Targeted motor and sensory reinnervation (TMSR) is used to improve the control of upper-limb prostheses by transferring residual nerves from the amputated limb to reinnervate and activate new muscle targets. Yet how does the brain encode and integrate such artificial touch and movements of the prosthetic limb? How does this impact the ability to better control prosthetic devices? Achieving and fine-tuning such control depends on knowing how the patient’s brain remaps various motor and somatosensory pathways in the motor cortex and the somatosensory cortex.
A study published in the November issue of Brain, showed that motor cortex maps of the amputated limb were similar in terms of extent, strength, and topography to individuals without limb amputation, but were different from patients who did not receive TMSR and used standard prostheses.
This is an amputee fitted with an advanced arm prosthetic following TMSR surgery.
Photograph courtesy of Irit Hacmun, Tel Aviv.
The Laboratory of Cognitive Neuroscience, directed by Olaf Blanke, MD, PhD, at Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in collaboration with Andrea Serino, PhD, at the University Hospital of Lausanne, Switzerland, and teams of clinicians and researchers have successfully mapped 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, MD, PhD, at the Shirley Ryan AbilityLab, Chicago.
The scientists used ultrahigh-field 7T functional magnetic resonance imaging (fMRI), a technique that measures brain activity by detecting changes in blood flow across it. This gave them insight into the cortical organization of primary motor and somatosensory cortex of each patient. The approach also identified 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.
The study also showed that TMSR still needs improvement, according to EPFL; the connections between the primary sensory and motor cortex with the higher-level embodiment regions in the frontoparietal 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 prostheses 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 provided the first detailed neuroimaging investigation in patients with bionic limbs based on the TMSR prosthesis, and showed that ultrahigh-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 suggested that TMSR may counteract poorly adapted plasticity in the cortex after losing a limb. According to the researchers, 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.
Editor’s note: This story was adapted from materials provided by Ecole Polytechnique Fédérale de Lausanne
