Our findings revealed that, following virtual reality exposure, VOR response decreased by 12%, indicating a reduced reliance on vestibular inputs. Conversely, after parabolic flight, OKN performance was diminished by 13%, indicating a diminished weight of visual inputs. These findings suggest that the sensory modality failing to detect the motion was considered less reliable and therefore assigned a reduced contribution during the integration process, regardless of its actual accuracy. Additionally, visual sensitivity was associated with increased susceptibility to cybersickness, whereas vestibular sensitivity seemed to correlate MS severity in parabolic flight. Altogether, our data suggest that the sensitivity of the most stimulated sensory modality during a sensory conflict may predict an individual's susceptibility to MS.