Dr Ben Seymour, Computational and Biological Learning Lab, Trumpington Street, Cambridge CB2 1PZ


Center for Information and Neural Networks, National Institute for Information and Communications Technology (NICT), 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan.

bjs49 AT / seymour AT

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Phantoms and robots.

Losing a limb is a double-edged sword – not only do you lose the motor function of your arm or leg, but often the absent limb is plagued with unbearable phantom pain. Theories of phantom limb pain place central importance on the notion of aberrant reogranisation of the deafferented cortical representation of the affected limb. Support for this comes from the apparent therapeutic benefit of procedures like the famous mirror box - they temporarily support less abnormal representations through previously learned sensory associations. Thus for this and many other types of pain syndrome caused by lesions to the pain system, restorative approaches are likely to offer significant promise.

But how do you restore a missing limb? From a motor perspective, you can use brain-machine interfaces: by decoding activity from micro-electrode arrays implanted over M1, it’s possible to control quite complex movements of a robotic arm. However most robotic limbs don't usually feel. Last year, O’Doherty and colleagues published a seminal paper in Nature showing that microstimulation of primate S1 can be used to guide sensory-motor exploration of visually identified targets. They showed that monkeys could guide virtual hand and ‘feel’ objects, and then grab them with a virtual arm controlled by simultaneous motor decoding over M1. This provided the first example of how so called brain-machine-brain interfaces can be designed to yield closed loop sensory-motor systems, yielding full avatar systems.

In their latest paper, the same group take these findings one step further, by showing that variation of the statistics of S1 cortical microstimulation – by altering the periodicity and regularity of a pulse train - leads to discriminable artificial touch. This is important, because it moves beyond a crude pulse of sensory stimulation to the demonstration that potentially quite sophisticated sensory information can be encoded in the stimulation statistics. This means that in principle, complex and multidimensional information could be fed back from sensor-containing robotic prosthetic limbs in amputee patients, allowing cortical sensory representation, and a subsequent physiological embodiment of the prosthesis. And if current theories are correct, this should abolish phantom limb pain.

It also raises the intriguing possibility that more advanced sensation could be added - why stop at traditional sensations of touch, temperature and vibration, when you could add a metal detector, or a sonar, to your fingertips...


O'Doherty, J.E., Lebedev, M.A., Ifft, P.J., Zhuang, K.Z., Shokur, S. Bleuler, H. & M.A.L. Nicolelis. (2011). 
Active tactile exploration using a brain-machine-brain interface. 
Nature, 479(7372): 228-231. 

O'Doherty, J.E., Lebedev, M.A., Li, Z. & M.A.L. Nicolelis. (2012). 
Virtual active touch using randomly patterned intracortical microstimulation. 
IEEE Transactions on Neural Systems and Rehabilitation Engineering, 20(1): 85-93. 

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