Movement speed of an autonomous prosthetic limb shapes embodiment, usability and robotic social attributes in virtual reality

Autonomous robotic prostheses can aid individuals with limb loss regain functionality and near-normal appearance. However, to psychologically integrate such limbs into one's body image, it is necessary to understand how movement characteristics, such as movement speed, affect the sense of embodiment. Using a VR simulation, we investigated how the speed of autonomous lower-arm movements affects embodiment and user-perception. Nineteen healthy participants embodied an amputated virtual avatar with a prosthetic lower left-arm moving autonomously at six different speeds following minimum-jerk trajectories. Participants rated sense of body ownership, sense of agency, usability, competence, warmth, and discomfort after performing a reaching task at each speed. Ownership, sense of agency, and usability were highest for moderate speeds (autonomous movements lasting 1 s), and were significantly lower for both faster (125 ms) and slower (4 s) movements (p < 0.05). Competence was significantly higher at moderate and moderately fast speeds compared to slower speeds. Discomfort was significantly higher at the fastest speed compared to moderate and slower speeds. Overall, the results show a tendency of moderate movement speeds being favorable for user perception of autonomous limbs and hint at the existence of an optimal speed or a speed range for enhancing embodiment and usability.