“Movement” here refers specifically to any motion through the virtual environment that is not the result of mapping the user’s real world movements into VR. Movement and acceleration most commonly come from the user’s avatar moving through the virtual environment (by locomotion or riding a vehicle) while the user’s real-world body is stationary. These situations can be discomforting because the user’s vision tells them they are moving through space, but their bodily senses (vestibular sense and proprioception) say the opposite. This illusory perception of self-motion from vision alone has been termed vection, and is a major underlying cause of simulator sickness.
Speed of movement through a virtual environment has been found to be proportional to the speed of onset for simulator sickness, but not necessarily the subsequent intensity or rate of increase. Whenever possible, we recommend implementing movement speeds near typical human locomotion speeds (about 1.4 m/s walking, 3 m/s for a continuous jogging pace) as a user-configurable—if not default—option.
For VR content, the visual perception of acceleration is a primary impetus for discomfort. This is because the human vestibular system responds to acceleration but not constant velocity. Perceiving acceleration visually without actually applying acceleration to your head or body can lead to discomfort. (See our section on simulator sickness for a more detailed discussion.)
Keep in mind that “acceleration” can refer to any change over time in the velocity of the user in the virtual world in any direction. Although we normally think of acceleration as “increasing the speed of forward movement,” acceleration can also refer to decreasing the speed of movement or stopping; rotating, turning, or tilting while stationary or moving; and moving (or ceasing to move) sideways or vertically. It is any change in direction or speed.
Instantaneous accelerations are more comfortable than gradual accelerations. Because any period of acceleration constitutes a period of conflict between the senses, discomfort will increase as a function of the frequency, size, and duration of acceleration. We generally recommend you minimize the duration and frequency of accelerations as much as possible.
Similar to how drivers are much less likely to experience motion sickness in a car than their passengers, giving users control over the motion they see can prevent simulator sickness. Let users move themselves around instead of taking them for a ride, and avoid jerking the camera around, such as when the user is hit or shot. This can be very effective on a monitor, but can cause simulator sickness. Similarly, do not freeze the display so that it does not respond to the user’s head movements, as this can create discomforting misperceptions of illusory motion. In general, avoid decoupling the user’s and camera’s movements for any reason.
Research suggests that providing users with an avatar that anticipates and foreshadows the visual motion they are about to experience allows them to prepare for it in a way that reduces discomfort. This can be a serendipitous benefit in 3rd-person games. If the avatar’s actions (e.g., a car begins turning, a character starts running in a certain direction) reliably predict what the camera is about to do, this may prepare the user for the impending movement through the virtual environment and make for a more comfortable experience.
Some first-person games apply a mild up-and-down movement to the camera to simulate the effects of walking. This can be effective to portray humanoid movement on a computer or television screen, but can be a problem for many people in immersive head-mounted VR. Every bob up and down is another bit of acceleration applied to the user’s view, which—as we already said above—can lead to discomfort. Do not use any head-bob or changes in orientation or position of the camera that were not initiated by the real-world motion of the user’s head.
In the real world, we most often stand still or move forward. We rarely back up, and we almost never strafe (move side to side). Therefore, when movement is a must, forward user movement is most comfortable. Left or right lateral movement is more problematic because we don’t normally walk sideways and it presents an unusual optic flow pattern to the user.
In general, you should respect the dynamics of human motion. There are limits to how people can move in the real world, and you should take this into account in your designs.
Moving up or down stairs (or steep slopes) can be discomforting for people. In addition to the unusual sensation of vertical acceleration, the pronounced horizontal edges of the steps fill the visual field of the display while all moving in the same direction. This creates an intense visual that drives a strong sense of vection. Users do not typically see imagery like this except for rare situations like looking directly at a textured wall or floor while walking alongside it. We recommend that developers use slopes and stairs sparingly. This recommendation applies to other images that strongly induce vection, as well, such as moving up an elevator shaft where stripes (of light or texture) are streaming downwards around the user.
Developers are advised to consider how these guidelines can impact one another in implementation. For example, eliminating lateral and backwards movement from your control scheme might seem like a reasonable idea in theory, but could cause users to engage in relatively more motions (i.e., turning, moving forward, and turning again) to accomplish the same changes in position. This results in more visual self-motion—and consequently more vection—than users would have seen if they simply stepped backwards or to the side. Environments and experiences should be designed to minimize the impact of these issues.
Consider also simplifying complex actions to minimize the amount of vection the user will experience, such as automating or streamlining a complex maneuver for navigating obstacles. One study had players navigate a virtual obstacle course with one of two control schemes: one that gave them control over 3 degrees of freedom in motion, or another that gave them control over 6. Although the 3-degrees-of-freedom control scheme initially seems to give the user less control (and therefore lead to more simulator sickness), it actually led to less simulator sickness because it saved them from having to experience extraneous visual motion.
This is one of those cases where a sweeping recommendation cannot be made across different types of content and situations. Careful consideration, user testing, and iterative design are critical to optimizing user experience and comfort.
Hettinger, L.J., Berbaum, K.S., Kennedy, R.S., Dunlap, W.P., & Nolan, M.D. (1990). Vection and simulator sickness. Military Psychology, 2(3), 171-181.
Stanney, K.M. & Hash, P. (1998). Locus of user-initiated control in virtual environments: Influences on cybersickness. Presence, 7(5), 447-459.