Enhancing upper limb prosthetic embodiment through virtual reality with eye-tracking and vibrotactile feedback

The advances in bionic prosthetics yearly show impressive achievements in the effort to improve the motor capabilities and the quality of life of people facing the difficulties due to a limb amputation or agenesis. However, aligning the actual and the desired experience of using an artificial limb in daily activities constitutes an additional challenge. Indeed, several prosthetic users tend to abandon their bionic limbs because of (for instance) their lack of reliability and sensory feedback. Consequently, strategies for increasing the prosthetic technology acceptance are explored in literature, often referring to the phenomenon of the embodiment: the incorporation of an external object in the individual body scheme (the neurocognitive representation of one's body). Among the solutions for stimulating the prosthetic embodiment, the literature offers heterogeneous interactive trainings, often based on Virtual Reality (VR) settings. By exploiting the capability of digital environments to engage the user in repetitive exercises (as in the exergames) and to accurately elicit expected reactions (like the ones of the Rubber Hand Illusion, RHI), the study presented in this paper explores the use of immersive VR to enhance the sense of embodiment, focusing on one of its components, the body ownership - the sensation of owning the external object. The VR application developed for accomplishing this study includes a vibro-tactile feedback to increase the sensory engagement and facilitate the integration of a bionic upper limb - in this case the "Hannes" hand. In particular, the experiment compares two conditions: in one, the users (healthy subjects) use their gaze for attaching components of the digital limb after a certain dwell time, and evoking the vibro-tactile feedback; in the other condition, they must press a button with the other hand for completing the task without any dwell time. The two conditions are assessed using measures of body ownership, such as the proprioceptive drift (estimating the real hand position after a session for triggering the RHI for the actual prosthesis). The findings suggest that immersive VR can improve the acceptance and integration of prosthetic limbs, especially without fully entrusting the gaze control for the interaction: probably, this condition misdirected the cognitive resources for evoking the illusion of owning the digital hand, apparently a requiremennt for "embodying" the actual prosthesis. Preliminary results show this approach could help the bionic limb users with a novel tool to improve their prosthetic technology acceptance.