are available for specifying the amount of sensorimotor adaptation that is achievable with different kinds of distortions using different types of training procedures. Similarly, no attention has been given in the research on adaptation to changes in resolution; attention has been focused almost exclusively on changes in response bias (i.e., the deviation between the mean response and the correct response). Furthermore, with only minor exceptions, interactions among different kinds of alterations (distortions, time delays, and noise), many of which are likely to be present simultaneously in SE systems, have been ignored. A few modality-specific comments on sensorimotor adaptation are available in Chapters 2, 3, and 4. Extensive further discussion of sensorimotor adaptation in the context of whole-body motion is available in Chapter 6.

DISCOMFORT

An important prerequisite for widespread use of SE systems is that they be comfortable for people to use. Independent of whether the discomfort caused by the system is most appropriately considered under the heading of "motion sickness," "poor ergonomics," or the ''sopite syndrome" (see Chapter 6 for a definition of this syndrome), such discomfort must be reduced sufficiently to permit individuals to make effective use of the system over extended periods of time. Despite significant previous research on some components of this problem, substantial further research in this area is warranted for a number of reasons.

First, as the situation now stands, discomfort is a real threat to the effective use of SEs. For example, quite apart from the deficiencies in currently available helmet-mounted displays with respect to the visual information provided, they tend to cause such a high degree of discomfort that daily long-term use seems almost out of the question. In fact, the combination of relatively limited visual information and relatively high discomfort is leading some individuals to seriously consider using off-head displays in their SE systems (discussion of both helmet-mounted displays and off-head displays is presented in Chapter 2).

Second, past work on the sources and effects of discomfort has not yet resulted in adequate understanding of the phenomena involved. More specifically, we do not yet know how the magnitude of each discomfort component depends on the characteristics of the system (the properties of the visual and auditory displays, the weight of the devices mounted on the head, the method by which movement through space is simulated, etc.) or on the characteristics of the individual user (including the user's prior experience with the system). Although some progress has been made in related areas (e.g., studies of motion sickness conducted in connection with flight simulators, sophisticated use of anthropometric measurements



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