Targeted motor and sensory reinnervation: fMRI maps connection

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  • Published: Nov 1, 2017
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Targeted motor and sensory reinnervation: fMRI maps connection

What a nerve!

An amputee fitted with an advanced arm prosthetic following TMSR surgery (credit: Irit Hacmun, Tel Aviv)

Functional MRI has been used to demonstrate how the brain requires its motor and sensory pathways when an amputee is given a robotic limb. Targeted motor and sensory reinnervation (TMSR) is a neuroprosthetic approach to connect residual limb nerves and reroute them to intact muscles and skin regions to give control the person control.

By its very nature, TMSR changes the way the brain processes motor control and somatosensory input. However, surgeons and the developers of robotic prosthetics are keen to understand more about the detailed brain mechanisms that allow this to occur. The success of TMSR prostheses and advances in this area of medical science will rely on our better understanding of the brain in this context. Now, researchers at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have used ultra-high field 7 Tesla fMRI to reveal how TMSR alters the representations of the patient's upper limb in the brain. The team placed particular emphasis on scanning the primary motor cortex and the somatosensory cortex and regions associated with the processing of other complex brain functions. They publish details of their findings in the journal Brain.

Touchy feely

TMSR involves reconnecting residual limb nerves from the site of amputation to remaining muscle and skin tissues. This way, the patient's brain can send nerve signals to their body as if their limb were intact and those signals are then "decoded" by the prosthetic's on-board computer which then sends its own electrical commands to its motors and servos to move the prosthetic limb and fingers. The reinnervated connections associated with the skin provide the necessary feedback, the haptic response from the remaining muscles and can induce touch perception indirectly from the prosthetic limb.

Medical science would like to understand how the brain encodes and integrates such artificial movements and touch in a prosthetic limb. In the optimal prosthetic the patient would be able to control the robotic limb in a manner as close as possible to their own limb. But, exactly how does this reinnervation and remapping of the brain affect the patient's ability to better integrate and control a prosthetic? 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 team suggests.

Team leader Olaf Blanke of EPFL and his colleagues collaborated with Andrea Serino at the University Hospital of Lausanne and teams of clinicians and researchers in Switzerland and abroad to study TMSR. The subjects of the study were proficient users of prosthetic limbs developed by Todd Kuiken and his group at the Rehabilitation Institute of Chicago, USA. The 7T fMRI gave the scientists an unprecedented view with high spatial resolution into the cortical organization of primary motor and somatosensory cortex of each patient.

Phantom menace

Perhaps not surprisingly, although gratifying nevertheless, the fMRI scans showed that motor cortex maps of the amputated limb were similar in terms of extent, strength, and topography to those recorded in people without limb amputation. However, they were starkly different to those seen in patients with amputations that had not had TMSR and were using conventional prostheses. This suggests that TMSR somehow has a unique effect on the brain’s motor map, the very nature of which allows those patients to have great control of their robotic limb and the requisite feedback between prosthesis and patient. Bizarrely, the approach could even reveal maps of missing (so-called phantom) fingers in the somatosensory cortex of the TMSR patients that were activated through the reinnervated skin regions from the chest or residual limb.

However, the study suggests that while ideally a TMSR-empowered artificial limb would move and feel like a real limb, the technology has not yet reached that point in its development. The study will help move the technology a step towards that ideal. In addition, the research also says something about TMSR and how it might counteract poorly adapted plasticity in the cortex after a person loses a limb. This might provide new insights into the nature and the reversibility of cortical plasticity in patients with amputations and how phantom limb syndrome and pain arises in amputees.

Related Links

Brain, 2017, 140, 2993-3011: "Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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