to be effective in an SE context, the sensory information need not exceed, and may often be less than, the sensory-processing capabilities of the average human observer. The latter observation is borne out by the experience of people with deficient vision (or hearing): people with many kinds of sensory disabilities can still experience both real and virtual environments (VEs). Indeed, perturbations of normal visual stimulation may be disturbing but not necessarily destructive of perception in either real or synthetic environments. When sensory conditions are maintained with minimal variation, various forms of adaptation can occur and can be used to advantage in overcoming technical limitations (Held and Durlach, 1991).

SE-Relevant Aspects of Visual System Organization

The superficial similarity between the human visual system and created imaging systems (such as a television camera) has the potential to misguide efforts to advance the state of the art of the visual channel of SE systems. One example of a fundamental difference between the design goals of the two systems lies in the manner in which images are collected. Created systems generally strive to collect a uniformly resolved image of a scene. For applications in which any area of the scene might be attended to (by the ultimate viewer), this is an appropriate goal. However, the ideal proximal stimulus in an SE system is one in which the information is presented in a manner that is complementary to the normal operation of the sense organ. In the case of the eye, an optimal system would not ignore the role of eye motions, the uneven distribution of photoreceptors in the retina, and the limitations of the eye's optics (Westheimer, 1986). Because the eyes are used to actively probe a scene (Gregory, 1991) in a way somewhat similar to an insect probing the world with antennae, the concentration of image quality at the center of the fixation point may be a highly appropriate use of resources. In a sense, the fovea is like the sensitive tip of a blind person's cane; the rest is just context.

Although these concepts have begun to be addressed by designers of eye-slaved displays, the design of the visual channel of SE systems is likely to benefit from more widespread conceptualization of the eye as an output device as well as an input device. There are a number of potential benefits of utilizing such fixation information. It can guide the allocation of resources by matching the information transfer rate to the receiver (e.g., high resolution can be limited to the fovea). It can reduce the computational burden of image generation in VE systems. The fixation point information collected can be used to adapt the information presented (in the case of teleoperator systems) or generated (in the case of VE systems) to more seamlessly match the interests of the viewer.

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