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Vergence-accommodation conflict

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Vergence-accommodation conflict, also known as VAC, or Accommodation-vergence conflict, occurs when your brain receives mismatching cues between the distance of a virtual 3D object (vergence), and the focusing distance (accommodation) required for the eyes to focus on that object. This occurs while looking at stereoscopic imagery, such as watching 3D TV or 3D cinema, as well as in all current, traditional HMDs.

Visual System Background

To understand vergence-accommodation conflict, it's important to first understand how the human visual system normally processes depth cues. The human visual system relies on multiple mechanisms to perceive depth and focus clearly on objects at varying distances.

Vergence

Vergence refers to the simultaneous movement of both eyes in opposite directions to maintain binocular vision. When looking at a nearby object, the eyes rotate toward each other (convergence). When looking at a distant object, the eyes rotate away from each other (divergence). The angle of vergence provides the brain with information about the distance to the object being viewed.[1]

Accommodation

Accommodation is the process by which the eye changes optical power to maintain a clear image of objects at different distances. This is achieved by the ciliary muscles adjusting the curvature of the crystalline lens within the eye. When viewing a near object, the ciliary muscles contract, allowing the lens to become more convex and increase its optical power. When viewing a distant object, the muscles relax, and the lens flattens.[2]

Natural Coupling

In natural viewing conditions, vergence and accommodation are neurologically coupled through the accommodation-convergence reflex. This coupling has developed through evolution and is reinforced throughout life. When the eyes converge to focus on a near object, accommodation automatically increases to focus the lens appropriately, and vice versa. This natural linkage helps make focusing quick and accurate in real-world environments.[3]

Causes in VR and AR Systems

Fixed Focal Planes

In traditional stereoscopic display technologies, including most current VR headsets, the virtual image is focused at a fixed depth away from the eyes (typically 1.5 to 2 meters), while the perceived depth of virtual objects, and thus the amount of eye convergence required, varies depending upon the content.[4] This creates a fundamental mismatch between these normally coupled visual processes.

Stereoscopic Presentation

Stereoscopy in VR and AR creates the perception of depth by presenting slightly different images to each eye, simulating binocular disparity. This effectively manipulates vergence while leaving accommodation unchanged, as the physical display remains at a fixed distance from the eyes.[5]

Display Optics

The optical design of HMDs typically uses lenses to place the virtual image at a comfortable viewing distance. These lenses help users focus on close physical screens, but they don't change the fact that all content appears at the same optical distance regardless of its virtual depth in the scene.[6]

Effects and Symptoms

Visual Fatigue

Visual fatigue is one of the most common symptoms of vergence-accommodation conflict. Users often report eye tiredness, discomfort, and reduced ability to focus after extended use of VR systems. This fatigue occurs because the visual system is continuously attempting to resolve the conflicting depth cues.[7]

Eyestrain

Eyestrain manifests as aching, burning, or tired eyes. The constant struggle between vergence and accommodation can lead to muscular strain in the eyes, particularly in the ciliary muscles responsible for accommodation and the extraocular muscles that control vergence.[8]

Reduced Visual Clarity

Users may experience temporary blurred vision or difficulty focusing, particularly when rapidly switching between virtual objects at different apparent depths. This occurs because the eye's natural focusing mechanism is being consistently overridden.[9]

Headaches

Headaches are commonly reported with prolonged exposure to vergence-accommodation conflicts, likely due to the sustained visual and cognitive effort required to process conflicting depth information.[10]

Reduced Performance

Some studies have shown that vergence-accommodation conflict can lead to reduced performance in visual tasks, including slower reaction times and decreased accuracy in depth judgment tasks.[11]

Aftereffects

The natural accommodation-vergence coupling will re-establish at some point after removing the HMD. Before this recoupling is complete, users might experience visual adaptation aftereffects, including temporary changes in their depth perception or focusing abilities in the real world.[12]

Measurement and Assessment

Zone of Comfort

Researchers have identified a "zone of comfort" within which vergence-accommodation conflicts are tolerable. This zone typically allows for a discrepancy between vergence and accommodation of approximately 0.5 to 1.0 diopters. Beyond this range, visual discomfort increases significantly.[13]

Objective Measurements

Objective measurements of vergence-accommodation conflict effects include:

- Accommodative response measurements using autorefractors - Eye tracking to measure vergence movements - Pupillometry to assess cognitive load and visual stress - Electroencephalography (EEG) to measure neural responses to visual conflicts[14]

Subjective Assessments

Subjective assessments commonly use validated questionnaires such as the Simulator Sickness Questionnaire (SSQ) or Visual Fatigue Scale (VFS) to quantify user discomfort and fatigue levels after exposure to VR/AR environments.[15]

Technological Solutions

Varifocal Displays

Varifocal displays dynamically adjust the focal distance of the virtual image to match the vergence distance. These systems typically use eye tracking to determine where the user is looking and then mechanically or optically adjust the display's focal distance accordingly.[16]

Light Field Displays

Light field displays present multiple focal planes simultaneously, allowing users to accommodate naturally to different depths. These displays attempt to recreate the full light field that would be produced by real objects, enabling both correct vergence and accommodation.[17]

Multifocal Displays

Multifocal displays present multiple focal planes at different distances simultaneously. By blending these planes appropriately, they can simulate continuous focal depth, allowing for more natural accommodation responses.[18]

Focal Surface Displays

Focal surface displays dynamically reshape the focal surface to match the 3D geometry of the virtual scene. This approach attempts to provide correct focus cues across the entire field of view simultaneously.[19]

Holographic Displays

True holographic displays recreate the wavefront of light as it would appear from real objects, theoretically solving the vergence-accommodation conflict entirely. However, practical implementations face significant technical challenges.[20]

Content Design Guidelines

Depth Budget Management

Depth budget refers to the range of virtual depths presented in a scene. Limiting this range can help reduce vergence-accommodation conflicts. Content creators are advised to keep most interactive elements within a comfortable depth range (typically within ±0.5 diopters of the display's focal distance).[21]

Focus Management

Strategic use of depth of field effects, motion blur, and other focus cues can guide user attention and potentially reduce the impact of vergence-accommodation conflicts by providing additional monocular depth cues.[22]

Comfortable Viewing Distances

Keeping critical virtual content at moderate distances (typically 1-3 meters virtual distance) can minimize vergence-accommodation conflicts, as this range often aligns better with the fixed focal distance of most displays.[23]

Clinical Implications

Individual Differences

Susceptibility to vergence-accommodation conflict varies significantly between individuals. Factors such as age, preexisting visual conditions, and the flexibility of one's visual system all contribute to how severely one experiences symptoms.[24]

Age-Related Considerations

Younger users typically have more robust accommodation ability (accommodative amplitude) and may therefore experience more severe conflicts. Conversely, older users with presbyopia (age-related loss of accommodation) may experience fewer symptoms because their visual system already relies less on accommodation cues.[25]

Pre-existing Conditions

Users with certain visual conditions such as strabismus, amblyopia, or accommodative insufficiency may have altered responses to vergence-accommodation conflicts, sometimes experiencing either reduced or exacerbated symptoms.[26]

Research and Future Directions

Adaptation Studies

Research has shown that some users can adapt to vergence-accommodation conflicts over time, with their visual systems learning to decouple these normally linked processes temporarily. Studies are ongoing to determine the extent, duration, and potential consequences of this adaptation.[27]

Perceptual Learning

Some researchers are investigating whether perceptual learning techniques could be employed to train users to better tolerate vergence-accommodation conflicts, potentially expanding the zone of comfort through systematic exposure.[28]

Hybrid Solutions

Research into hybrid solutions combines multiple approaches—such as eye tracking with partial correction of focus—to achieve practical implementations that balance technical feasibility with effective reduction of vergence-accommodation conflict.[29]

Practical Workarounds

Temporary Solutions

For users experiencing difficulty focusing on nearby virtual objects, closing one eye is an effective temporary solution. With one eye closed, there is no vergence signal, eliminating the conflict with accommodation. This technique is particularly useful when reading text or examining fine details in VR.[30]

Session Duration Management

Limiting VR/AR session duration and taking regular breaks allows the visual system to reset and can reduce the buildup of fatigue symptoms. Many practitioners recommend following the "20-20-20 rule": every 20 minutes, look at something 20 feet away for at least 20 seconds.[31]

Proper IPD Adjustment

Ensuring that the interpupillary distance (IPD) setting on the HMD matches the user's actual IPD can reduce unnecessary strain and improve comfort, though it doesn't directly solve the vergence-accommodation conflict.[32]

Conclusion

Vergence-accommodation conflict remains one of the most significant challenges in current VR and AR systems. While technological solutions are advancing rapidly, complete resolution of this conflict is still not widely implemented in consumer devices. Understanding the physiological basis, symptoms, and mitigation strategies for vergence-accommodation conflict is crucial for both developers creating VR/AR content and users seeking comfortable and effective immersive experiences. As display technologies continue to evolve, addressing this fundamental perceptual conflict will likely be a key factor in the wider adoption and sustained use of immersive technologies.

References

  1. ↑ Schor, C. M. (1986). Adaptive regulation of accommodative vergence and vergence accommodation. American Journal of Optometry and Physiological Optics, 63(8), 587-609.
  2. ↑ Ciuffreda, K. J. (1991). Accommodation and its anomalies. In Vision and visual dysfunction (Vol. 1, pp. 231-279). London: Macmillan.
  3. ↑ Fincham, E. F., & Walton, J. (1957). The reciprocal actions of accommodation and convergence. The Journal of Physiology, 137(3), 488-508.
  4. ↑ Kramida, G. (2016). Resolving the vergence-accommodation conflict in head-mounted displays. IEEE Transactions on Visualization and Computer Graphics, 22(7), 1912-1931.
  5. ↑ Hoffman, D. M., Girshick, A. R., Akeley, K., & Banks, M. S. (2008). Vergence–accommodation conflicts hinder visual performance and cause visual fatigue. Journal of Vision, 8(3), 33-33.
  6. ↑ Hua, H. (2017). Enabling focus cues in head-mounted displays. Proceedings of the IEEE, 105(5), 805-824.
  7. ↑ Shibata, T., Kim, J., Hoffman, D. M., & Banks, M. S. (2011). The zone of comfort: Predicting visual discomfort with stereo displays. Journal of Vision, 11(8), 11-11.
  8. ↑ Lambooij, M., Fortuin, M., Heynderickx, I., & IJsselsteijn, W. (2009). Visual discomfort and visual fatigue of stereoscopic displays: A review. Journal of Imaging Science and Technology, 53(3), 30201-1.
  9. ↑ Vilela, M. A. P., Pellanda, L. C., Fassa, A. G., & Castagno, V. D. (2015). Prevalence of asthenopia in children: a systematic review with meta-analysis. Jornal de Pediatria, 91(4), 320-325.
  10. ↑ Kooi, F. L., & Toet, A. (2004). Visual comfort of binocular and 3D displays. Displays, 25(2-3), 99-108.
  11. ↑ Wann, J. P., Rushton, S., & Mon-Williams, M. (1995). Natural problems for stereoscopic depth perception in virtual environments. Vision Research, 35(19), 2731-2736.
  12. ↑ Mon-Williams, M., Wann, J. P., & Rushton, S. (1993). Binocular vision in a virtual world: visual deficits following the wearing of a head-mounted display. Ophthalmic and Physiological Optics, 13(4), 387-391.
  13. ↑ Shibata, T., Kim, J., Hoffman, D. M., & Banks, M. S. (2011). The zone of comfort: Predicting visual discomfort with stereo displays. Journal of Vision, 11(8), 11-11.
  14. ↑ Banks, M. S., Kim, J., & Shibata, T. (2013). Insight into vergence-accommodation mismatch. In Stereoscopic Displays and Applications XXIV (Vol. 8648, p. 86480E). International Society for Optics and Photonics.
  15. ↑ Kennedy, R. S., Lane, N. E., Berbaum, K. S., & Lilienthal, M. G. (1993). Simulator sickness questionnaire: An enhanced method for quantifying simulator sickness. The International Journal of Aviation Psychology, 3(3), 203-220.
  16. ↑ Dunn, D., Tippets, C., Torell, K., Kellnhofer, P., Akşit, K., Didyk, P., Myszkowski, K., Luebke, D., & Fuchs, H. (2017). Wide field of view varifocal near-eye display using see-through deformable membrane mirrors. IEEE Transactions on Visualization and Computer Graphics, 23(4), 1322-1331.
  17. ↑ Huang, F. C., Chen, K., & Wetzstein, G. (2015). The light field stereoscope: immersive computer graphics via factored near-eye light field displays with focus cues. ACM Transactions on Graphics (TOG), 34(4), 1-12.
  18. ↑ Mercier, O., Sulai, Y., Mackenzie, K., Zannoli, M., Hillis, J., Nowrouzezahrai, D., & Lanman, D. (2017). Fast gaze-contingent optimal decompositions for multifocal displays. ACM Transactions on Graphics (TOG), 36(6), 1-15.
  19. ↑ Matsuda, N., Fix, A., & Lanman, D. (2017). Focal surface displays. ACM Transactions on Graphics (TOG), 36(4), 1-14.
  20. ↑ Maimone, A., Georgiou, A., & Kollin, J. S. (2017). Holographic near-eye displays for virtual and augmented reality. ACM Transactions on Graphics (TOG), 36(4), 1-16.
  21. ↑ Cutting, J. E., & Vishton, P. M. (1995). Perceiving layout and knowing distances: The integration, relative potency, and contextual use of different information about depth. In W. Epstein & S. Rogers (Eds.), Perception of space and motion (pp. 69-117). Academic Press.
  22. ↑ Vinnikov, M., Allison, R. S., & Fernandes, S. (2016). Impact of depth of field simulation on visual fatigue: Who are impacted? and how? International Journal of Human-Computer Studies, 91, 37-51.
  23. ↑ Padmanaban, N., Konrad, R., Stramer, T., Cooper, E. A., & Wetzstein, G. (2017). Optimizing virtual reality for all users through gaze-contingent and adaptive focus displays. Proceedings of the National Academy of Sciences, 114(9), 2183-2188.
  24. ↑ Read, J. C., & Bohr, I. (2014). User experience while viewing stereoscopic 3D television. Ergonomics, 57(8), 1140-1153.
  25. ↑ Johnson, P. V., Kim, J., & Banks, M. S. (2016). The visibility of the depth cue defocus blur is affected by the accommodation-vergence response. Journal of Vision, 16(6), 22-22.
  26. ↑ Bando, T., Iijima, A., & Yano, S. (2012). Visual fatigue caused by stereoscopic images and the search for the requirement to prevent them: A review. Displays, 33(2), 76-83.
  27. ↑ Vienne, C., Sorin, L., Blondé, L., Huynh-Thu, Q., & Mamassian, P. (2014). Effect of the accommodation-vergence conflict on vergence eye movements. Vision Research, 100, 124-133.
  28. ↑ Kim, J., Kane, D., & Banks, M. S. (2014). The rate of change of vergence–accommodation conflict affects visual discomfort. Vision Research, 105, 159-165.
  29. ↑ Konrad, R., Padmanaban, N., Molner, K., Cooper, E. A., & Wetzstein, G. (2016). Accommodation-invariant computational near-eye displays. ACM Transactions on Graphics (TOG), 36(4), 1-12.
  30. ↑ Kreylos, O. (2016). Accommodation and Vergence in Head-mounted Displays. Doc-Ok.org.
  31. ↑ Yan, Z., Hu, L., Chen, H., & Lu, F. (2008). Computer vision syndrome: A widely spreading but largely unknown epidemic among computer users. Computers in Human Behavior, 24(5), 2026-2042.
  32. ↑ Ippolito, B. L., & Jones, K. S. (2020). Interpupillary distance and visual discomfort in head-mounted displays. Applied Ergonomics, 88, 103149.
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