Move Like an Ammonite: Personalizing Force Feedback for Avatar Embodiment in Virtual Reality

In virtual environments, users can embody diverse avatars beyond physical constraints. The avatar-induced cognitive transformation (i.e. Proteus effect) can contribute to engineer self-perception, promote empathy and augment human capabilities. However, when the physicality of the avatar differs greatly from that of the user, the discrepancy between bodily sensation and prediction will inhibit embodiment. Therefore, it's challenging to evoke a high sense of embodiment in avatars that have different body structures or textures based on visual feedback alone. We propose a method to modulate movement impedance (inertia, viscosity, stiffness) of our own body in response to embodied avatars using a wearable haptic device, enhancing the sense of body ownership and the plausibility of interaction using that new avatar. Using Bayesian optimization, we identified individually optimized haptic parameters that maximize subjective plausibility for each user. Our results revealed that Bayesian optimization significantly enhanced users' perceived plausibility of the avatar via haptic feedback. The optimal parameters exhibited substantial inter-individual variation, highlighting the importance of characterizing user-specific motor sensations. On the other hand, regression analysis found no clear correlation between avatar impression ratings and optimal haptic parameters, suggesting subjective plausibility formation involves personal interpretations beyond measurable impressions.