Target motor and sensory innervation (TMSR) is a surgical procedure practiced on patients with amputations that reroutes residual limb nerves towards intact muscles and skin in order to fit them with a limb prosthesis that allows unprecedented control. By nature, TSMR changes the way the brain processes motor control and somatosensory input. But the detailed brain mechanisms have never been investigated before. The success of TMST prostheses will depend on the ability to understand the ways the brain re-maps these pathways.

EPFL scientists have used ultra-high field 7 Tesla fMRI to show how TSMR affects upper-limb representations in the brains of patients with amputations in particular in primary motor cortex in the somatosensory cortex and regions processing more complex brain functions.

This is an amputee fitted with an advanced arm prosthetic following TMSR surgery. (Tel Aviv)This is an amputee fitted with an advanced arm prosthetic following TMSR surgery. (Tel Aviv)

Targeted muscle sensors innervation (TMSR) is used to improve the control of upper limb prostheses. Residual nerves from an amputated limb are transferred to reinnervates 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, which induces touch perception on the missing limb.

How does the brain encode and integrate artificial touch and movements of a prosthetic limb? How does this impact our ability to better integrate and control prosthetics? Achieving and fine-tuning 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 with Andrea Serino at the University Hospital of Lausanne and team 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 were proficient users of prosthetic limbs developed by Todd Kuiken and his group at Rehabilitation Institute of Chicago.

The researchers used ultra-high field 7T functional MRI, a technique that measures brain activity by detecting changes in blood flow across it. This gave them insight at great spatial resolution into the cortical organization of primary motor and somatosensory cortex of each patient.

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 had standard prosthetics. This shows the impact of the surgical TMSR procedure on the brain’s motor map.

The approach was able to identify maps of missing 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 it is to a lesser degree than in healthy subjects. 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. But preservation of original mapping was reduced in non-TMSR patients, showing that TMSR procedure preserves strong functional connections between primary sensory and motor cortex.

But TMSR is still in need of improvement. The connections between the primary sensory and motor cortex with 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 TMSR-empowered artificial limbs don’t move and feel like a real limb, and are not encoded by the patient’s brain as a real limb. Scientists conclude that future TMSR prosthetics should implement systematic somatosensory feedback linked to the robotic hand movements. This enables 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 the ultra-high field 7 Tesla fMRI is an exceptional tool for studying the upper-limb maps of the motor and somatosensory cortex after amputation.

The findings also suggest that TMSR may counteract poorly adapted plasticity in the cortex after losing a limb. This may provide new insights into the nature and reversibility of cortical plasticity in patients with amputations and its link to phantom limb syndrome and pain.

The study also shows that there is a need for further engineering advanced like the integration of somatosensory feedback into current prosthetics that can enable them to move and feel like real limbs.

The paper on this research was published in Brain.