Embodied avatar movement distortions in virtual reality: effects on gait symmetry and goal-directed reaching movements of the upper and lower limbs

Abstract

In immersive virtual reality, when full-body tracking is enabled, a self-avatar can mimic the user’s movements from a first-person perspective and can serve as a representation of the user’s body within the virtual environment. If the avatar’s movements are sufficiently synchronized with the user’s actual movements, the user can experience a sense of embodiment over the avatar. Recent studies have shown that the sense of embodiment can persist even when the avatar is programmed to spatially deviate from the user’s actual movements. While such studies have primarily focused on upper-limb movements, which are typically deliberate, visually guided, and goal-directed, gait is a rhythmic and automatic motor behavior, and it remains unclear whether spatial distortions would persist in this context. Furthermore, in these prior studies, the avatar’s movements were directed toward a visible target, which may have contributed to the observed motor alignment, reflecting a response which was influenced by the goal and task structure, rather than a tendency to follow the distorted avatar’s movements. Therefore, the main objective of this thesis is to better understand how spatial distortions applied to an embodied avatar influence individuals’ tendency to follow the avatar’s movements, and to explore the interrelation between these distortions, motor adjustments, and the sense of embodiment. To better understand how gait responds to these distortions, we developed an algorithm to manipulate one of the avatar’s step lengths in real time by gradually increasing or decreasing the distortion. The first study assessed at which point the step length distortion became consciously noticeable and evaluated the impact of this detection on the sense of embodiment. Results showed that, on average, increasing distortions up to 12% could go undetected, while decreasing distortions were no longer noticed once they dropped below 9%. The sense of embodiment decreased with increasing distortion but was not disrupted by conscious detection alone. Furthermore, embodiment could be induced at high levels by gradually decreasing the distortion applied to an avatar, rather than the traditional method that consists of inducing embodiment without distortions and gradually applying them. A secondary analysis of the same experimental data examined whether participants would adjust their gait to follow the avatar’s distorted step length, and whether such adjustments were influenced by conscious detection or changes in embodiment. However, despite perceiving the distortion, participants showed no changes in gait symmetry, suggesting that gait may be resistant to visual feedback manipulation. In contrast, earlier studies reporting an effect where participant align their movements with their distorted avatars typically involved discrete upper-limb movements, which differ from gait in being less automatic and more visually guided. Moreover, as those previous studies typically involved reaching movements toward visible targets, it remained unclear whether participants would align with a distorted avatar in the absence of such visual cues. To verify whether participants would follow the avatar in more deliberate actions without explicit visual guidance, we designed a third study focusing on upper and lower-limb reaching movements performed without visual targets. An algorithm was developed that progressively increased the avatar’s lateral deviation relative to a fixed straight-ahead reference, with the magnitude of distortion increasing gradually after each reach. Contrary to our hypothesis, participants did not align their movements with the avatar’s distorted reaching trajectory but instead actively counteracted the distortion. Together, these findings provide new insights into the boundaries of embodiment and the mechanisms supporting motor adaptation. They demonstrate that a strong sense of embodiment can persist even in the presence of spatial distortions, but that this does not necessarily lead to behavioral motor alignment with the avatar. During gait, participants largely disregarded the spatial distortions, likely due to the automatic and rhythmical nature of walking. In contrast, during a goal-directed reaching task, when distortions conflicted with expectations about how a movement should unfold, participants actively counteracted them.

Date	15 Oct 2025
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	David Labbé (Supervisor) & Rachid Aissaoui (Co-supervisor)