Vergence-accommodation conflict

Vergence-accommodation conflict (VAC), also known as accommodation-vergence conflict, is a visual phenomenon that occurs when the brain receives mismatching cues between vergence and accommodation of the eye. This commonly occurs in virtual reality devices, augmented reality devices, 3D movies, and other types of stereoscopic displays and autostereoscopic displays. The effect can be unpleasant and cause eye strain. Two main ocular responses can be distinguished - vergence of eyes and accommodation. Both of these mechanisms are crucial in stereoscopic vision. Vergence or independent inward/outward rotation of eyes is engaged to fixate on objects and perceive them as single. Incorrect vergence response can cause double vision. Accommodation is the eye’s focusing mechanism and it is engaged to produce a sharp image on a retina. Both of these mechanisms are neurally linked forming the accommodation-convergence reflex of eyes. One can distinguish vergence distance – a distance of a point towards which both eyes are converging, and an accommodation distance – a distance of a region in space towards which the focus or refractive power of the crystalline lens has been adjusted to produce a sharp image on the retina. In normal conditions the human visual system expects vergence and accommodation distances to match. When viewing most artificial 3D images or displays, vergence and accommodation distances for the most part are mismatched. The human visual system has not evolved to view these types of artificial 3D images comfortably, so VAC can be a very unpleasant sensation for the viewer. VAC is often encountered when viewing stereograms, 3D movies, or virtual reality (VR). It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to virtual reality sickness. The phenomenon can make it impossible to focus on objects close to the eye in VR, limiting the development of VR software. VAC is very difficult to overcome when designing new types of 3D displays.