Effects of unexpected visual motion on postural sway and motion sickness.

Motion sickness is thought to occur when the brain's assumptions about incoming sensory information do not match the actual signals received. These signals must involve the vestibular system for motion sickness to occur. In this paper, we describe an experiment in which subjects experienced unexpected visual motions, or perturbations, as they navigated a virtual environment (VE) while standing and wearing a head mounted display (HMD) or while viewing a monitor. We found that postural instability, as measured by a balance board, increased with time only when perturbations were present. HMD users exhibited greater sway when exposed to visual perturbations than did monitor users. Yet motion sickness increased only when an HMD was used and occurred with or without participants undergoing perturbations. These results suggest that the postural instability which is generated by unexpected visual perturbation does not necessarily increase the likelihood of motion sickness in a virtual environment.