|1||Accomodation-convergence conflict||In the real world, our eyes work to both focus and converge on a point in space when we look toward it through a natural process called the accommodation-convergence reflex. Because of the flat nature of the screen in front of the eyes in the HMD environment, it causes a conflict in the human visual system. Principles like Monovision are being researched to resolve this .|
|2||Stereopsis inhibition||The visual facility that creates depth perception is still evolving in the case of children. Being in a VR environment for a significant period of time can cause lack of depth perception (ie, causes depth inhibition) in children upto 6-7 year old.|
|3||Strabismus||Prolonged exposure to VR environments can accentuate lazy-eye syndrome, or Strabismus, where the brain does not acknowledge (for yet unknown reasons) the availability of input from an eye. While VR can itself be used as a therapy to correct Strabismus to a certain extent, the full impact of VR on this phenomenon is not yet known.|
|4||Pattern burning||Staring at a virtual grid for a significant period of time can give the sensation of burning the pattern into a person's retinal system, quite similar to the other display mechanisms. This effect can get accentuated because of the perceived immersion and/or brightness levels. This can be considered to be the equivalent of continued exposure to loud sounds in the auditory system. Audio devices may be capable of inducing a permanent bilateral sensorineural hearing loss.|
|5||Past pointing||The brain adapts to the constraints of a simulated world. When the simulation is over, for a brief period of time, the brain has to re-adapt. If the human reaches for something he/she may reach past it because VR may affect the perception of objects at a distance. Past-pointing can significantly affect an individual's ability to carry out everyday tasks, per research done by Biocca et al. The fact that the human visual system can adapt to the "new" mechanism of seeing and the associated pseudo-reality that comes along with it, can cause temporary or permanent alteration in hand-eye coordination, for example in AR-VR environments. This is especially true in the case of people operating in a medical/ industrial world, like Doctors performing surgery.|
|6||Flicker and FoV, Brightness||
Wider field-of-view displays enhance performance but increase the likelihood of simulator sickness/ flicker. Flicker is more readily perceived in peripheral than foveal vision, so that characteristics such as refresh rate and luminance that give an acceptable display for a narrow field-of-view may cause unacceptable symptoms with a wide field-of-view.
Flicker is related to the luminance, or brightness, of the display, with flicker sensitivity increasing as the brightness increases.
|7||Judder||The FOV in an HMD is much wider compared to other displays causing judder to be much more evident on an HMD. In addition, virtual images on a monitor appear to be on a surface in the world, in contrast to virtual images on an HMD, which appear to be directly in the world; this causes the perceptual system to have higher expectations for HMD images and to more readily detect deviations from what we’re used to when looking at the real world.|
|8||Pseudo-corialis effect||Head movements made in the presence of only visual cues of self-motion is termed the pseudo-Coriolis effect, and causes motion sickness|
|9||Other factors||Motion sickness is accentuated by the weight of the head-gear. Also, non-meaningful object rendering (ex, wrong wind/shadow direction), decreases the sense of presence.|