performance were highly correlated, it still would not be clear that the concept of telepresence has practical significance. In particular, it is not clear that it would enable one to design better SE systems. In order for this to be the case, it would be necessary to show that models and measurements of telepresence can be usefully substituted for models and measurements of performance, or at the very least, that models and measurements of telepresence provide significant added value to the results that can be achieved solely through the use of models and measurements of performance.

ALTERATIONS OF SENSORIMOTOR LOOPS

In practically all SE systems, the human operator's normal sensorimotor loops will be altered by the presence of distortions, time delays, and noise (statistical variability) in the system. In many cases, such alterations will be introduced unintentionally and will degrade performance. For example, time delays may result from the need to communicate over long distances, and time delays, noise, and unwanted distortions may result from the inclusion of imperfect system components. In some cases, however, these alterations (specifically distortions) may be introduced intentionally in an attempt to achieve performance that is better than normal—for example, in a teleoperator system that incorporates a telerobot that is intentionally nonanthropomorphic. Because of the lack of isomorphism (i.e., structural and functional similarity) between the operator and the telerobot in such systems, the mapping between the human operator and the telerobot will necessarily result in altered sensorimotor loops for the operator. Similar conditions will exist in all virtual environment systems in which special features of the environment are artificially emphasized and unrealistic methods for interacting with this environment are employed in an effort to achieve superior task performance. Attempts to achieve improved resolution by magnification of perceptual cues represent only one line of investigation in this area.

Independent of the nature and origin of the alteration, in order for a system designer to predict the performance of a candidate system, theoretical models must be available for characterizing human responses to the alterations associated with the use of the system. Such models should be able to predict the effect of the alterations on such variables as simulator sickness (and also, perhaps, telepresence), as well as on objective task performance, and to describe how the various response components change over time due to sensorimotor adaptation and learning.

Although considerable work has been performed in this area (e.g., see the extensive review by Welch, 1978), there are as yet no adequate models available for predicting performance. For example, no adequate models



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