SE system from hearing a door slam in the room that houses the SE system) or from the system itself (e.g., the helmet used for the visual display is too intrusive to be ignored).

A second set of such factors concerns the existence of user-predictable interactivity. Telepresence is likely to increase when the user's actions, and the consequences of these actions as represented by the subsequent stimuli sensed by the user (i.e., the feedback), constitute a rich and easily perceived and influenced interaction pattern. When the synthetic world is highly realistic, such conditions will be satisfied automatically. When it is unrealistic, the extent to which they are satisfied will depend on the extent to which the user can adapt to the new world. As the user adapts, the degree of telepresence (and the transparency of the interface) will generally tend to increase. The extent to which the user can adapt to the new world, however, will depend strongly on both the nature of this world and the nature of the user's exposure to it. If the world is incomprehensible (either because the relations between the user's actions and the effects of these actions are random or simply because they are so complicated that they appear random), adaptation will not occur. Further discussion of adaptation is presented in the next subsection.

A third set of such factors relates to higher-level, more cognitive features of telepresence and to similar experiences that occur outside the domain of SE technology. In general, the ability of humans to be transported into unreal worlds is the basis of most art, literature, theater, and entertainment, not to mention hypnosis and the use of hallucinogenic drugs. The extent to which consideration of these other forms of transportation will prove useful in the study of SE telepresence is not yet clear. It does appear likely, however, that the variation among individuals in their susceptibility to transportation by these other methods is likely to also occur with SE telepresence.

The question "What is telepresence good for?" has not yet been adequately answered. The interest in the concept of telepresence is due in part to intrinsic philosophical and scientific interest in issues concerned with reality and illusion. It is also due in part, however, to an implicit assumption that a high degree of telepresence is positively correlated with good performance. That this is not generally the case, however, can be easily demonstrated merely by noting that one of the primary motivations for the use of teleoperator systems in hazardous environments is to prevent the operator from experiencing noxious stimuli present in the real environment (i.e., reducing the sense of presence in the real environment). In general, the relationship between telepresence and performance has not yet been determined. Furthermore, even if telepresence were adequately defined in an operational sense, and even if it were determined using this operational definition that in most situations telepresence and



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