Motion sickness is a condition in which a disagreement exists between visually perceived movement and the vestibular system’s sense of movement. Depending on the cause, it can also be referred to as seasickness, car sickness, simulation sickness, cyber sickness or airsickness.

The vestibular system is the sensory system that contributes to the sense of balance and spatial orientation for the purpose of coordinating movement with balance. [Wikipedia]

In VR, motion sickness is one of the most occurring negative health effects, the symptoms of which include nausea, dizziness, fatigue, headaches, and general discomfort. In some occasions, users can also experience disorientation, vertigo, drowsiness, pallor, sweating, vomiting, etc. [LaViola]

Scene Motion in VR

Scene motion in VR is visual movement of the whole virtual scene that would not normally occur in the real world [Jerald].

Scene motion in a VR environment may be intentional or unintentional. Intentional scene motion would generally be created to make the virtual world behave differently than the real world; whereas unintentional scene motion is normally caused by shortcomings of technology, such as latency or inaccurate calibration, mismatched field of view, optical distortion, etc.

Noticeable scene motion can degrade a VR experience by causing motion sickness, reducing task performance, lowering visual acuity, and decreasing the sense of presence.

Theories of Motion Sickness

There are several theories as to why motion sickness occurs: sensory conflict, eye movement, evolutionary, and postural instability theories. It is helpful to know all of them when creating VR experiences, as they give a good understanding of why our bodies react adversely to VR, what’s causing motion sickness, and how to avoid it.

Sensory Conflict Theory

The most widely accepted theory of motion sickness is the sensory conflict theory. According to it, motion sickness occurs as a result of a sensory disagreement between expected motion and motion that is actually experienced.

Overflight experience in mobile VR

In VR, expected motion is generally synthesized by visual and auditory cues, whereas the motion that we experience is sensed by our body through vestibular and proprioceptive cues (the sense through which we perceive the position and movement of our body, including the sense of equilibrium and balance). [Reason, Brand]

So then, due to nature of many VR experiences, the visual system would often sense motion, and vestibular system would not. This leads to a disagreement between the synthesized expected motion and real-world experienced motion, which then may result in motion sickness.

Eye Movement Theory

The eye movement theory suggests that motion sickness occurs because of the unnatural eye motion that is required to keep the scene’s image stable on the retina. If the image moves differently than expected, which often happens in VR, then a conflict happens between what the eyes expect and what actually occurs. The eyes then must move differently than they do in the real world in order to stabilize the image on the retina. As a result, motion sickness occurs. [Flanagan, May, Dobie]

To solve this problem in VR, providing a single rest frame to focus on will help to reduce eye movement which can also minimize the risk of motion sickness.

Sprint Vector experience in VR

Evolutionary Theory

First proposed by Treismann, the evolutionary theory (or poison theory) explains why such motion can make us sick. Evolutionary, it has been critical for our survival to properly perceive the motion of our body and of the world around us. If we get conflicting information from our senses, it means something is not right with our perceptual and motor systems. Our bodies have evolved to protect us by minimising physiological disturbances produced by absorbed toxins.

Such protection causes the movement discouragement ( like laying down until we recover), ejecting the poison via sweating and vomiting, and causing nausea in order to discourage us from ingesting similar toxins in the future. So that motion sickness may occur because the brain interprets sensory mismatch as a sign of intoxication, and triggers nausea/vomiting as a self-defense response.

Postural Instability Theory

The postural instability theory elaborates more on which experiences will cause a mismatch between simulated and real motions or how severe the sickness will be.

This theory is explaining motion sickness as a result of when an animal lacks or has not yet learned strategies to keep postural stability. Maintaining posture is one of the main goals of animals, and in circumstances where they have not yet learned how to keep balance, they tend to become sick. Similarly, people need to learn new patterns to control their postural stability in novel situations. Until this learning is completed, sickness may occur. [Riccio & Stoffregen]

The length of time one is unstable and the magnitude of that unstableness influence motion sickness and the intensity of symptoms.

For example, if the visual scene is moving forward, users often lean forward to compensate. Since the user is standing still instead of moving forward as perceived, then the leaning forward makes the user less stable; postural instability and motion sickness increase. Getting one’s “sea legs” while traveling by boat to adjust to the boat’s motion is an example of learning to adapt to keep postural stability in the real world. Similarly, VR users learn how to better control posture and balance over time, resulting in less motion sickness.

Reducing motion sickness in VR

Considering motion sickness theories listed above, good practices to reduce the risks of motion sickness might be: