This section is concerned with simulator sickness, an unfortunate side-effect of virtual interfaces mentioned in Chapter 1. The literature summarized here is relevant to the research reported in Chapter 6. This section borrows in part from .
Simulator sickness refers to the experience of malaise arising from exposure to computer-generated or computer-mediated environments. It is a major issue impeding the introduction of advanced interfaces for both non-inclusive simulators as well as inclusive virtual environments. Simulator sickness imposes limitations on the use of these interfaces for training and entertainment applications as well as raising liability concerns. An extensive survey of the literature on simulator sickness is available in . Also available are abstracts from a recent conference on motion sickness .
One can think of simulator sickness as having two components. The first arises from imperfections in the technology, such as lag or geometric distortions. This component might be termed ``interface sickness'' (see Appendix F for a discussion of this terminology). Interface sickness is a serious problem, but one which is likely to become less so as technology advances. For an extended discussion, including a technique for measuring the effect of display imperfections on the vestibulo-ocular reflex, see .
The second component of simulator sickness arises from the accurate presentation of stimuli which are inherently nauseogenic. This component is called ``motion sickness''. Thus, a part of simulator sickness is believed to be closely related to other types of motion sickness such as sea sickness, car sickness, and space sickness. The relationship between various forms of motion sickness is discussed in .
Motion sickness refers to a pattern of symptoms including nausea, headaches and disorientation. According to the standard sensory rearrangement theory, ``all situations which provoke motion sickness are characterized by a condition of sensory rearrangement in which the motion signals transmitted by the eyes, the vestibular system and the non-vestibular proprioceptors are at variance either with one another or with what is expected from previous experience'' [29,85,86].
Crampton has restated the sensory rearrangement theory in a manner which closely parallels an application of the rest frame construct to be introduced in Section 3.3.5. According to Crampton  (p. 30):
Animals have an orientation constancy such that brain mechanisms support a perception of up and down, right and left, of the stable environment in which they maneuver. This perception is a complex one that includes the distinction between the animal moving its eyes or body and the movement of objects in its space. The perception is supported by neural mechanism or memory that is continually updated or tested by vestibular, visual, auditory, chemical, kinesthetic, and somatosensory inputs that are often associated with motor activity such as visual search, limb movement, and locomotion. This orientation constancy may be extended to include geographical features important to migration and navigation.
In the course of a species' history, orientation constancy is assembled through selection over generations and then modified by each individual's experience until almost all combinations of inputs are interpreted such that the animal can effectively perform within its space environment to sustain, defend and reproduce its own kind. If a new sensory input is so discordant with the orientation constancy (a mismatch) that the performance of the animal is degraded, certain species display symptoms of lethargy, anorexia, salivation, and then vomit. In addition, man shows facial pallor and reports nausea.
In accordance with sensory rearrangement theory, motion sickness is likely to be an ever-increasing problem as virtual interfaces become more compelling. More compelling stimuli from the virtual interface cause the nervous system to give the stimuli more weight, increasing the sickness caused by any sensory conflicts.
Finally, while this dissertation is the first full-scale presentation of the rest frame construct and its implications, its parts have been aired previously and have begun to work their way into the literature. A group led by Deborah Harm at NASA has applied the ideas from Chapter 3 to develop the first successful predictor for space sickness based on perceptual style [34,33]. Astronauts were divided into two groups: those whose selected rest frame in weightlessness was determined more by the visual scene (VS), and those whose selected rest frame was determined more by their own body axis (IZ). ``It was found that orientation reference type had a significant effect, resulting in an estimated 3-fold increase in the expected motion sickness score on flight day 1 for VS astronauts. Estimated probabilities of no symptoms ranged from 0.46 (Flight Day 1) to 0.79 (Flight Day 7) for IZ astronauts, and from 0.31 (Flight Day 1) to 0.62 (Flight Day 7) for VS astronauts.''