Preventing Breaks in Embodiment in Immersive Virtual Reality

Virtual reality (VR) is immersive not only because of visual integration, but because we can act and perform in a virtual environment (VE). Beyond the mere fact that a VR system provides images computed directly from the users' viewpoint in stereoscopy, it can also provide a natural way to interact within the VE using the entire body. Full-body motion capture allows the user's movement to be mapped to a virtual body. Such a mapping may let users feel in control of (sense of agency), own (sense of body ownership) and locate themselves inside this virtual body (sense of self-location), thus leading them to feel embodied in this virtual body whenever these conditions are met (sense of embodiment (SoE)). Hence the integration of a full body avatar in VR is essential, as it brings even more; experiencing being someone else, train to recover mobility or having superhuman abilities. Most importantly, the avatar mediates the virtual nature of the VE to make it seamless for the user. Thus, this thesis focuses on helping the users execute complex movements in VR by applying a distortion to their movements. However, such a distortion is efficient only if users, when assisted, do not experience a break in embodiment (BiE), i.e., do not abruptly lose their sense of embodiment. Moreover, the way the virtual body is perceived is not consistent between users and some users might accept or reject a virtual body more easily than others. Therefore, this thesis explores how to prevent a BiE while distorting users' movement and adapting the VR experience to each individual. To this end, we designed a system combining brain-computer interface and machine learning to detect when users experience a BiE, compute users' preferences, and adapt the VR application's parameters. We first identified the user`s threshold for perceiving a movement distortion in some critical contexts, such as articular limits. We designed a new distortion to help users execute a complex movement. We introduced a method based on machine learning to find users' preferences when using our new distortion. We also established a link between the brain's error monitoring mechanism and the cognitive process of embodiment. This link exposed a new implicit EEG marker indicating when users experience a BiE. Finally, we designed a system to calibrate the distortion implicitly based on users' preferences with all these elements. This system first consists of detecting a BiE during a continuous movement thanks to state-of-the-art brain-computer interface algorithms. Our machine learning method then computes users' preferences, and the distortion is adapted dynamically. This original approach demonstrates the use of an implicit feedback loop to help users while preventing any BiE. Our framework opens an exciting perspective for personalized and self-adapting embodiment systems.

Embodiment Sensitivity to Movement Distortion and Perspective Taking in Virtual Reality

Henrique Galvan Debarba

Despite recent technological improvements of immersive technologies, Virtual Reality suffers from severe intrinsic limitations, in particular the immateriality of the visible 3D environment. Typically, any simulation and manipulation in a cluttered environment would ideally require providing feedback of collisions to every body parts (arms, legs, trunk, etc.) and not only to the hands as has been originally explored with haptic feedback. This thesis addresses these limitations by relying on a cross modal perception and cognitive approach instead of haptic or force feedback. We base our design on scientific knowledge of bodily self-consciousness and embodiment. It is known that the instantaneous experience of embodiment emerges from the coherent multisensory integration of bodily signals taking place in the brain, and that altering this mechanism can temporarily change how one perceives properties of their own body. This mechanism is at stake during a VR simulation, and this thesis explores the new venues of interaction design based on these fundamental scientific findings about the embodied self. In particular, we explore the use of third person perspective (3PP) instead of permanently offering the traditional first person perspective (1PP), and we manipulate the user-avatar motor mapping to achieve a broader range of interactions while maintaining embodiment. We are guided by two principles, to explore the extent to which we can enhance VR interaction through the manipulation of bodily aspects, and to identify the extent to which a given manipulation affects the embodiment of a virtual body. Our results provide new evidence supporting strong embodiment of a virtual body even when viewed from 3PP, and in particular that voluntarily alternating point of view between 1PP and 3PP is not detrimental to the experience of ownership over the virtual body. Moreover, detailed analysis of movement quality show highly similar reaching behavior in both perspective conditions, and only obvious advantages or disadvantages of each perspective depending on the situation (e.g. occlusion of target by the body in 3PP, limited field of view in 1PP). We also show that subjects are insensitive to visuo-proprioceptive movement distortions when the nature of the distortion was not made explicit, and that subjects are biased toward self-attributing distorted movements that make the task easier.

EPFL2017

Manipulation of the participants' self-localization in a lying position

Michaël Mouthon

Healthy human beings are aware of their current position space and are able to distinguish their own body from that of other persons or objects. This capability arises from a coupling of sensory information with cognitive processes, and leads to a representation of the person's self. However, in some clinical cases these processes fail and a person can feel himself to be outside of his physical body, as in the out-of-body experience. These phenomena are interesting to study as they lend insight to certain aspects of bodily self-consciousness, such as self-localization and self-attribution. These aspects can be experimentally manipulated in healthy participants through exposure to conflicting multisensory input, as has been demonstrated with the creation of an ownership's feeling with rubber hand stroked synchronously with the real hand. Here, we worked on the full body by manipulate the self-localization of healthy participants. We used similar protocols as in the study of Lenggenhager et al. (2007) and Ehrsson (2007b) but with a lying position, rather a seated or standing as in the aforementioned studies, which are the commune position when out-of-body experience occurs. Participants were put in a comfortable lying position and saw an image of their body through 3D head mounted display, recorded by a video camera located two meters above them. By providing different temporal and spatial information about a visual-tactile event (stroking on the participants' back or chest) self-localization and identification could be modified in a predictable way. We measured self-localization using a mental ball dropping task and measured phenomenological aspects using a questionnaire. The results indicated that a significant variations in self-localization occurred during the experiment. As a follow-up study, we used mental imagery (in our case we used fMRI) to study the regions of the brain involved in this illusion

2008

Neural Mechanisms of the Embodied Self

Ravi Tej Sai Giridhar Tadi

What is it like to have a body? The unitary experience of the self and the body accompanied by unambiguous self-location and egocentric visuo-spatial perspective are essential aspects of the embodied self. They play a vital role for dynamic whole body human perception and interactions in our daily lives and help distinguish our body from others and the environment. In the current thesis, we used an amalgam of virtual reality techniques and cognitive neuroscience methods in combination with electrical neuroimaging to investigate multisensory, sensorimotor, and cognitive processes involved in mental own body imagery, visuo-spatial perspective taking, the feeling of ownership and agency and their contribution to different aspects of the bodily self and bodily processing.