Motion sickness is a common by-product of exposure to optical depictions of inertial motion. This phenomenon, called visually induced motion sickness (VIMS), has been reported in a variety of virtual environments, such as fixed-base flight and automobile simulation (Frank, Casali, & Wierwille, 1988; Regan & Price, 1994; Yoo, Lee, & Jones, 1997) and in a variety of nonvehicular simulations (DiZio & Lackner, 1992; Ellis, 1991). Improvements in simulation fidelity are associated with increases in the likelihood of sickness (Crowley, 1987; McGuiness, Bouwman, & Forbes, 1981; Miller & Goodson, 1960). The effectiveness of virtual environments and simulation systems, and their acceptance by users, can be reduced if they produce motion sickness (Biocca, 1992). This is true especially if sickness in simulations occurs in situations in which it does not occur in the simulated system. This problem provides a practical motivation for understanding visually induced motion sickness. Prevention of visually induced motion si ckness would be facilitated if objective measures could be developed to predict it and if the factors that cause it could be identified and eliminated.
Explanations of motion sickness typically have been grounded in the concept of sensory conflict (e.g., Oman, 1982; Reason, 1978; Reason & Brand, 1975). However, the sensory conflict theory of motion sickness has low predictive validity (Draper, Viirre, Gawron, & Furness, 2001; Stoffregen & Riccio, 1991), which reduces the extent to which this theory can guide the design of simulators and other virtual environments. The present study does not attempt to evaluate the sensory conflict theory of motion sickness; rather, one of our goals was to evaluate a new, alternative theory of motion sickness etiology.
Postural Sway and Imposed Vibration
The occurrence of motion sickness is influenced by the frequency of imposed oscillation. In laboratory studies, motion sickness occurs in the presence of imposed periodic motion at frequencies from 0.08 to 0.40 Hz (Guignard & McCauley, 1990; Lawther & Griffin, 1988). Motion at other frequencies produces little or no sickness, even with long exposure durations (Guignard & McCauley, 1990). These data are consistent with what is known about operational vehicles that are associated with motion sickness: Vibration or oscillation in this frequency range is characteristic of ships, trains, aircraft, and vehicular rides (Guignard & McCauley, 1990; Lawther & Griffin, 1988). The consistency of the laboratory and operational data might suggest that motion sickness is caused by motion in the 0.08- to 0.40-Hz range. However, the spectral power of normal standing sway is concentrated between 0.1 and 0.4 Hz (Bensel & Dzendolet, 1968), yet people are not sickened by their own postural sway. Thus it cannot be the case that vi bration in this frequency range is inherently nauseogenic.
Destabilization of Posture
Riccio and Stoffregen (1991) suggested that motion sickness results from instability in control of the posture of the body or its segments. They defined postural stability as "the state in which uncontrolled movements of the perception and action systems are minimized" (p. 202). This means that stability may be degraded rather than lost outright; there can be variation in the magnitude of instability, and instability can persist over long periods without necessarily leading to frank loss of control.
What could cause postural stability to be degraded? Stoffregen and Smart (1998) suggested that instability might occur when posture is controlled in the presence of imposed oscillations of a frequency between 0.08 and 0.40 Hz through a form of wave interference (Tipler, 1987). When independently generated waveforms interact, the resulting waveform is a function of the relative frequencies of the components. …