obviously contains elements outside the domain of psychology, it is included here for convenience.

RESOLUTION, ILLUSIONS, AND INFORMATION TRANSFER

Perhaps the most obvious kinds of knowledge about human perception and performance that are needed to design cost-effective SE systems concern the resolution of the human's input and output systems and the way in which effective resolving power is changed as these systems are integrated with SE interface systems having various kinds of displays and controls. (The term resolution refers here to the ability to separate out and independently sense different signals as well as to detect small changes in isolated signals.) Given such knowledge, one can then examine implications for task performance for various types of tasks, and the cost-performance trade-offs for these tasks.

Knowledge of normal human resolving power on the input side, i.e., the sensory side, allows one to predict the display resolution beyond which finer resolution could not be perceived and would therefore be wasted. A similar statement holds for the output, i.e., control, side. Although knowledge of human resolving power in vision and audition is incomplete, it is sufficiently advanced to provide designers of SE systems with solid background for design choices. Areas in which current knowledge is considerably less adequate include both the input (sensory) side and the output (motor) side of the haptic system, as well as the ways in which performance is degraded when displays and controls (in any of the modalities) with less-than-human resolution are used. Information on resolution for specific modalities (e.g., vision) is provided in the chapters concerned with these modalities.

A further and related set of issues that is important to consider in the design of SE systems concerns perceptual illusions. Generally speaking, a given perception is thought of as illusory to the extent that it appears to be generated by a stimulus configuration that is different from the actual one. VEs themselves can be regarded as integrated sets of illusions. Detailed study of both intrasensory and intersensory illusions is important because, in many cases, the existence of illusions enables SE system design to be simplified and therefore to increase its cost-effectiveness. At the opposite end of the spectrum, the occurrence of unexpected illusions can seriously interfere with the expected performance of the system. Elicitation of motion sickness often involves the occurrence of illusions concerning the position, orientation, and movements of various portions of the body.

It is possible to regard certain types of illusions, such as the illusion of continuous motion that can be generated by sequences of static images at



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