Virtual reality displays introduce spatial distortions that are very hard to correct because of the difficulty of precisely modelling the camera from the nodal point of each eye. How significant are these distortions for spatial perception in virtual reality? In this study, we used a helmet-mounted display and a mechanical head tracker to investigate the tolerance to errors between head motions and the resulting visual display. The relationship between the head movement and the associated updating of the visual display was adjusted by subjects until the image was judged as stable relative to the world. Both rotational and translational movements were tested, and the relationship between the movements and the direction of gravity was varied systematically. Typically, for the display to be judged as stable, subjects needed the visual world to be moved in the opposite direction to the head movement by an amount greater than the head movement itself, during both rotational and translational head movements, although a large range of movement was tolerated and judged as appearing stable. These results suggest that it not necessary to model the visual geometry accurately and suggest circumstances when tracker drift can be corrected by jumps in the display which will pass unnoticed by the user.

To enhance presence, facilitate sensory motor performance, and avoid disorientation or nausea, virtual-reality applications require the perception of a stable environment. End-end tracking latency (display lag) degrades this illusion of stability and has been identified as a major fault of existing virtual-environment systems. Oscillopsia refers to the perception that the visual world appears to swim about or oscillate in space and is a manifestation of this loss of perceptual stability of the environment. The effects of end-end latency and head velocity on perceptual stability in a virtual environment were investigated psychophysically. Subjects became significantly more likely to report oscillopsia during head movements when end-end latency or head velocity were increased. It is concluded that perceptual instability of the world arises with increased head motion and increased display lag. Oscillopsia is expected to be more apparent in tasks requiring real locomotion or rapid head movement.