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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 [[HMD]]s.
'''Vergence-accommodation conflict''' ('''VAC'''), also known as '''accommodation-vergence conflict''' or sometimes '''accommodation–vergence mismatch''', is a visual and perceptual phenomenon that occurs when the [[Brain|brain]] receives mismatching cues between the distance to which the eyes are pointed or converged ([[Vergence|vergence]]) and the distance at which the eyes' lenses are focused ([[Accommodation (eye)|accommodation]]).<ref name="Hoffman2008">{{cite journal |last=Hoffman |first=D.​M. |last2=Girshick |first2=A. R. |last3=Akeley |first3=K. |last4=Banks |first4=M. S. |title=Vergence–Accommodation Conflicts Hinder Visual Performance and Cause Visual Fatigue |journal=Journal of Vision |volume=8 |issue=3 |pages=33 |year=2008 |doi=10.1167/8.3.33 |pmid=18481983 |url=https://jov.arvojournals.org/article.aspx?articleid=2192424}}</ref><ref name="Kreylos2014VAC">{{cite web |last=Kreylos |first=Oliver |title=Accommodation and Vergence in Head-mounted Displays |url=http://doc-ok.org/?p=1602 |website=Doc-Ok.org |date=2014-04-13 |access-date= [Insert Access Date Here] }}</ref> Because natural viewing conditions tightly couple these two mechanisms, breaking that link is a primary cause of visual discomfort and performance issues in modern [[Virtual reality|virtual reality]] (VR), [[Augmented reality|augmented reality]] (AR), and other [[Stereoscopy|stereoscopic]] 3-D displays, including nearly all mainstream [[Head-Mounted Display|head-mounted displays]] (HMDs).<ref name="Hoffman2008" />


== Visual System Background ==
==Physiological Basis==
When fixating on an object in the real world, the human [[Visual system|visual system]] simultaneously performs two key actions:
*  '''[[Vergence]]''': The two eyes rotate inwards ([[Convergence (eye)|convergence]]) or outwards ([[Divergence (eye)|divergence]]) via the [[Extraocular muscles|extraocular muscles]] so their lines of sight intersect at the target object, enabling single [[Binocular vision|binocular vision]]. This response is primarily driven by [[Binocular disparity|retinal disparity]].
*  '''[[Accommodation (eye)|Accommodation]]''': The [[Ciliary muscle|ciliary muscle]] adjusts the shape and thus the [[Optical power|optical power]] of the [[Crystalline lens|crystalline lens]] within each eye to bring the image of the target object into sharp focus on the [[Retina|retina]]. This response is primarily driven by retinal blur.


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.
In natural vision, these two systems are tightly linked through fast, reciprocal neurological signals known as the [[Accommodation reflex|accommodation–vergence reflex]].<ref name="Kreylos2014VAC" /><ref name="Kramida2016">{{cite journal |last=Kramida |first=G. |title=Resolving the Vergence–Accommodation Conflict in Head-Mounted Displays |journal=IEEE Transactions on Visualization and Computer Graphics |volume=22 |issue=7 |pages=1912–1931 |year=2016 |doi=10.1109/TVCG.2015.2473859 |pmid=26353219}}</ref> This coupling ensures that the eyes focus at the same distance they are pointed, allowing for clear, comfortable, and efficient vision. Stereoscopic displays disrupt this natural coupling because binocular disparity cues drive the vergence system to the ''simulated'' depth of a virtual object, while the accommodation system is driven by blur cues to focus on the ''physical'' display surface, which is typically at a fixed optical distance.<ref name="Kramida2016Resolving">{{cite web |last=Kramida |first=Gregory |last2=Varshney |first2=Amitabh |title=Resolving the Vergence-Accommodation Conflict in Head Mounted Displays |url=https://www.cs.umd.edu/sites/default/files/scholarly_papers/Kramidarev.pdf |website=Department of Computer Science, University of Maryland |year=2016 |access-date= [Insert Access Date Here] }}</ref>


=== Vergence ===
==Causes / Occurrence in Display Technologies==
The vergence-accommodation conflict is inherent in display technologies where the perceived depth of content differs from the physical or optical distance of the display surface:


[[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.<ref>Schor, C. M. (1986). Adaptive regulation of accommodative vergence and vergence accommodation. American Journal of Optometry and Physiological Optics, 63(8), 587-609.</ref>
*  '''Fixed-focus HMDs''': Nearly all consumer VR and many AR headsets use lenses to place a virtual image of the display screens at a fixed focal distance, typically between 1.3 and 2 meters (though this varies).<ref name="Kreylos2013HMD">{{cite web |last=Kreylos |first=Oliver |title=Head-mounted Displays and Lenses |url=http://doc-ok.org/?p=1360 |website=Doc-Ok.org |date=2013-07-24 |access-date= [Insert Access Date Here] }}</ref> Viewers must accommodate to this fixed plane to see a sharp image. However, stereoscopic rendering creates virtual objects that appear at various depths, requiring vergence changes. Objects rendered virtually nearer than the fixed focal plane induce a ''positive VAC'' (eyes converge more than they accommodate), while objects rendered virtually farther induce a ''negative VAC'' (eyes converge less than they accommodate).<ref name="Shibata2011">{{cite journal |last=Shibata |first=T. |last2=Kim |first2=J. |last3=Hoffman |first3=D. M. |last4=Banks |first4=M. S. |title=The Zone of Comfort: Predicting Visual Discomfort With Stereo Displays |journal=Journal of Vision |volume=11 |issue=8 |pages=11 |year=2011 |doi=10.1167/11.8.11 |pmid=21841115 |url=https://jov.arvojournals.org/article.aspx?articleid=2192969}}</ref>
*  '''[[3D television|3D Cinema and Television]]''': VAC also occurs here, but symptoms are often milder. The screen is typically farther away, the [[Field of view|field of view]] is smaller, and content creators can limit disparities to keep virtual objects within a "zone of comfort" relative to the screen distance.<ref name="ISO2015">{{cite standard |title=Ergonomics of human-system interaction — Part 392: Ergonomic requirements for the reduction of visual fatigue from stereoscopic images |standard=ISO 9241-392:2015 |publisher=International Organization for Standardization |year=2015}}</ref>
*  '''[[Optical see-through display|Optical See-Through (OST) AR]]''': In OST AR glasses, virtual images (often at a fixed focus) are overlaid onto the real world. This creates a conflict not only between vergence and accommodation for virtual objects but also a potential mismatch between focusing on real-world objects at various distances and the fixed focus of the virtual overlay. This can introduce depth discontinuities, reduce the perceived registration accuracy of virtual objects, and cause discomfort.<ref name="Zhou2021">{{cite journal |last=Zhou |first=Y. |last2=Li |first2=X. |last3=Yuan |first3=C. |title=Vergence-Accommodation Conflict in Optical See-Through Display: Review and Prospect |journal=Results in Optics |volume=5 |pages=100160 |year=2021 |doi=10.1016/j.rio.2021.100160}}</ref>


=== Accommodation ===
==Effects and Symptoms==
The sustained conflict between vergence and accommodation forces the visual system and brain to work harder, potentially leading to a range of negative effects:<ref name="Kramida2016Resolving" /><ref name="Hoffman2008" />
*  '''[[Visual fatigue]] / Eyestrain''': Tired, aching, or burning eyes resulting from the prolonged effort to resolve conflicting cues.
*  '''[[Headache]]s'''.
*  '''Blurred Vision''': Difficulty maintaining sharp focus on virtual objects, especially those perceived as very near or very far relative to the display's fixed focal plane.
*  '''[[Diplopia]] (Double Vision)''': Incorrect vergence responses due to the conflict can sometimes lead to seeing double images.
*  '''Focusing Problems''': Difficulty rapidly refocusing between virtual objects at different apparent depths because the natural reflex is disrupted. Users may also experience lingering focus issues or unusual visual sensations after removing the HMD.
*  '''[[Virtual Reality Sickness|VR Sickness]] / Discomfort''': VAC is considered a significant contributor to symptoms like nausea, dizziness, and general discomfort associated with VR/AR use.
*  '''Reduced Visual Performance''': Measurable degradation in tasks requiring fine depth judgments, reduced reading speed, slower visuomotor reaction times, and increased time required to fuse binocular images.<ref name="Hoffman2008" /><ref name="Lin2022">{{cite journal |last=Lin |first=C-J. |last2=Chi |first2=C-F. |last3=Lin |first3=C-K. |last4=Chang |first4=E-C. |title=Effects of Virtual Target Size, Position and Parallax on Vergence–Accommodation Conflict as Estimated by Actual Gaze |journal=Scientific Reports |volume=12 |pages=20100 |year=2022 |doi=10.1038/s41598-022-24450-9 |pmid=36418601 |pmc=9684603}}</ref>
*  '''[[Focal Rivalry]]''': Particularly in AR, the conflict between focusing on a real-world object and a virtual object projected at a different focal distance can make it difficult or impossible to see both sharply simultaneously.


[[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.<ref>Ciuffreda, K. J. (1991). Accommodation and its anomalies. In Vision and visual dysfunction (Vol. 1, pp. 231-279). London: Macmillan.</ref>
The severity of these symptoms varies significantly between individuals and depends on factors such as the magnitude of the VAC (the difference between vergence and accommodation distances), the duration of exposure, the nature of the visual content, and individual visual capabilities. Conflicts below approximately 0.4 to 0.6 [[Diopter|diopters]] are often tolerated, but larger conflicts, especially for near-field virtual objects (within arm's reach), become increasingly problematic.<ref name="Shibata2011" /><ref name="ISO2015" />


=== Natural Coupling ===
==Measurement==
VAC can be quantified by comparing the optical power (measured in [[Diopter|diopters]], D, which is the reciprocal of distance in meters) required for accommodation versus the optical power corresponding to the vergence distance.<ref name="Kramida2016Resolving" />
*  `VAC (Diopters) = | (1 / Accommodation Distance (m)) - (1 / Vergence Distance (m)) |`


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.<ref>Fincham, E. F., & Walton, J. (1957). The reciprocal actions of accommodation and convergence. The Journal of Physiology, 137(3), 488-508.</ref>
For example, if an HMD has a fixed focus set to 2 meters (requiring 1/2.0 = 0.5 D of accommodation) and displays a virtual object that appears to be 0.5 meters away (requiring 1/0.5 = 2.0 D of vergence), the VAC magnitude is |0.5 D - 2.0 D| = 1.5 D.


== Causes in VR and AR Systems ==
==Mitigation Strategies==
Addressing VAC is a major focus of VR and AR research and development. Strategies fall into two main categories: content design and technological solutions.


=== Fixed Focal Planes ===
===Content and Interaction Guidelines===
Careful design can minimize VAC-induced discomfort in fixed-focus displays:
#  '''Limit Depth Range''': Keep critical interactive content and prolonged visual targets within the "zone of comfort," typically corresponding to less than ~0.6 D of VAC for foreground objects (closer than the focal plane) and ~1.0 D for background objects (farther than the focal plane).<ref name="ISO2015" /><ref name="Shibata2011" />
#  '''Avoid Rapid Depth Changes''': Avoid sudden disparity jumps (> 1 D) or rapid oscillations in depth for prominent objects. Allow the visual system time (at least 500 ms) to adjust to significant depth changes.<ref name="Kramida2016" />
#  '''Optimize UI Placement''': Present user interface elements, text, and critical information at or slightly behind the display's native focal plane where VAC is zero or negative (which is generally better tolerated).<ref name="Shibata2011" />
#  '''Simulate Blur''': When hardware cannot provide correct focus cues, incorporate [[Gaze-contingent display|gaze-contingent]] [[Depth of field|depth-of-field]] rendering (simulating blur for objects not being looked at) to provide [[Monocular cues|monocular]] depth information that aligns better with vergence, potentially reducing cue conflicts.<ref name="Koulieris2017">{{cite conference |last=Koulieris |first=G-A. |last2=Buhler |first2=K. |last3=Drettakis |first3=G. |last4=Banks |first4=M. S. |title=Accommodation and Comfort in Head-Mounted Displays |booktitle=ACM SIGGRAPH 2017 Courses |year=2017 |doi=10.1145/3084873.3084901}}</ref>


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.<ref>Kramida, G. (2016). Resolving the vergence-accommodation conflict in head-mounted displays. IEEE Transactions on Visualization and Computer Graphics, 22(7), 1912-1931.</ref> This creates a fundamental mismatch between these normally coupled visual processes.
===Technological Solutions===
These hardware approaches aim to create displays where the accommodation distance can dynamically match the vergence distance demanded by the virtual content:
{| class="wikitable plainrowheaders"
! Approach !! Principle !! Representative prototypes / Research !! Strengths / Limitations
|-
! [[Varifocal display|Varifocal]]
| [[Eye tracking|Eye-tracking]] determines the user's gaze depth, and the display system adjusts a single focal plane to match that depth using [[Tunable lens|tunable lenses]] (e.g., liquid crystal, liquid lens, Alvarez) or mechanically moving components (screen or lens). | Meta Reality Labs Butterscotch Varifocal (2023);<ref name="DisplayDaily2023">{{cite web |title=Meta’s Going to SIGGRAPH 2023 and Showing Flamera and Butterscotch VR Technologies |url=https://displaydaily.com/metas-going-to-siggraph-2023-and-showing-flamera-and-butterscotch-vr-technologies/ |website=Display Daily |date=2023-08-04}}</ref> UNC Wide-FOV deformable-mirror NED.<ref name="Dunn2017">{{cite journal |last=Dunn |first=D. |last2=Tippets |first2=C. |last3=Torell |first3=K. |last4=Kellnhofer |first4=P. |last5=Akşit |first5=K. |last6=Didyk |first6=P. |last7=Myszkowski |first7=K. |last8=Luebke |first8=D. |last9=Fuchs |first9=H. |title=Wide Field-of-View Varifocal Near-Eye Display Using See-Through Deformable Membrane Mirrors |journal=IEEE Transactions on Visualization and Computer Graphics (TVCG) |volume=23 |issue=4 |pages=1322–1331 |year=2017 |doi=10.1109/TVCG.2017.2657058 |pmid=28113666}}</ref> | Delivers correct focus cue at the depth of fixation. Challenges include eye-tracking latency and accuracy, depth switching speed, limited depth range, and potentially incorrect blur cues for objects not at the fixation depth.<ref name="UNC2019">{{cite web |title=Dynamic Focus Augmented Reality Display |url=https://telepresence.web.unc.edu/research/dynamic-focus-augmented-reality-display/ |website=UNC Graphics and Virtual Reality Group |year=2019 |access-date=[Insert Access Date Here]}}</ref>
|-
! [[Multifocal display|Multifocal / Multiplane]]
| Presents images on several fixed focal planes simultaneously (e.g., using stacked LCDs, beam splitters) or time-sequentially. Content is rendered on the plane closest to its virtual depth. | Stanford light-field HMD research;<ref name="Wired2015">{{cite news |last=Zhang |first=S. |title=The Obscure Neuroscience Problem That’s Plaguing VR |work=Wired |date=2015-08-11 |url=https://www.wired.com/2015/08/obscure-neuroscience-problem-thats-plaguing-vr/}}</ref> Magic Leap 1 (2 planes). | Provides more correct focus cues across multiple depths simultaneously without necessarily requiring eye-tracking. Challenges include complexity, cost, reduced brightness/contrast, potential visible transitions between planes, and limited number of planes.
|-
! [[Light field display|Light Field]]
| Attempts to reconstruct the 4D light field of the scene (rays of light with position and direction). This allows the eye's lens to naturally focus at different depths within the reproduced volume. | Research using lenslet arrays, parallax barriers, holographic optical elements, super-multi-view displays. | Potentially provides true continuous focus cues without eye-tracking. Challenges include extremely high resolution and bandwidth requirements, computational complexity, limited field of view, and tradeoffs between spatial and angular resolution.
|-
! [[Holography|Holographic Displays]]
| Aims to fully reconstruct the wavefront of light from the virtual scene using diffraction patterns generated by [[Spatial light modulator|spatial light modulators]]. | Research by Microsoft Research, VividQ, Light Field Lab. | Theoretically the ultimate solution, providing all depth cues including accommodation correctly. Challenges include high computational cost ("speckle" noise), limited field of view, and hardware complexity for real-time, high-quality HMDs.
|-
! [[Retinal projection|Retinal Projection / Scanning]]
| Scans modulated light (often laser) directly onto the retina, potentially creating an image that is always in focus regardless of the eye's accommodation state (Maxwellian view). | Research systems; formerly North Focals (acquired by Google). | Can bypass VAC by eliminating the need for accommodation. Challenges include small [[Eyebox|eyebox]], potential for visual artifacts (e.g., [[Floater|floaters]] becoming more visible), safety concerns, and achieving high resolution/FOV.
|-
! Emerging Optics
| Novel optical components like Alvarez freeform lenses,<ref name="Liu2024">{{cite journal |last=Liu |first=Y. |last2=Cheng |first2=D. |last3=Wang |first3=Y. |last4=Hua |first4=H. |title=A Varifocal Augmented-Reality Head-Up Display Using Alvarez Freeform Lenses |journal=Journal of the Society for Information Display |volume=32 |issue=4 |pages=272-282 |year=2024 |doi=10.1002/jsid.1286}}</ref> tunable fluidic lenses, and deformable membranes are being explored for compact, low-power varifocal or multifocal elements. | Primarily research stage. | Aim for integration into smaller form factors. Manufacturing challenges, response time, optical quality, and control complexity remain active research areas.
|}


=== Stereoscopic Presentation ===
==Temporary Workarounds / Adaptation==
*  '''Closing One Eye''': Viewing with only one eye eliminates [[Binocular disparity|binocular disparity]] cues, thereby removing the vergence signal and the conflict. This can sometimes make it easier to focus on virtual objects, particularly near ones, at the cost of losing stereoscopic depth perception.<ref name="Kreylos2014VAC" />
*  '''Optimal Optical Correction''': Ensuring users wear their correct [[Eyeglass prescription|prescription glasses]] or [[Contact lens|contact lenses]] minimizes any additional strain on the visual system from uncorrected [[Refractive error|refractive errors]].
*  '''Adaptation''': The visual system can exhibit some adaptation to VAC during prolonged use, temporarily weakening the coupling between vergence and accommodation.<ref name="Kreylos2014VAC" /> However, this adaptation might be slow, incomplete, and can lead to the lingering aftereffects mentioned earlier when returning to natural viewing conditions.


[[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.<ref>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.</ref>
==Current Research Frontiers==
*  '''High-Resolution Varifocal Displays''': Prototypes like Meta’s Butterscotch demonstrate progress towards retinal resolution (e.g., 60 pixels per degree) combined with reasonably fast depth switching, suggesting potential commercial viability.<ref name="DisplayDaily2023" />
*  '''Focus-Correct Passthrough AR''': Integrating varifocal or multifocal optics into [[Video passthrough|video-see-through]] AR systems to correctly render both real-world and virtual imagery at appropriate focal depths.<ref name="UNC2019" />
*  '''Standards and Health Implications''': Ongoing work by standards bodies (e.g., ISO TC159, IEC TC100) to develop guidelines for extended VR/AR use, particularly concerning children and workplace applications.
*  '''Perceptual Modeling''': Research using large-sample studies to better understand individual variability in the accommodation-vergence relationship, potentially enabling personalized comfort settings or adaptive display parameters.<ref name="Lin2022" />


=== Display Optics ===
==See Also==
*  [[Accommodation (eye)|Accommodation]]
*  [[Depth perception]]
*  [[Eye tracking]]
*  [[Head-Mounted Display]]
*  [[Light field display]]
*  [[Stereoscopy]]
*  [[Varifocal display]]
*  [[Vergence]]
*  [[Visual fatigue]]
*  [[Virtual Reality Sickness]]


The [[optical design]] of [[HMD]]s 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.<ref>Hua, H. (2017). Enabling focus cues in head-mounted displays. Proceedings of the IEEE, 105(5), 805-824.</ref>
==References==
 
<references />
== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>Kooi, F. L., & Toet, A. (2004). Visual comfort of binocular and 3D displays. Displays, 25(2-3), 99-108.</ref>
 
=== 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.<ref>Wann, J. P., Rushton, S., & Mon-Williams, M. (1995). Natural problems for stereoscopic depth perception in virtual environments. Vision Research, 35(19), 2731-2736.</ref>
 
=== 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.<ref>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.</ref>
 
== 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.<ref>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.</ref>
 
=== 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<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>Matsuda, N., Fix, A., & Lanman, D. (2017). Focal surface displays. ACM Transactions on Graphics (TOG), 36(4), 1-14.</ref>
 
=== 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.<ref>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.</ref>
 
== 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).<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
== 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.<ref>Read, J. C., & Bohr, I. (2014). User experience while viewing stereoscopic 3D television. Ergonomics, 57(8), 1140-1153.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>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.</ref>
 
== 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.<ref>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.</ref>
 
=== 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.<ref>Kim, J., Kane, D., & Banks, M. S. (2014). The rate of change of vergence–accommodation conflict affects visual discomfort. Vision Research, 105, 159-165.</ref>
 
=== 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.<ref>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.</ref>
 
== 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.<ref>Kreylos, O. (2016). Accommodation and Vergence in Head-mounted Displays. Doc-Ok.org.</ref>
 
=== 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.<ref>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.</ref>
 
=== 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.<ref>Ippolito, B. L., & Jones, K. S. (2020). Interpupillary distance and visual discomfort in head-mounted displays. Applied Ergonomics, 88, 103149.</ref>
 
== 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 ==
<references/>


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Revision as of 04:16, 27 April 2025

Vergence-accommodation conflict (VAC), also known as accommodation-vergence conflict or sometimes accommodation–vergence mismatch, is a visual and perceptual phenomenon that occurs when the brain receives mismatching cues between the distance to which the eyes are pointed or converged (vergence) and the distance at which the eyes' lenses are focused (accommodation).[1][2] Because natural viewing conditions tightly couple these two mechanisms, breaking that link is a primary cause of visual discomfort and performance issues in modern virtual reality (VR), augmented reality (AR), and other stereoscopic 3-D displays, including nearly all mainstream head-mounted displays (HMDs).[1]

Physiological Basis

When fixating on an object in the real world, the human visual system simultaneously performs two key actions:

In natural vision, these two systems are tightly linked through fast, reciprocal neurological signals known as the accommodation–vergence reflex.[2][3] This coupling ensures that the eyes focus at the same distance they are pointed, allowing for clear, comfortable, and efficient vision. Stereoscopic displays disrupt this natural coupling because binocular disparity cues drive the vergence system to the simulated depth of a virtual object, while the accommodation system is driven by blur cues to focus on the physical display surface, which is typically at a fixed optical distance.[4]

Causes / Occurrence in Display Technologies

The vergence-accommodation conflict is inherent in display technologies where the perceived depth of content differs from the physical or optical distance of the display surface:

  • Fixed-focus HMDs: Nearly all consumer VR and many AR headsets use lenses to place a virtual image of the display screens at a fixed focal distance, typically between 1.3 and 2 meters (though this varies).[5] Viewers must accommodate to this fixed plane to see a sharp image. However, stereoscopic rendering creates virtual objects that appear at various depths, requiring vergence changes. Objects rendered virtually nearer than the fixed focal plane induce a positive VAC (eyes converge more than they accommodate), while objects rendered virtually farther induce a negative VAC (eyes converge less than they accommodate).[6]
  • 3D Cinema and Television: VAC also occurs here, but symptoms are often milder. The screen is typically farther away, the field of view is smaller, and content creators can limit disparities to keep virtual objects within a "zone of comfort" relative to the screen distance.[7]
  • Optical See-Through (OST) AR: In OST AR glasses, virtual images (often at a fixed focus) are overlaid onto the real world. This creates a conflict not only between vergence and accommodation for virtual objects but also a potential mismatch between focusing on real-world objects at various distances and the fixed focus of the virtual overlay. This can introduce depth discontinuities, reduce the perceived registration accuracy of virtual objects, and cause discomfort.[8]

Effects and Symptoms

The sustained conflict between vergence and accommodation forces the visual system and brain to work harder, potentially leading to a range of negative effects:[4][1]

  • Visual fatigue / Eyestrain: Tired, aching, or burning eyes resulting from the prolonged effort to resolve conflicting cues.
  • Headaches.
  • Blurred Vision: Difficulty maintaining sharp focus on virtual objects, especially those perceived as very near or very far relative to the display's fixed focal plane.
  • Diplopia (Double Vision): Incorrect vergence responses due to the conflict can sometimes lead to seeing double images.
  • Focusing Problems: Difficulty rapidly refocusing between virtual objects at different apparent depths because the natural reflex is disrupted. Users may also experience lingering focus issues or unusual visual sensations after removing the HMD.
  • VR Sickness / Discomfort: VAC is considered a significant contributor to symptoms like nausea, dizziness, and general discomfort associated with VR/AR use.
  • Reduced Visual Performance: Measurable degradation in tasks requiring fine depth judgments, reduced reading speed, slower visuomotor reaction times, and increased time required to fuse binocular images.[1][9]
  • Focal Rivalry: Particularly in AR, the conflict between focusing on a real-world object and a virtual object projected at a different focal distance can make it difficult or impossible to see both sharply simultaneously.

The severity of these symptoms varies significantly between individuals and depends on factors such as the magnitude of the VAC (the difference between vergence and accommodation distances), the duration of exposure, the nature of the visual content, and individual visual capabilities. Conflicts below approximately 0.4 to 0.6 diopters are often tolerated, but larger conflicts, especially for near-field virtual objects (within arm's reach), become increasingly problematic.[6][7]

Measurement

VAC can be quantified by comparing the optical power (measured in diopters, D, which is the reciprocal of distance in meters) required for accommodation versus the optical power corresponding to the vergence distance.[4]

  • `VAC (Diopters) = | (1 / Accommodation Distance (m)) - (1 / Vergence Distance (m)) |`

For example, if an HMD has a fixed focus set to 2 meters (requiring 1/2.0 = 0.5 D of accommodation) and displays a virtual object that appears to be 0.5 meters away (requiring 1/0.5 = 2.0 D of vergence), the VAC magnitude is |0.5 D - 2.0 D| = 1.5 D.

Mitigation Strategies

Addressing VAC is a major focus of VR and AR research and development. Strategies fall into two main categories: content design and technological solutions.

Content and Interaction Guidelines

Careful design can minimize VAC-induced discomfort in fixed-focus displays:

  1. Limit Depth Range: Keep critical interactive content and prolonged visual targets within the "zone of comfort," typically corresponding to less than ~0.6 D of VAC for foreground objects (closer than the focal plane) and ~1.0 D for background objects (farther than the focal plane).[7][6]
  2. Avoid Rapid Depth Changes: Avoid sudden disparity jumps (> 1 D) or rapid oscillations in depth for prominent objects. Allow the visual system time (at least 500 ms) to adjust to significant depth changes.[3]
  3. Optimize UI Placement: Present user interface elements, text, and critical information at or slightly behind the display's native focal plane where VAC is zero or negative (which is generally better tolerated).[6]
  4. Simulate Blur: When hardware cannot provide correct focus cues, incorporate gaze-contingent depth-of-field rendering (simulating blur for objects not being looked at) to provide monocular depth information that aligns better with vergence, potentially reducing cue conflicts.[10]

Technological Solutions

These hardware approaches aim to create displays where the accommodation distance can dynamically match the vergence distance demanded by the virtual content:

Approach Principle Representative prototypes / Research Strengths / Limitations
Varifocal Eye-tracking determines the user's gaze depth, and the display system adjusts a single focal plane to match that depth using tunable lenses (e.g., liquid crystal, liquid lens, Alvarez) or mechanically moving components (screen or lens). | Meta Reality Labs Butterscotch Varifocal (2023);[11] UNC Wide-FOV deformable-mirror NED.[12] | Delivers correct focus cue at the depth of fixation. Challenges include eye-tracking latency and accuracy, depth switching speed, limited depth range, and potentially incorrect blur cues for objects not at the fixation depth.[13]
Multifocal / Multiplane Stanford light-field HMD research;[14] Magic Leap 1 (2 planes). | Provides more correct focus cues across multiple depths simultaneously without necessarily requiring eye-tracking. Challenges include complexity, cost, reduced brightness/contrast, potential visible transitions between planes, and limited number of planes.
Light Field Research using lenslet arrays, parallax barriers, holographic optical elements, super-multi-view displays. | Potentially provides true continuous focus cues without eye-tracking. Challenges include extremely high resolution and bandwidth requirements, computational complexity, limited field of view, and tradeoffs between spatial and angular resolution.
Holographic Displays Aims to fully reconstruct the wavefront of light from the virtual scene using diffraction patterns generated by spatial light modulators. | Research by Microsoft Research, VividQ, Light Field Lab. | Theoretically the ultimate solution, providing all depth cues including accommodation correctly. Challenges include high computational cost ("speckle" noise), limited field of view, and hardware complexity for real-time, high-quality HMDs.
Retinal Projection / Scanning Research systems; formerly North Focals (acquired by Google). | Can bypass VAC by eliminating the need for accommodation. Challenges include small eyebox, potential for visual artifacts (e.g., floaters becoming more visible), safety concerns, and achieving high resolution/FOV.
Emerging Optics Primarily research stage. | Aim for integration into smaller form factors. Manufacturing challenges, response time, optical quality, and control complexity remain active research areas.

Temporary Workarounds / Adaptation

  • Closing One Eye: Viewing with only one eye eliminates binocular disparity cues, thereby removing the vergence signal and the conflict. This can sometimes make it easier to focus on virtual objects, particularly near ones, at the cost of losing stereoscopic depth perception.[2]
  • Optimal Optical Correction: Ensuring users wear their correct prescription glasses or contact lenses minimizes any additional strain on the visual system from uncorrected refractive errors.
  • Adaptation: The visual system can exhibit some adaptation to VAC during prolonged use, temporarily weakening the coupling between vergence and accommodation.[2] However, this adaptation might be slow, incomplete, and can lead to the lingering aftereffects mentioned earlier when returning to natural viewing conditions.

Current Research Frontiers

  • High-Resolution Varifocal Displays: Prototypes like Meta’s Butterscotch demonstrate progress towards retinal resolution (e.g., 60 pixels per degree) combined with reasonably fast depth switching, suggesting potential commercial viability.[11]
  • Focus-Correct Passthrough AR: Integrating varifocal or multifocal optics into video-see-through AR systems to correctly render both real-world and virtual imagery at appropriate focal depths.[13]
  • Standards and Health Implications: Ongoing work by standards bodies (e.g., ISO TC159, IEC TC100) to develop guidelines for extended VR/AR use, particularly concerning children and workplace applications.
  • Perceptual Modeling: Research using large-sample studies to better understand individual variability in the accommodation-vergence relationship, potentially enabling personalized comfort settings or adaptive display parameters.[9]

See Also

References

  1. ↑ 1.0 1.1 1.2 1.3 Template:Cite journal
  2. ↑ 2.0 2.1 2.2 2.3 Kreylos, Oliver (2014-04-13). "Accommodation and Vergence in Head-mounted Displays". http://doc-ok.org/?p=1602.
  3. ↑ 3.0 3.1 Template:Cite journal
  4. ↑ 4.0 4.1 4.2 Kramida, Gregory; Varshney, Amitabh (2016). "Resolving the Vergence-Accommodation Conflict in Head Mounted Displays". https://www.cs.umd.edu/sites/default/files/scholarly_papers/Kramidarev.pdf.
  5. ↑ Kreylos, Oliver (2013-07-24). "Head-mounted Displays and Lenses". http://doc-ok.org/?p=1360.
  6. ↑ 6.0 6.1 6.2 6.3 Template:Cite journal
  7. ↑ 7.0 7.1 7.2 Template:Cite standard
  8. ↑ Template:Cite journal
  9. ↑ 9.0 9.1 Template:Cite journal
  10. ↑ Template:Cite conference
  11. ↑ 11.0 11.1 "Meta’s Going to SIGGRAPH 2023 and Showing Flamera and Butterscotch VR Technologies". 2023-08-04. https://displaydaily.com/metas-going-to-siggraph-2023-and-showing-flamera-and-butterscotch-vr-technologies/.
  12. ↑ Template:Cite journal
  13. ↑ 13.0 13.1 "Dynamic Focus Augmented Reality Display". 2019. https://telepresence.web.unc.edu/research/dynamic-focus-augmented-reality-display/.
  14. ↑ Template:Cite news
  15. ↑ Template:Cite journal
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