The study examined the effects of varying the accommodation-convergence conflict created by stereoscopic displays which are now commonly used for the viewing of virtual environments, television and cinema. These displays will dissociate the naturally co-varying accommodation (focusing) and convergence (eye position) demands by placing an image geometrically behind or in front of the screen, and it has been suggested that the unnatural conflict between these demands will cause discomfort. Commercially available stereoscopic equipment was used to create a stimulus with four different levels of conflict, one of which was a control condition of zero conflict. Sixteen participants, each with normal visual systems, were presented with all four conditions in a balanced experimental design. The changes in visual discomfort, near heterophoria, distance heterophoria and visual acuity were assessed. Clear changes in comfort were observed, although no significant associated physiological changes were observed. The model which best describes the relationship between the conflict and the discomfort is one in which a small amount of conflict does not cause visual discomfort, whereas a larger amount will do so. This finding is consistent with expectations based on historical optometric experiments, which indicate that the normal visual system can maintain comfortable vision whilst experiencing small discrepancies between the accommodation and convergence demands. Our results indicate that visual discomfort occurs beyond a given conflict threshold and continues to rise as the conflict increases. They are consistent with the idea that this threshold is idiosyncratic to the individual. The principal implication of these findings is that people with normal visual systems should not experience asthenopic symptoms as a consequence of the accommodation-convergence conflict if the difference between the stimulus to each system is small.
['Peter A. Howarth', 'Pete Underwood']
The impact of viewing stereoscopic displays on the visual system