Summary: Following targeted motor and sensory reinnervation, a procedure that reroutes residual limb nerves to intact muscles and skin in amputees, the brain remaps both motor and sensory pathways. Additionally, researchers note, TMSR may help counteract poorly adapted cortical plasticity following amputation.
EPFL scientists from the Center for Neuroprosthetics have used functional MRI to show how the brain re-maps motor and sensory pathways following targeted motor and sensory reinnervation (TMSR), a neuroprosthetic approach where residual limb nerves are rerouted towards intact muscles and skin regions to control a robotic limb.
Targeted motor and sensory reinnervation (TMSR) is a surgical procedure on patients with amputations that reroutes residual limb nerves towards intact muscles and skin in order to fit them with a limb prosthesis allowing unprecedented control. By its nature, TMSR changes the way the brain processes motor control and somatosensory input; however the detailed brain mechanisms have never been investigated before and the success of TMSR prostheses will depend on our ability to understand the ways the brain re-maps these pathways. Now, EPFL scientists have used ultra-high field 7 Tesla fMRI to show how TMSR affects upper-limb representations in the brains of patients with amputations, in particular in primary motor cortex and the somatosensory cortex and regions processing more complex brain functions. The findings are published in Brain.
Targeted motor and sensory reinnervation (TMSR) is used to improve the control of upper limb prostheses. Residual nerves from the amputated limb are transferred to reinnervate and activate new muscle targets. This way, a patient fitted with a TMSR prosthetic “sends” motor commands to the re-innervated muscles, where his or her movement intentions are decoded and sent to the prosthetic limb. On the other hand, direct stimulation of the skin over the re-innervated muscles is sent back to the brain, inducing touch perception on the missing limb.
But how does the brain encode and integrate such artificial touch and movements of the prosthetic limb? How does this impact our ability to better integrate and control prosthetics? Achieving and fine-tuning such control depends on knowing how the patient’s brain re-maps various motor and somatosensory pathways in the motor cortex and the somatosensory cortex.
The lab of Olaf Blanke at EPFL, in collaboration with Andrea Serino at the University Hospital of Lausanne and teams of clinicians and researchers in Switzerland and abroad have successfully mapped out these changes in the cortices of three patients with upper-limb amputations who had undergone TMSR and were proficient users of prosthetic limbs developed by Todd Kuiken and his group at the Rehabilitation Institute of Chicago.
The scientists used ultra-high field 7T functional magnetic resonance imaging (fMRI), a technique that measures brain activity by detecting changes in blood flow across it. This gave them an unprecedented insight at great spatial resolution into the cortical organization of primary motor and somatosensory cortex of each patient.
Surprisingly, the study showed that motor cortex maps of the amputated limb were similar in terms of extent, strength, and topography to individuals without limb amputation, but they were different from patients with amputations that did not receive TMSR, but were using standard prostheses. This shows the unique impact of the surgical TMSR procedure on the brain’s motor map.
The approach was even able to identify maps of missing (phantom) fingers in the somatosensory cortex of the TMSR patients that were activated through the reinnervated skin regions from the chest or residual limb.
The somatosensory maps showed that the brain had preserved its original topographical organization, although to a lesser degree than in healthy subjects. Moreover, when investigating the connections between upper-limb maps in both cortices, the researchers found normal connections in the TMSR patients, which were comparable with healthy controls. However, preservation of original mapping was again reduced in non-TMSR patients, showing that the TMSR procedure preserves strong functional connections between primary sensory and motor cortex.
The study also showed that TMSR is still in need of improvement: the connections between the primary sensory and motor cortex with the higher-level embodiment regions in fronto-parietal cortex were as weak in the TMSR patients as in the non-TMSR patients, and differed with respect to healthy subjects.
This suggests that, despite enabling good motor performance, TMSR-empowered artificial limbs still do not move and feel like a real limb and are still not encoded by the patient’s brain as a real limb. The scientists conclude that future TMSR prosthetics should implement systematic somatosensory feedback linked to the robotic hand movements, enabling patients to feel the sensory consequences of the movements of their artificial limb.
The findings provide the first detailed neuroimaging investigation in patients with bionic limbs based on the TMSR prosthesis, and show that ultra-high field 7 Tesla fMRI is an exceptional tool for studying the upper-limb maps of the motor and somatosensory cortex following amputation.
In addition, the findings suggest that TMSR may counteract poorly adapted plasticity in the cortex after losing a limb. According to the authors, this may provide new insights into the nature and the reversibility of cortical plasticity in patients with amputations and its link to phantom limb syndrome and pain.
Finally, the study also shows that there is a need of further engineering advances such as the integration of somatosensory feedback into current prosthetics that can enable them to move and feel as real limbs.
- University Hospital Lausanne (CHUV)
- The Gonda Multidisciplinary Brain Research Center (Bar-Ilan University)
- Foundation Campus Biotech Geneva
- EPFL Biomedical Imaging Research Center
- Spinoza Centre for Neuroimaging
- Clinique Romande de Réadaptation – SUVA
- Centro Protesi INAIL
- Rehabilitation Institute of Chicago
- University Hospital Geneva (HUG)
Funding: Fuding provided by Swiss National Science Foundation and Bertarelli Foundation.
Source: Elaine Schmidt – EPFL
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Irit Hacmun, Tel Aviv.
Original Research: Full open access research for “Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation” by Andrea Serino, Michel Akselrod, Roy Salomon, Roberto Martuzzi, Maria Laura Blefari, Elisa Canzoneri, Giulio Rognini, Wietske van der Zwaag, Maria Iakova, François Luthi, Amedeo Amoresano, Todd Kuiken, and Olaf Blanke in Brain. Published online October 27 2017 doi:10.1093/brain/awx242
Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation
Neuroprosthetics research in amputee patients aims at developing new prostheses that move and feel like real limbs. Targeted muscle and sensory reinnervation (TMSR) is such an approach and consists of rerouting motor and sensory nerves from the residual limb towards intact muscles and skin regions. Movement of the myoelectric prosthesis is enabled via decoded electromyography activity from reinnervated muscles and touch sensation on the missing limb is enabled by stimulation of the reinnervated skin areas. Here we ask whether and how motor control and redirected somatosensory stimulation provided via TMSR affected the maps of the upper limb in primary motor (M1) and primary somatosensory (S1) cortex, as well as their functional connections. To this aim, we tested three TMSR patients and investigated the extent, strength, and topographical organization of the missing limb and several control body regions in M1 and S1 at ultra high-field (7 T) functional magnetic resonance imaging. Additionally, we analysed the functional connectivity between M1 and S1 and of both these regions with fronto-parietal regions, known to be important for multisensory upper limb processing. These data were compared with those of control amputee patients (n = 6) and healthy controls (n = 12). We found that M1 maps of the amputated limb in TMSR patients were similar in terms of extent, strength, and topography to healthy controls and different from non-TMSR patients. S1 maps of TMSR patients were also more similar to normal conditions in terms of topographical organization and extent, as compared to non-targeted muscle and sensory reinnervation patients, but weaker in activation strength compared to healthy controls. Functional connectivity in TMSR patients between upper limb maps in M1 and S1 was comparable with healthy controls, while being reduced in non-TMSR patients. However, connectivity was reduced between S1 and fronto-parietal regions, in both the TMSR and non-TMSR patients with respect to healthy controls. This was associated with the absence of a well-established multisensory effect (visual enhancement of touch) in TMSR patients. Collectively, these results show how M1 and S1 process signals related to movement and touch are enabled by targeted muscle and sensory reinnervation. Moreover, they suggest that TMSR may counteract maladaptive cortical plasticity typically found after limb loss, in M1, partially in S1, and in their mutual connectivity. The lack of multisensory interaction in the present data suggests that further engineering advances are necessary (e.g. the integration of somatosensory feedback into current prostheses) to enable prostheses that move and feel as real limbs.
“Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation” by Andrea Serino, Michel Akselrod, Roy Salomon, Roberto Martuzzi, Maria Laura Blefari, Elisa Canzoneri, Giulio Rognini, Wietske van der Zwaag, Maria Iakova, François Luthi, Amedeo Amoresano, Todd Kuiken, and Olaf Blanke in Brain. Published online October 27 2017 doi:10.1093/brain/awx242