In a previous post, we discussed the three most widely recognized theories about simulation sickness (and their scenario design implications) in detail. In general, these older understandings of what causes simulation sickness tend to argue that we must remove or attenuate motion signals in our simulations to prevent discomfort. The “Rest-Frame Hypothesis” is a somewhat more recent explanation for the mechanism underlying simulator sickness and suggests some novel alternate ways to minimize it.
An Introduction to the “Rest-Frame Hypothesis”
In 1999 researchers led by Jerrold Prothero suggested that, at base, simulator sickness may be better understood as arising from a cognitive conflict rather than a strictly sensory conflict. Prothero pointed out that, at any instant, the nervous system can establish any number of “reference frames” in which to track events. During any task with multiple reference frames, the brain selects one to be the “rest-frame”—that is, the reference frame that is “stationary.” For example, when riding in a car on the freeway, you might have three reference frames: the interior of the vehicle, the landscape outside your window, and a passing truck. If you were to awake from a nap and mistake the passing truck for a stationary object, you’d experience a possibly panicky moment of vertigo because it would briefly seem like your car was suddenly rolling slowly backward down the freeway.
As Prothero explained in his 1998 dissertation (which formed the basis of his later work):
“The rest-frame construct suggests that motion sickness does not arise from conflicting motion signals per se, but rather from conflicting rest frames deduced from those motion signals. That is, what is crucial is not the full set of motion cues in an environment, but rather how those motion cues are interpreted to influence one’s sense of what is and is not stationary.”
He thus argues that there is no need to remove conflicting sensory cues, but simply to remove the confusion about which frame is the true rest-frame—by, for example, changing the emphasis among cues to give the brain “hints” as to what it should consider truly stationary.
Using the Rest-Frame Construct to Address Simulator Sickness
In a chapter on simulator sickness in the Handbook of Driving Simulation for Engineering, Medicine, and Psychology, researcher Heather Stoner (Realtime Technologies lead engineer and general manager) noted several different “rest frame”-based approaches have proven promising.
The key is in breaking the visual information in the simulation into two elements: the simulation content and an independent visual background (IVB). If the IVB—for example, the horizon and clouds in the sky—match the larger room’s movement (i.e., remains still). It is easier for the participant’s brain to identify the true rest-frame and avoid discomfort correctly.
There are many ways to achieve this. The visible horizon and/or earth-fixed clouds are one (although not possible for all simulations, for example, low-visibility scenarios). Others include projecting the entire video simulation content onto a semi-transparent surface so that the participant can dimly see the actual room through the simulation at all times. Or, a stationary grid pattern can be overlaid overtop the video. Even something as “low tech” as having a grid border around the simulation video, or limiting the vertical field of view so that participants can see past the screen to the rest of the room has proven effective.