When a sound source is visible and auditory cues conflict with visual cues, vision always predominates over audition. When a sound source presents no visible cues, the most important role of vision is in the monitoring of head movements. The visual field provides important feedback about the nature and degree of head movement so that deliberate disturbance to the visual field causes significant disruption in the ability to localize and invisible sound source. This is essential in ensuring the success of virtual reality. All the applications of the technology of television have been dominated by entertainment. Because of this, the vast majority of televised scenes have been of familiar landscapes; domestic rooms; churches; courtrooms and even the open prairie.

In each case, the viewer is able to bring a vast amount of his or her experience to bear in interpreting the missing third dimension. However, as applications of television broaden to include displays for molecular modeling programs for surgery for instance, the televised scenes become unfamiliar and interpretation of the depth cues become more difficult. Just as in the cinema, the human eye's persistence of vision means that we experience consecutive still images as a continuously flowing moving image. But, despite constant development in video display technology (better colour, higher definition images etc.

), all television systems present a completely 'flat' 2-D image. Therefore, it is essential to understand various techniques used by virtual reality engineers to ensure a realistic visual stimulation. Depth Enhancement Techniques Many applications of television technology, which would benefit from conventional 2-D display to 3-D systems, are deprived of this transformation due to other considerations. Nature has solved the lack of binocular vision in animals not equipped with two forward-facing eyes by furnishing them with a mechanism for extracting depth information from motion parallax cues.

Humans cannot do this without artificial aid and so require the assistance of video depth enhancement systems like the Pulfrich Effect and Depth Enhanced Vision (DEV). The Pulfrich Effect is named after a nineteenth century discoverer who noticed that if a neutral density filter is placed over one eye, a swinging pendulum appears to swing in an ellipse. Depending on the direction of motion, objects on a video screen also appear to recede into the screen or advance out of the screen. This effect is due to the filter that causes the brain to process two identical retinal images at different speeds, which results in motion parallax being misinterpreted by the nervous system. 8 The experimental depth enhancement system known as DEV for short with a built in laboratory prototype used a similar technique to generate a left eye image and a right eye image from an existing 2-D video picture. The prototyped DEV system utilized a two-monitoring polarizing display and passive glasses.

The only unique feature was that one of the screens was offset from the other by an amount so that the viewer's eyes, when viewing with the polarized spectacles, converged at a point at the front of the unit. This eradicated the impression of viewing the image 'down a tunnel'. 9.