Human Factors Issues in Virtual Environments: A Review of the Literature

In keeping with our promise earlier in this review, we summarize here the process by which we believe spatial cues are used for localizing a sound source in a free-field listening situation. We believe it entails two parallel processes: 1. The azimuth of the source is determined using differences in interaural time or interaural intensity, whichever is present. Wightman and colleagues (1989) believe the low-frequency temporal information is dominant if both are present. 2. The elevation of the source is determined from spectral shape cues. The received sound spectrum, as modified by the pinna, is in effect compared with a stored set of directional transfer functions. These are actually the spectra of a nearly flat source heard at various elevations. The elevation that corresponds to the best-matching transfer function is selected as the locus of the sound. Pinnae are similar enough between people that certain general rules (e.g. Blauert's boosted bands or Butler's covert peaks) can describe this process. Head motion is probably not a critical part of the localization process, except in cases where time permits a very detailed assessment of location, in which case one tries to localize the source by turning the head toward the putative location. Sound localization is only moderately more precise when the listener points directly toward the source. The process is not analogous to localizing a visual source on the fovea of the retina. Thus, head motion provides only a moderate increase in localization accuracy. Finally, current evidence does not support the view that auditory motion perception is anything more than detection of changes in static location over time.

Sensory substitution systems provide their users with environmental information through a human sensory channel (eye, ear, or skin) different from that normally used, or with the information processed in some useful way. We review the methods used to present visual, auditory, and modified tactile information to the skin. First, we discuss present and potential future applications of sensory substitution, including tactile vision substitution (TVS), tactile auditory substitution, and remote tactile sensing or feedback (teletouch). Next, we review the relevant sensory physiology of the skin, including both the mechanisms of normal touch and the mechanisms and sensations associated with electrical stimulation of the skin using surface electrodes (electrotactile (also called electrocutaneous) stimulation). We briefly summarize the information-processing ability of the tactile sense and its relevance to sensory substitution. Finally, we discuss the limitations of current tactile display technologies and suggest areas requiring further research for sensory substitution systems to become more practical.