Returning control to paralyzed limbs one nerve at a time.

A few months ago we reported on a fascinating study undertaken by Andy Schwartz and colleagues at the University of Pittsburgh, who developed a brain-machine interface that when implanted into the motor cortex, the part of the brain responsible for controlling voluntary muscle movements,  of monkeys allowed then to control a robot arm with surprising precision.  This week Chet Moritz and colleagues at Washington National Primate Research Center have published another exciting paper  (1) online in the journal Nature that describes an alternative approach to the use of brain-machine interfaces to overcome paralysis.

Rather than use an implant that monitors the activity of groups of nerve cells in the motor cortex and then use complex algorithms to decode this activity and calculate appropriate control signals for external devices, the approach used by the University of Pittsburg group, Chet Moritz and colleagues used a brain implant that could detect the activity of a single nerve cell and then home in on it and measure its activity. These implants were placed in the part of the monkey motor cortex responsible for controlling the wrist muscle and used implanted wires to directly stimulate the wrist muscles using a technique known as functional electrical stimulation (FES).  Monkeys whose wrist muscles had been temporarily paralyzed by injection of anaesthetic to the nerves that control them, quickly learned to control their wrist muscles again using the brain implant-FES system. The wrist muscle movements in turn controlled the location of a cursor on a screen, and by moving the cursor to particular locations in a screen the monkeys could gain rewards in the form of a tasty snack. More surprisingly the scientists found that monkeys could also learn to use motor cortex neurons that were not normally involved in controlling the wrist muscles to control the wrist muscles.

This research has caught the attention of the mainstream press, and it’s good to see that the welcome it has received is accompanied by cautionary notes.  There’s no doubt that this is a significant advance, the Washington National Primate Research Center team have shown that a relatively simple device can be used to restore control to paralyzed muscles, but they have so far only demonstrated control of one muscle group whereas a useful limb will require the simultaneous and accurate control of many muscle groups by many nerve cells.  I’m optimistic that this won’t be as much of a problem as it may initially appear since this study, the previous work at the University of Pittsburgh, and indeed the frequently observed ability of patients with brain damage to recover lost functions, all demonstrate that the brain is surprisingly adaptable. Whether using individual nerve cells to control muscles or groups of nerve cells to control robots will prove must useful in the clinic several years down the line is impossible to say right now. It’s quite likely that elements of both techniques will be used in future systems and that he decision as to which approach should be used in an individual paralysis patient will be determined by the nature of the injury and duration of subsequent paralysis.
Several scientists involved in this work have also stressed the importance of sensory feedback, the ability of a patient to “feel” what a paralyzed or robotic limb is doing, and this is an area under investigation by several research groups that will no doubt see further advances in the coming years.  Even without the ability to feel objects, and consequently the ability to more precisely manipulate objects, I’m of the opinion that the ability to use a robotic arm, or even a patients own arm,  has the potential to greatly increase the independence of paralysis patients. For that reason I expect that we will see this technology in the clinic sooner than many people think, and will be a therapeutic advance that many paralysis patients will welcome.


Paul Browne

1) Moritz C.T., Perlmutter S.I. and Fetz E.E. “Direct control of paralyzed muscles by cortical neurons” Nature. 2008 October 15. DOI: 10.1038/nature07418 [Epub ahead of print]

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