A new study describes an experiment that led participants to perceive an artificial foot as if it were their own. The results could be used to restore some of the missing information from an amputated limb and perhaps improve the embodiment, control, and acceptability of lower-limb prostheses.
In 1998, researchers began studying the rubber hand illusion, involving an interaction between vision, touch, and proprioception, during which study subjects were able to “feel” sensations they saw taking place on an artificial hand that led them to perceive it as their own. For the experiment, subjects couldn’t see their left hands, but rather saw an artificial hand in front of them next to their right hands. Researchers then stroked each participant’s hidden hand and the artificial hand with a paintbrush. Iterations of the illusion study concluded that if the hidden hand and the artificial hand were stroked at the same time and in the same direction, the participants began to experience the rubber hand as their own. Researchers also observed increased activity in the parietal lobe of the participants’ brains as this embodiment occurred. The illusion has been repeated with various body parts and different scenarios, including with upper-limb prosthesis users, which demonstrated that the illusion can be elicited in individuals with upper-limb amputations by stimulating the referred phantom fingers on the residual limb.
A new study, published September 4 in the Journal of Neuroengineering and Rehabilitation (JNER), describes a variation of the experiment using a rubber foot that led to the participants perceiving it as if it were their own. The results of the study could be used to help restore some of the missing information from an amputated limb and perhaps improve the embodiment, control, and acceptability of lower-limb prostheses, the researchers said.
In the JNER study, 19 healthy subjects (ten women) sat on a bed in a supine position with his or her right leg out of sight behind a screen. Each participant was instructed to fix his or her sight on the single-axis prosthetic foot used during the experiment. Synchronous (modality-matched) and asynchronous (modality-mismatched) stimulation conditions were tested. The researchers used vibrotactile stimulation on each participant’s hidden foot and on the prosthetic foot. Participants’ perceptions of possessing an artificial foot occurred most strongly when the stimulations were provided synchronously on the biological foot and to the corresponding areas of the prosthetic foot. The study’s authors said they demonstrated that it is possible to enhance the illusion even with mismatched stimulation, even though the illusion response was lower than in case of matched stimulation.
According to the authors, the first goal of the study was to investigate the possibility of promoting body ownership of an artificial foot using matched stimulation; the second goal was to investigate the possibility of enhancing the feeling of possessing a rubber foot when applying mismatched vibrotactile stimulation, which could potentially be integrated in a sensory feedback system for lower-limb prostheses.
Future studies will be devoted to investigating whether the proposed mismatched-stimulation paradigm can promote the embodiment of the prosthesis in the body schema of individuals with lower-limb amputations, said the authors.
