## 8 Conclusions and Future Work  
This study examined the effects of head-centric rest-frames in terms of VRISE, user experience, and presence in virtual environments. The study results were somehow mixed, and more future work must be done to research the effects of head-centric rest-frames in more depth. The study showed that head-centric rest-frames are not helpful in terms of VRISE gaming contents where translation and rotation is slow (walking) and there is no vertical movement in the y-axis. In more action gaming contents with fast translation and rotation and vertical movement (jumps), the study showed that head-centric rest-frames are helpful for users who are familiar with virtual reality technology. For all users, there were differences in the mean VRISE levels, but we could not confirm that as statistically significant. Our study also showed that virtual glasses are helpful for users who use distance spectacles but not when the real glasses are worn underneath the HMD. It is obviously disturbing to have \"double\" glasses when experiencing a virtual environment. They are not recommended for new users of virtual reality, as we found statistically significant differences with a positive impact on the SSQ Disorientation levels only for participants who had previous experience with VR technology. Additional objects in the virtual environment for users experiencing virtual reality for the first time and are being overwhelmed with new visual information, perception, and immersion are just too much information to be processed by the nervous system. New users of virtual reality should not be exposed to highly provocative content during their first sessions. They should go through the process of adaptation/habituation to reduce VRISE. Incremental exposure and progressively increasing the intensity of stimulations over multiple exposures is a very effective way to reduce motion sickness [19]. Users who are wearing distance spectacles should choose glasses as the rest-frames (and wear contact lenses under the HMD to avoid the effects of \"double\" glasses). Non-wearers of distance spectacles should choose a baseball hat. Head-centric rest-frames is a promising solution to be implemented in an application or in a game where there is an emphasis on reducing the VRISE levels. They are easily implemented, and they do not drastically change the design and appearance of the game. We propose the usage of rest-frames as an optional setting to be activated as preferred by the user, based on their personal preferences and experiences. In future works, the effects of head-centric rest-frames should be performed in more depth, with more participants, as this was a potential limitation of this study. Since rest-frames can be disturbing for some participants, dynamic rest-frames could be implemented. That means that, in low action contents or while being still, rest-frames should not be visible or be transparent, but in high action contents or when moving or rotating fast, rest-framesshould be displayed. In our study, rest-frames were tested in the case of smooth artificial locomotion. Besides other types of contents, it should also be tested on other locomotion interfaces in order to compare and analyze the effects of head-centric rest-frames effectively. One interesting idea is also the usage of darkened lenses, which become darker when the action gets more provocative. Conceptualization, A.S., M.P. and J.G. ; data curation, A.S.; formal analysis, A.S.; investigation, A.S., M.P. and J.G. ; methodology, A.S. and J.G. ; project administration, J.G. ; software, J.G. ; supervision, J.G. ; validation, A.S., M.P. and J.G. ; visualization, A.S.; writing--original draft, A.S.; and writing--review and editing, A.S., M.P. and J.G. All authors have read and agreed to the published version of the manuscript. This research received no external funding.