Vergence is a binocular eye-movement that directs the two eyes onto a target: you converge or diverge both eyes as the target moves closer/farther away. Accommodation is an adjustment of the focal power of the eye lens to acquire a clear and sharp retinal image: the focal power increases/decreases as a target moves closer or farther away. Because both mechanisms occur consistently in natural environments, the human visual system evolved to associate them neurologically: stimulating vergence will accompany an accommodative response, and vice versa.
In this sense, a stereoscopic 3D display stimulates vergence and accommodation in an unnatural way. The figure below compares the vergence and accommodative stimuli under three viewing conditions: natural viewing, wearing glasses, and stereoscopic 3D viewing. The left three panels illustrate the three viewing conditions, and the right panel plots the accommodative stimuli as a function of the vergence stimuli. Natural viewing stimulates consistent vergence and accommodation, as explained earlier. Optical corrections usually offset accommodative stimuli, thus shifting the stimulus graph without changing its slope. In case of stereoscopic 3D viewing, the vergence stimulus varies depending on the disparity of targets, while the accommodative stimulus is fixed at the screen distance. The presence of such conflicting stimuli is called vergence-accommodation conflict, making it more difficult for the visual system to respond quickly and accurately compared to natural, consistent vergence and accommodative stimuli.
About a hundred years ago, optometrists proposed a zone of comfort based on their clinical experiences. The zone of comfort is an area defined on a 2D space of vergence and accommodative stimuli. It usually contains the consistent match between vergence and accommodative stimuli (a 45-degree diagonal line), which means that the natural environment is comfortable to watch. However, as the graph above suggests, visual experience with eyewear differs from the experience of watching stereoscopic 3D. We performed a series of psychophysical experiments to assess the zone of comfort for watching stereoscopic 3D contents. Our measurements suggest that the uncrossed/crossed disparity is more comfortable when the viewing distance is close or far away. The zone was narrower at a far viewing distance but wider at a near viewing distance. The standard clinical measurements (Percival's zone of comfort, Sheard's zone of comfort, and phoria) were able to predict the zone of comfort for watching stereoscopic 3D. Percival's zone of comfort was better at a far viewing distance while phoria was better at a near viewing distance.
We also tested how temporal variation of vergence-accommodation conflict affects visual discomfort. Nature is full of fast moving objects, but we also need to stare at stationary targets for a long time. Thus it is natural that the visual system consists of a fast, responsive component and a slow, adapting component. Researchers have investigated dynamic models for vergence and accommodation, and the figure below shows an exemplary response to a step input. Here, the input is consistent (vergence and accommodation are stimulated by the same input) and goes from 1 D to 2 D at 2 sec. The phasic component is the fast, responsive component and the tonic component is the slow, adaptive component. Interestingly, the cross-link is known to be driven by the phasic component of the other mechanism (phasic vergence/accommodation drives the cross-link of accommodation/vergence). The overall response is the sum of the three components.
If the phasic component drives cross-link of the other mechanism, we can assume that more frequent changes in the vergence-accommodation conflict would cause more visual discomfort. We tested this hypothesis through a psychophysical experiment. In three viewing conditions, we varied the rate of temporal variation of the conflict while maintaining the same total conflict. The experimental results confirmed our hypothesis: vergence-accommodation conflict caused more severe visual discomfort when the conflict changes faster or more often.
Collaborator: Takashi Shibata, David Hoffman, David Kane, Martin Banks