versus impulsive angular accelerations in a rotating chair. This means that, to determine reliably an individual's susceptibility in a given situation, it may well be necessary to test him or her in that situation. The one exception to this generalization is that a small percentage of the population, perhaps 10 percent, seems highly susceptible in all motion situations. Such individuals have histories of persistent sickness in cars, boats, and other vehicles and show little change with repeated exposure.
The issue of adaptability, as well as retention and transfer of adaptation, can be as important as basic susceptibility in a given exposure situation. A person with high susceptibility who has rapid adaptation and abatement of symptoms, shows retention of adaptation between widely spaced exposure periods, and shows generalization of adaptation of motion sickness responses to other situations may be better suited for certain situations than an individual of moderate susceptibility who adapts slowly, has poor retention, and shows little transfer (Graybiel and Lackner, 1983). Adaptability and retention of adaptation have not been explored adequately, and only a few studies have even attempted to assess them (Graybiel and Lackner, 1983).
Adaptation to provocative motion can generally be enhanced if exposure is gradual and incremental in intensity. For example, adaptation to rotating environments can be achieved by initially exposing individuals to a very low rate of rotation, one that neither disrupts motor control nor elicits motion sickness, and having them make many head and body movements (Graybiel et al., 1968; Graybiel and Knepton, 1976). By repeating the movements after additional 1 rpm increases in velocity, it is possible to adapt individuals to quite high velocities of rotation without significant performance decrements and without eliciting motion sickness. If the final velocity, say, 10 rpm, were introduced in a single step, most individuals would be incapacitated by motion sickness and unable to adapt regardless of exposure duration. The principle of incremental exposure facilitating adaptation seems to be a general one applicable to all situations so far evaluated (Lackner, 1985; Lackner and Lebovits, 1978) and is likely to apply to VEs.
In thinking about the relevance of motion sickness in VEs, it is critical to remember that it is a complex syndrome with multiple etiological factors, the relative importance of which varies for different individuals and for different intensities of exposure (Kennedy et al., 1992). The presence of more than one eliciting factor (e.g., making head movements as well as looking at a moving visual display) is almost always synergistic in bringing on symptoms. Although nausea and vomiting are often viewed as the most severe manifestations of motion sickness, they can generally be dealt with using antimotion sickness drugs, and in cases of extreme sickness, drug injections (Graybiel and Lackner, 1987). From the standpoint of