Evaluation of a Multimodal Tactile Feedback System for Upper Limb-loss Users: Embodiment and Performance

The aim of this study is to evaluate the effect of integrating tactile feedback systems into a robotic upper-limb prosthesis. These systems deliver pressure stimuli (through small silicone chambers that inflate), vibration stimuli (through small circular actuators), or a combination of both to the arm, in order to improve the feeling of owning and controlling the artificial hand. In this way, when the robotic hand touches, grasps, and holds an object, the user receives sensory feedback that may make prosthesis use more natural, intuitive, and functional in everyday life. This is expected to improve the sense of bodily integration of the prosthesis, particularly by enhancing the perception of owning the bionic limb and the feeling of control over it, thereby improving the ability to perform daily activities with the prosthesis. In addition, the study aims to investigate whether the simultaneous delivery of multiple stimuli may confuse or discomfort the user or they are well integrated by the sensitive system improving the experience of tactile sensation. This is a pilot, open-label study, meaning that both the researchers and the participants will be aware of the different phases of the study. The study population will include individuals with unilateral transradial upper-limb loss, either acquired or congenital. The planned sample size is 9 participants who meet the inclusion and exclusion criteria and who provide written informed consent to take part in the study. The study consists of two phases. Phase 1: Rubber Hand Illusion experiment During this phase, the feedback devices called WISH (pressure sensation provided by the inflation of silicone chambers), VIBES (vibration sensation), and PUSE (both devices applied and activated together to provide both sensations, either synchronously or with minimal delay) will be placed on the residual limb and secured with elastic Velcro straps. A robotic hand, controlled by the participant through electromyographic sensors, will be positioned on a table in front of the participant. The participant will see the robotic hand move while receiving sensory feedback synchronized with its movements. Different stimulation conditions (pressure only, vibration only, and combined feedback) will be tested. At the end of each condition, a questionnaire will be administered to assess the perception of ownership and agency. Phase 2: Upper-limb prosthesis use In the second phase, the actuators will be integrated into the socket of a SoftHand robotic prosthesis, a myoelectric upper-limb prosthesis. Participants will be asked to wear the prosthesis and perform tasks under each of the feedback conditions tested in Phase 1. After a free-use familiarization period of approximately 10 minutes, participants will be asked to perform tasks involving object and surface recognition, as well as activities of daily living, which will be timed. The results of the different conditions will be compared to identify the feedback configuration associated with the best performance, defined as fewer errors and shorter execution time. At the end of each condition, a questionnaire will be administered to assess ease of use and tolerability of the prosthesis.