## 2 Background and Related Works
At the dim level, the subjects were just able to detect the IVB, and at a bright level, the subjects were easily able to detect the IVB. For low-frequency scene movements, subjects exhibited less balance disturbances when the IVB was presented. The authors suggested that an IVB may alleviate disturbances when conflicting visual and inertial cues are likely to result in a simulator or VE sickness. The effect of IVB on VRISE and presence was researched in a study by [PERSON] et al. [22], where subjects were exposed to a complex motion on prerecorded trajectories through a simulated environment in a driving simulator in a projection-based system. There were three IVB conditions: grid (8 horizontal and 35 vertical grid lines), less cloud (seven clouds), and many clouds (28 clouds). Using the Revised Simulator Sickness Questionnaire (RSSQ), the subjects reported less nausea when the many clouds IVB was used relative to the grid IVB condition. The results indicated that a natural IVB composed of meaningful objects is more effective than a grid for alleviating VRISE levels. The results also showed that different types of IVBs do not significantly influence subjects' presence and enjoyment levels. A virtual guiding avatar (VGA), which combines self-motion prediction cues and an IVB, was proposed in a within-subjects design study by [PERSON] et al. [25] to alleviate the VRISE levels. The VGA was presented as an abstract airplane, and its purpose was to lead the participant along a horizontal motion trajectory through a VE. In a complex visual motion in a cartoon-like environment, the participants were exposed to a driving simulator. The VGA had three motion properties: fixed, rotation only, and rotation plus translation. The results showed that the VRISE levels were reduced when the VGA that presented rotational cues alone or rotation plus translation were utilized. Additionally, the VGA increased the participants' sense of presence and enjoyment. An alternative to grid lines is to permit the participant to partially see the real environment behind the virtual environment. [PERSON] et al. [26] used a partially occluded HMD so the virtual room could be seen overlaid on the actual environment in two similar experiments. The primary difference was that the second experiment required focusing on the actual room. Their first experiment reported lower SSQ-T scores and fewer postural stance breaks in the independent visual background condition. The second experiment showed no difference with the condition on the SSQ-T but fewer stance breaks for the independent visual background condition. The differing results could mean the additional focus on the background or that the elimination of a participant from part of the analysis had an effect. The median score on the SSQ-T was identical between the two studies, but the standard deviation was different. [PERSON] and [PERSON] [13] showed that the IVB reduced self-reported simulator sickness, while it did not affect the section illusion. Reviewing the background and related works shows the importance of rest-frames and their effects on VRISE, the user experience, and presence in the domain of VR. The RFH has been verified in VR, but most studies have been done in projection-based systems [15], using IVB or allocentric rest-frames. Stable rest-frames (RFs) have historically not been possible with HMDs due to the need for low latency and high-quality tracking and calibration. However, RFs can now be rather stable with modern VR technology [4]. Although RFH and the effects of rest-frames to alleviate VRISE levels is a proven concept, there are few studies that have analyzed the impact more deeply, with different kinds, sizes and shapes, and positions of rest-frames. More studies were done to assess the effects of rest-frames in virtual reality on spatial perception and navigation, and no study analyzed the impact of rest-frames on presence and performance. Therefore, we proposed head-centric rest-frames, which correspond to real objects in a real world, and participants are used to wearing them.