Visually induced motion sickness

At times, people exposed to moving visual scenes may perceive themselves as moving even though they are, in fact, stationary. This sensation is sometimes experienced by people sitting in a railway carriage, in a station, when a neighbouring train slowly pulls away. Rather than sensing that the other train is leaving the station, they have the compelling feeling that their own train is moving in the opposite direction. This phenomenon, the feeling of moving brought about solely by a change in the visual scene, is called vection. Sustained exposure to moving visual scenes may not only produce vection, but can also provoke signs and symptoms of motion sickness such as dizziness, sweating, stomach awareness, and nausea and these adverse effects are now generally termed "visually induced motion sickness" (VIMS). VIMS is frequently reported in a variety of simulated or virtual environments such as flight and driving simulators, as well as in other contexts, such as at the cinema. It not only constitutes a nuisance to the user of these technologies, but also limits the usability of these technologies. Unlike other forms of motion sickness, such as seasickness, little is known about what conditions, or what aspects of moving visual scenes, are particularly provocative. Furthermore, research conducted thus far has generally investigated rotational motion patterns that are not representative of motion typically encountered in the real world. As a consequence, the work presented here has investigated the interrelationship between visual stimulus characteristics, VIMS, and vection during simulated forward and backward selfmotion (Le. along the fore-and-aft axis). In the first study, individuals were exposed to moving visual scenes that induced an illusion of motion in the fore-and-aft axis. These were presented either at a constant speed, or at a sinusoidally varying speed. Although varying the speed was expected to lead to higher levels of VIMS, this was not observed. The absence of an increased level of VIMS was hypothesised to be a consequence of the particular frequency employed (0.025 Hz). The frequency dependence of VI MS was then tested in a series of experiments. Noting that amplitude and acceleration covaried with frequency, it was found that within the range 0.025 - 1.6 Hz, VIMS peaked at 0.2 Hz. Using motion profiles with varying amplitude and acceleration, studies employing angular motion stimulation, on the other hand, had previously shown a peak in VIMS to occur at a frequency of approximately 0.06 Hz. This suggests that results obtained with angular motion stimulation cannot be extrapolated to scenarios involving linear motion stimulation in the fore-and-aft axis. The studies thus far isolated the effect of stimulus characteristics by preventing eye movements from occurring by means of fixation. A further study was conducted with the express purpose of investigating the effect of gaze shifting. It was found that the level of VIMS significantly increased with fixation away from the focus of expansion of a radial display. This suggests that the visual stimulus interacts differently with different portions of the retina. Real-world motion scenarios generally entail motion along different axes simultaneously. Most studies into VIMS have been restricted to single-axis motion and, although VIMS is assumed to increase with more complex motion scenarios, little is known about how VIMS changes with·increasing complexity. Comparing single- versus dual-axis motion, it was unexpectedly found that dualaxis motion did not lead to higher levels of VIMS, challenging the generally held assumption that VIMS is proportional to the degree of sensory conflict. The feasibility of predicting the incidence of VIMS based on an individual's motion sickness history as assessed by the revised Motion Sickness Susceptibility Questionnaire (MSSQ) was finally explored. Correlation coefficients were comparable to those observed with true motion suggestive of a common underlying mechanism between different forms of motion sickness. For the prediction of individual behaviour, the MSSQ was found to be of limited value in its current form. . A general finding was that vection consistently preceded the occurrence of VIMS, in line with the idea that vection is a necessary condition for VIMS to occur. This implies that future displays optimising the simulation of self-motion are likely to result in higher levels of VIMS. In addition, the findings that frequency, gaze direction, and multi-axis motion affected VIMS differently with simulated motion in the fore-and-aft axis as compared to angular motion profiles, indicate that angular motion commonly used to study VIMS may be of limited value.