When walking, our head does not travel on a straight path but oscillates in a swaying pattern. This pattern has been implemented in Virtual Reality (VR) as "viewpoint oscillations" - which can be defined as periodic changes in position and/or orientation of the point of view to enhance walking simulations and make them feel closer to real walking. Viewpoint oscillations are especially beneficial when users cannot physically walk because of limitations of space or hardware, disability, or to avoid fatigue. In this paper, we provide new experimental work on the effects of viewpoint oscillations on walking sensation, as well as cybersickness and virtual embodiment, since such results are scarce in immersive VR, especially when using an avatar in first-person view. To do so, we also propose a technical improvement of viewpoint oscillations in embodied VR. Our technique makes use of an HMD-embedded gaze tracker to artificially add rotations that stabilize the target of the gaze in the users' field of view. We conducted a user study on 24 participants, which showed that our implementation of viewpoint oscillations successfully increased walking sensation and did not impact cybersickness or agency, compared to a linear motion without oscillations. In addition, a novel positive effect of stabilized viewpoint oscillations was found on virtual body ownership. As such, this study demonstrates the feasibility and viability of implementing gaze tracking-based stabilization with standard commercial HMDs, and, taken together, our results promote the use of viewpoint oscillations during walking simulations in embodied VR with an HMD.

Viewpoint oscillations are periodic changes in the position and/or orientation of the point of view in a virtual environment. They can be implemented in Virtual Reality (VR) walking simulations to make them feel closer to real walking. This is especially useful in simulations where users remain in place because of space or hardware constraints. As for today, it remains unclear what exact benefit they bring to user experience during walking simulations, and with what characteristics they should be implemented. To answer these questions, we conduct a systematic literature review focusing on five main dimensions of user experience (walking sensation, vection, cybersickness, presence and embodiment) and discuss 44 articles from the fields of VR, Vision, and Human-Computer Interaction. Overall, the literature suggests that viewpoint oscillations benefit vection, and with less evidence, walking sensation and presence. As for cybersickness, the literature contains contrasted results. Based on these results, we recommend using viewpoint oscillations in applications that require accurate distance or speed perception, or that aim to provide compelling walking simulations without a walking avatar, and a particular attention should be paid to cybersickness. Taken together, this work gives recommendations for enhancing walking simulations in VR, which may be applied to entertainment, virtual visits, and medical rehabilitation.

In this paper, we investigate the use of a motorized bike to support the walk of a self-avatar in virtual reality (VR). While existing walking-in-place (WIP) techniques render compelling walking experiences, they can be judged repetitive and strenuous. Our approach consists in assisting a WIP technique so that the user does not have to actively move in order to reduce effort and fatigue. We chose to assist a technique called walking-by-cycling, which consists in mapping the cycling motion of a bike onto the walking of the user's self-avatar, by using a motorized bike. We expected that our approach could provide participants with a compelling walking experience while reducing the effort required to navigate. We conducted a within-subjects study where we compared "assisted walking-by-cycling" to a traditional active walking-by-cycling implementation, and to a standard condition where the user is static. In the study, we measured embodiment, including ownership and agency, walking sensation, perceived effort and fatigue. Results showed that assisted walking-by-cycling induced more ownership, agency, and walking sensation than the static simulation. Additionally, assisted walking-by-cycling induced levels of ownership and walking sensation similar to that of active walking-by-cycling, but it induced less perceived effort. Taken together, this work promotes the use of assisted walking-by-cycling in situations where users cannot or do not want to exert much effort while walking in embodied VR such as for injured or disabled users, for prolonged uses, medical rehabilitation, or virtual visits.

Technological developments provide solutions to alleviate the tremendous impact on the health and autonomy due to the impact of dementia on navigation abilities. We systematically reviewed the literature on devices tested to provide assistance to people with dementia during indoor, outdoor and virtual navigation (PROSPERO ID number: 215585). Medline and Scopus databases were searched from inception. Our aim was to summarize the results from the literature to guide future developments. Twenty-three articles were included in our study. Three types of information were extracted from these studies. First, the types of navigation advice the devices provided were assessed through: (i) the sensorial modality of presentation, e.g., visual and tactile stimuli, (ii) the navigation content, e.g., landmarks, and (iii) the timing of presentation, e.g., systematically at intersections. Second, we analyzed the technology that the devices were based on, e.g., smartphone. Third, the experimental methodology used to assess the devices and the navigation outcome was evaluated. We report and discuss the results from the literature based on these three main characteristics. Finally, based on these considerations, recommendations are drawn, challenges are identified and potential solutions are suggested. Augmented reality-based devices, intelligent tutoring systems and social support should particularly further be explored.

A seated user watching his avatar walking in Virtual Reality (VR) may have an impression of walking. In this paper, we show that such an impression can be extended to other postures and other locomotion exercises. We present two user studies in which participants wore a VR headset and observed a first-person avatar performing virtual exercises. In the first experiment, the avatar walked and the participants (n=36) tested the simulation in 3 different postures (standing, sitting and Fowler's posture). In the second experiment, other participants (n=18) were sitting and observed the avatar walking, jogging or stepping over virtual obstacles. We evaluated the impression of locomotion by measuring the impression of walking (respectively jogging or stepping) and embodiment in both experiments. The results show that participants had the impression of locomotion in either sitting, standing and Fowler's posture. However, Fowler's posture significantly decreased both the level of embodiment and the impression of locomotion. The sitting posture seems to decrease the sense of agency compared to standing posture. Results also show that the majority of the participants experienced an impression of locomotion during the virtual walking, jogging, and stepping exercises. The embodiment was not influenced by the type of virtual exercise. Overall, our results suggest that an impression of locomotion can be elicited in different users' postures and during different virtual locomotion exercises. They provide valuable insight for numerous VR applications in which the user observes a self-avatar moving, such as video games, gait rehabilitation, training, etc.

We investigate how underfoot vibrotactile feedback can be used to increase the impression of walking and embodiment of static users represented by a first- or third-person avatar. We designed a multi-sensory setup involving avatar displayed on an HMD, and a set of vibrotactile effects displayed at every footstep. In a first study (N = 44), we compared the impression of walking in 3 vibrotactile conditions : 1) with a "constant" vibrotactile rendering reproducing simple contact information, 2) with a more sophisticated "phase-based" vibrotactile rendering the successive contacts of a walking cycle and 3) without vibrotactile feedback. The results show that overall both constant and phase-based rendering significantly improve the impression of walking in first and third-person perspective. Interestingly, the more realistic phase-based rendering seems to increase significantly the impression of walking in the third-person condition, but not in the first-person condition. In a second study (N=28), we evaluated the embodiment towards first- and third-person avatar while receiving no vibrotactile feedback or by receiving vibrotactile feedback. The results show that vibrotactile feedback improves embodiment in both perspectives of the avatar. Taken together, our results support the use of vibrotactile feedback when users observe first- and third-person avatar. They also suggest that constant and phase-based rendering could be used with first-person avatar and support the use of phase-based rendering with third-person avatar. They provide valuable insight for stimulations in any VR applications in which the impression of walking is prominent such as for virtual visits, walking rehabilitation, video games, etc.

In this paper we explore the multi-sensory display of self-avatars' physiological state in Virtual Reality (VR), as a means to enhance the connection between the users and their avatar. Our approach consists in designing and combining a coherent set of visual, auditory and haptic cues to represent the avatar's cardiac and respiratory activity. These sensory cues are modulated depending on the avatar's simulated physical exertion. We notably introduce a novel haptic technique to represent respiratory activity using a compression belt simulating abdominal movements that occur during a breathing cycle. A series of experiments was conducted to evaluate the influence of our multi-sensory rendering techniques on various aspects of the VR user experience, including the sense of virtual embodiment and the sensation of effort during a walking simulation. A first study ($\mathrm{N}=30$) that focused on displaying cardiac activity showed that combining sensory modalities significantly enhances the sensation of effort. A second study ($\mathrm{N}=20$) that focused on respiratory activity showed that combining sensory modalities significantly enhances the sensation of effort as well as two sub-components of the sense of embodiment. Interestingly, the user's actual breathing tended to synchronize with the simulated breathing, especially with the multi-sensory and haptic displays. A third study ($\mathrm{N}=18$) that focused on the combination of cardiac and respiratory activity showed that combining both rendering techniques significantly enhances the sensation of effort. Taken together, our results promote the use of our novel breathing display technique and multi-sensory rendering of physiological parameters in VR applications where effort sensations are prominent, such as for rehabilitation, sport training, or exergames.