(McCauley, 1984; McCauley and Sharkey, 1992). Research should be directed toward identifying the factors that determine which SEs are especially provocative and how to minimize this while preserving the efficacy of the system. Mechanical factors, such as altered inertial loading of the head by HMDs, as well as sensory factors, need to be considered. Also, attention must be given to elements of the sopite syndrome that are more subtle then those usually associated with motion sickness.
Sensorimotor loops Many SE systems introduce distortions, time delays, gain changes, and statistical variability (noise) between voluntary movements and associated patterns of sensory feedback. Systematic research is necessary to determine the extent to which these factors degrade performance and the subjective state of the user. Acceptable tolerances should be determined for these factors, as well as for the extent to which sensory feedback across different modalities must be in temporal synchrony.
Multisensory and motor influences on orientation This is a critical research area for designing effective VEs that involve locomotion and haptic exploration. Very little is known at present about these influences, except that they are highly complex and pervasive. They are difficult to identify as such because so much of what we take for granted in our everyday activities, such as the perceptual stability of our environment and our bodies during movement, is due to their action.