Neuroprosthesis research in amputee patients aims to develop new prostheses that move and feel like real limbs. controlled directly by patients using their brains to provide sensory feedback. Targeted muscle and sensory reinnervation (TMSR) does exactly this by rerouting motor and sensory nerves from the amputated limb towards intact muscles and skin regions, changing the way the brain processes motor control and somatosensory input, impetus based on sensation, such as pressure, pain, or warmth. However, these detailed brain mechanisms have never been mapped before. Now, a study from researchers led by EPFL identifies how TMSR affects upper-limb representations in the brains of patients with amputations, in particular in the primary motor cortex and the somatosensory cortex. The team states the success of TMSR prostheses will depend on the ability to understand the ways the brain re-maps the pathways between these regions. The opensource study is published in the journal Brain.
Previous studies show a patient fitted with a TMSR prosthetic sends motor commands to re-innervated muscles, where movement intentions are decoded and sent to the prosthetic limb. Direct stimulation of the skin over the re-innervated muscles is then sent back to the brain, inducing touch perception for the missing limb. However, it is unclear as to what extent TMSR-based prostheses recruit and reinstate cortical representations of the missing limb. The current study maps changes in the cortices of three patients with upper-limb amputations who have undergone TMSR.
The current study utilizes fMRI to show how a bionic prosthesis for upper limb amputees based on TMSR impacts the functional organization of the missing limb in the cortices. Results show motor cortex maps of the amputated limb were similar in terms of extent, strength, and topography to individuals with all of their limbs. Data findings show motor cortex maps were different from patients with amputations who were using standard prostheses showing the unique impact of the surgical TMSR procedure on the brain’s motor map.
Results show the connections between upper-limb maps in both cortices are normal in the TMSR patients and compared with healthy controls. Data findings show the preservation of original mapping was reduced in the control group, showing the TMSR procedure preserves strong functional connections between the primary sensory and motor cortex. The lab stresses despite enabling good motor performance, TMSR-powered artificial limbs still do not move and feel like a real limb, and are not encoded by the patient’s brain as a real limb. The group concludes future TMSR prosthetics should implement systematic somatosensory feedback linked to robotic hand movements, enabling patients to feel the haptics of the movements involving their artificial limb.
The team surmises their findings provide the first neuroimaging investigation in patients with TMSR-based bionic limbs. For the future, the researchers state there is a need for further engineering advances such as the integration of somatosensory feedback into current prosthetics enabling them to move and feel like real limbs.