Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) have developed a new technique to help amputees use their prostheses. The method involves virtual reality and neural stimulation to help change an amputee’s “phantom limb” to more closely match their prosthetic limb, making it easier and more natural to use.
Many amputees frequently experience sensations from their missing limbs. Typically, such “phantom limbs” are perceived as much smaller than the original limb. Commercially available prosthetic limbs do not yet provide sensory feedback, other than what the patient sees. That means that the patient has no sense of touch from the prosthetic limb and must constantly watch it for correct use.
Tricking the brain
Now, EPFL scientists show that amputees can actually be convinced that the prosthetic hand belongs to their own body. They do this by going beyond the “seeing is believing”. Instead of using the sense of sight alone, they used an astute combination of two senses: sight and touch.
“The brain regularly uses its senses to evaluate what belongs to the body and what is external to the body. We showed exactly how vision and touch can be combined to trick the amputee’s brain into feeling what it sees, inducing embodiment of the prosthetic hand with an additional effect that the phantom limb grows into the prosthetic one,” explains Giulio Rognini of EPFL’s Laboratory of Cognitive Neuroprosthetics led by Olaf Blanke, in a collaboration with Silvestro Micera of EPFL and Scuola Superiore Sant’Anna in Italy. “The setup is portable and could one day be turned into a therapy to help patients embody their prosthetic limb permanently.”
The technique involves amputees wearing a virtual reality goggles through which they can view a virtual prosthetic limb. By applying an artificial tactile sensations to a nerve in the amputee’s stump, the researchers could create the illusion that a digit in their phantom limb was experiencing a sensation. Simultaneously, the index finger in the virtual prosthetic glowed in synchrony with the administered touch sensations. That helped to create the illusion that the phantom limb and the prosthetic are one and the same.
The patients reported feeling as though the prosthetic hand belonged to their own body. Moreover, when asked to evaluate the position of their hands, both patients felt as though their phantom limb had extended into the prosthetic limb. Previous to the experiment, they both reported that the phantom hand was small and directly connected to the stump. For the patients it felt as if the phantom limb had no forearm, a change in size referred to as “telescoping”. In fact, their phantom limb extended during the experiment, and remained extended for up to 10 minutes afterwards.
The experiment simply requires the patient to passively observe two sensations on the fingertip, the visual glow and the artificial touch happening in synchrony, in order for embodiment and extension of the phantom limb to take place. This is the first time that the principles of multisensory integration, in particular how the brain integrates bodily multisensory information to create the coherent and compelling experience of having a body, have been tailored to provoke embodiment of the prosthetic hand and reduction of telescoping.
Source: École Polytechnique Fédérale de Lausanne