Robotic Leg Illusion: System Design and Human-in-the-Loop Evaluation

The question how humans integrate artificial lower limb devices into their body schema has distinct potential for engineering motion assistance systems, e.g., the design of robotic prostheses. Adding robotic technology to existing psychological experiments enables a deeper investigation of multisensory interaction between proprioceptive, visual, and tactile stimuli during motion. This paper reports the design and control of a robot to investigate embodiment with regard to the lower limbs. In an evaluation study, the rubber hand illusion is transferred to the whole leg for the first time. Participants performed knee bends according to three different conditions being imitated by a robotic leg. The occurrence of a robotic leg illusion was subjectively assessed by a questionnaire and objectively measured by the proprioceptive drift. Considering both metrics, the results show a successful integration of the robotic leg into the body schema. Motion synchronization appears to be a paramount factor, whereby the study indicates that acoustical stimulation might also be relevant. The interrelation between mechatronic design and control of the human-in-the-loop experiment and the factors influencing the illusion are discussed and alternative experimental setups are suggested.