## 1 Introduction
An egocentric rest-frame is centered on the navigator, whereas an allocentric rest-frame is centered externally in the environment and defined by features of the environment [17]. Egocentric rest-frames are also called player-fixed rest-frames and are defined with respect to the person and are required for the current state of VR technology, which allows players to look around. Earth-fixed (allocentric) rest-frames start to move if the subject is looking around in VR, and it might disappear and, thus, not be available as a rest-frame anymore. Player-fixed rest-frames might be challenging in regard to occluding or being occluded by other objects in the scene [18]. Egocentric rest-frames can be divided into body-centric, head-centric, and coulo-centric [13]. In the virtual environment (VE), the rest-frames are integrated into the scene, remain fixed in relation to the real world or body part, and do not move as the user or the body part virtually moves. Rest-frames can be utilized as backgrounds, and also, smaller foreground cues can help to serve as a rest-frame. The Rest Frame Hypothesis states that sickness does not directly arise from conflicting orientation and motion cues but, rather, from conflicting rest-frames implied by those cues [19]. What is crucial is not the full set of cues in an environment but, rather, how those cues are interpreted to influence one's sense of what is and is not stationary. Sickness is inextricably tied to one's internal mental model of what should be stable. Although this is only a slight refinement to sensory conflict theory [20], it suggests that attempts to reduce sickness may usefully focus on the particular stimuli that influence the selected rest-frame rather than on all orientation and motion stimuli. The RFH provides an explanation for the occurrence of VRISE and, also, an approach to its instant reduction. The RFH assumes the brain has an internal mental model of which objects are stationary and which are moving. When new incoming sensory motion cues do not fit the current mental model of the rest-frame, or the subject has difficulty in selecting a consistent rest-frame, sickness results. The rest-frame must remain congruent with inertial and visual cues. The RFH allows for the existence of sensory conflicts without causing motion sickness if those conflicting cues are not essential to the rest-frame's stability. The RFH predicts simulator sickness from the dependency on the match between the selected rest-frame and the subject's motion. [PERSON] and [PERSON] [13] also suggested that motion sickness occurs when there are too many rest (reference)-frames to choose from, and the individual becomes confused and conflicted. A possible way to combat motion sickness, in this case, would be to introduce one rest-frame, an independent visual background (IVB), salient enough to attenuate focus from other competing and conflicting rest-frames [21]. The rest-frame salience is an important factor, but a study by [PERSON] et al. [18] showed no significant impact of the rest-frame salience or on the VRISE levels or on the game experience. VRISE is much less of an issue for optical see-through augmented reality HMDs, because users can directly see the real world, which acts as a rest-frame consistent with vestibular cues. VRISE is also reduced when the real world can be seen in the periphery of the outside edges of an HMD. An independent visual background (IVB) is a visual scene made to appear behind the content-of-interest of a virtual environment and controlled independently from the content-of-interest [14]. It is a different kind of rest-frame and has been shown to be effective in alleviating VRISE [15]. A visual scene can be divided into components, including one labeled the \"content-of-interest\" and another called the \"independent visual background\". IVB could provide visual motion and orientation cues that match those from the vestibular system, which inclusion in a VE should reduce sickness [22].